GIANT CLUSTERFUCK OF A COMMIT

OK, this is epic.  I was basically just trying to a) support Clang and
b) add more test coverage.  _THREE MONTHS_ and a few hundred CI runs
later, this is what I came up with.  I don't know how it ended up being
what it is, but here we go.

Some highlights of the changes:

1) CI builds has been moved to GitHub Actions,
2) the entire notion of a toolchain has been reworked; it now supports
Clang on all platforms.

* .github: this directory contains the GitHub Actions workflow
scripts/actions.  In the process, I created like 6 external GitHub
actions, but it's still pretty massive.  An upside is that it covers
much more platform/toolchain combinations _and_ check a lot of the
expected post-conditions.  TODO: .ci/Makefile is obsolete now, as well
as .travis.yml and .appveyor.yml.

* common.cmake: added Clang support.  In the process, a great deal has
been learned about how CMake works; in particular, static runtime
support has been reworked to be more robust.

* project: the entire notion of a "toolchain" has been reworked.
Instead of a measly --mingw parameter, there's now a separate --toolset
parameter, which allows you to choose between GCC, Clang, MSVC, etc.
Both Boost and CMake build scripts were enhanced greatly to support
Clang and other toolchains in a more robust way.

10 files changed, 787 insertions, 139 deletions

diff --git a/project/boost/build.py b/project/boost/build.py
index a9fe8da..3a073f3 100644
--- a/project/boost/build.py
+++ b/project/boost/build.py
@@ -5,8 +5,8 @@
 
 R'''Build Boost.
 
-This script builds the Boost libraries.  It's main utility is setting the
-correct --stagedir parameter value to avoid name clashes.
+This script builds the Boost libraries.  It main utility is setting the correct
+--stagedir parameter value to avoid name clashes.
 
 Usage example:
 
@@ -46,6 +46,7 @@ import sys
 import tempfile
 
 from project.boost.directory import BoostDir
+from project.toolchain import ToolchainType
 from project.boost.toolchain import Toolchain
 from project.configuration import Configuration
 from project.linkage import Linkage
@@ -60,14 +61,14 @@ DEFAULT_CONFIGURATIONS = (Configuration.DEBUG, Configuration.RELEASE,)
 # binaries from a CI, etc. and run them everywhere):
 DEFAULT_LINK = (Linkage.STATIC,)
 DEFAULT_RUNTIME_LINK = Linkage.STATIC
-# Shut compilers up:
-COMMON_B2_ARGS = ['-d0']
+B2_QUIET = ['-d0']
+B2_VERBOSE = ['-d2', '--debug-configuration']
 
 
 class BuildParameters:
     def __init__(self, boost_dir, build_dir=None, platforms=None,
                  configurations=None, link=None, runtime_link=None,
-                 mingw=False, b2_args=None):
+                 toolset=None, verbose=False, b2_args=None):
 
         boost_dir = normalize_path(boost_dir)
         if build_dir is not None:
@@ -76,10 +77,12 @@ class BuildParameters:
         configurations = configurations or DEFAULT_CONFIGURATIONS
         link = link or DEFAULT_LINK
         runtime_link = runtime_link or DEFAULT_RUNTIME_LINK
+        toolset = toolset or ToolchainType.AUTO
+        verbosity = B2_VERBOSE if verbose else B2_QUIET
         if b2_args:
-            b2_args = COMMON_B2_ARGS + b2_args
+            b2_args = verbosity + b2_args
         else:
-            b2_args = COMMON_B2_ARGS
+            b2_args = verbosity
 
         self.boost_dir = boost_dir
         self.build_dir = build_dir
@@ -88,17 +91,20 @@ class BuildParameters:
         self.configurations = configurations
         self.link = link
         self.runtime_link = runtime_link
-        self.mingw = mingw
+        self.toolset = toolset
         self.b2_args = b2_args
 
     @staticmethod
     def from_args(args):
         return BuildParameters(**vars(args))
 
+    def get_bootstrap_args(self):
+        return self.toolset.get_bootstrap_args()
+
     def enum_b2_args(self):
         with self._create_build_dir() as build_dir:
             for platform in self.platforms:
-                with Toolchain.detect(platform, mingw=self.mingw) as toolchain:
+                with Toolchain.detect(self.toolset, platform) as toolchain:
                     for configuration in self.configurations:
                         for link, runtime_link in self._linkage_options():
                             yield self._build_params(build_dir, toolchain,
@@ -138,9 +144,9 @@ class BuildParameters:
         params.append(self._stagedir(toolchain, configuration))
         params.append('--layout=system')
         params += toolchain.get_b2_args()
+        params.append(self._variant(configuration))
         params.append(self._link(link))
         params.append(self._runtime_link(runtime_link))
-        params.append(self._variant(configuration))
         params += self.b2_args
         return params
 
@@ -204,8 +210,10 @@ def _parse_args(argv=None):
                         type=Linkage.parse, default=DEFAULT_RUNTIME_LINK,
                         help=f'how the libraries link to the runtime ({linkage_options})')
 
-    parser.add_argument('--mingw', action='store_true',
-                        help='build using MinGW-w64')
+    toolset_options = '/'.join(map(str, ToolchainType.all()))
+    parser.add_argument('--toolset', metavar='TOOLSET',
+                        type=ToolchainType.parse, default=ToolchainType.AUTO,
+                        help=f'toolset to use ({toolset_options})')
 
     parser.add_argument('--build', metavar='DIR', dest='build_dir',
                         type=normalize_path,
@@ -214,6 +222,8 @@ def _parse_args(argv=None):
                         type=normalize_path,
                         help='root Boost directory')
 
+    parser.add_argument('-v', '--verbose', action='store_true',
+                        help='verbose b2 invocation (quiet by default)')
     parser.add_argument('b2_args', metavar='B2_ARG',
                         nargs='*', default=[],
                         help='additional b2 arguments, to be passed verbatim')
diff --git a/project/boost/directory.py b/project/boost/directory.py
index 17448b6..9b35194 100644
--- a/project/boost/directory.py
+++ b/project/boost/directory.py
@@ -6,6 +6,7 @@
 import logging
 import os.path
 
+from project.boost.toolchain import BootstrapToolchain
 from project.utils import cd, run
 from project.os import on_windows
 
@@ -21,18 +22,19 @@ class BoostDir:
 
     def build(self, params):
         with self._go():
-            self._bootstrap_if_required()
+            self._bootstrap_if_required(params)
             self._b2(params)
 
-    def _bootstrap_if_required(self):
+    def _bootstrap_if_required(self, params):
         if os.path.isfile(self._b2_path()):
             logging.info('Not going to bootstrap, b2 is already there')
             return
-        self.bootstrap()
+        self.bootstrap(params)
 
-    def bootstrap(self):
+    def bootstrap(self, params):
         with self._go():
-            run(self._bootstrap_path())
+            toolchain = BootstrapToolchain.detect(params.toolset)
+            run([self._bootstrap_path()] + self._bootstrap_args(toolchain))
 
     def _b2(self, params):
         for b2_params in params.enum_b2_args():
@@ -50,6 +52,12 @@ class BoostDir:
         return f'bootstrap{ext}'
 
     @staticmethod
+    def _bootstrap_args(toolchain):
+        if on_windows():
+            return toolchain.get_bootstrap_bat_args()
+        return toolchain.get_bootstrap_sh_args()
+
+    @staticmethod
     def _b2_path():
         return os.path.join('.', BoostDir._b2_name())
 
diff --git a/project/boost/download.py b/project/boost/download.py
index c3451a8..ca113a6 100644
--- a/project/boost/download.py
+++ b/project/boost/download.py
@@ -5,8 +5,8 @@
 
 R'''Download & bootstrap Boost.
 
-This script downloads and bootstraps a Boost distribution.  It's main utility
-is that it's supposed to be cross-platform.
+This script downloads and unpacks a Boost distribution archive.  Its main
+utility is that it's supposed to be cross-platform.
 
 Usage examples:
 
@@ -97,7 +97,6 @@ def download(params):
     with _download_if_necessary(params.version, params.storage) as path:
         archive = Archive(params.version, path)
         boost_dir = archive.unpack(params.unpack_dir)
-        boost_dir.bootstrap()
         params.rename_if_necessary(boost_dir)
 
 
diff --git a/project/boost/toolchain.py b/project/boost/toolchain.py
index a77c073..cce06a4 100644
--- a/project/boost/toolchain.py
+++ b/project/boost/toolchain.py
@@ -3,78 +3,455 @@
 # For details, see https://github.com/egor-tensin/cmake-common.
 # Distributed under the MIT License.
 
-R'''Compiler detection.
-
-It is assumed that Boost.Build is good enough to detect both GCC on Linux and
-MSVC on Windows.  From that point on, it's just a matter of setting the correct
-address-model= value.
-
-But I also frequently use MinGW-w64, and the most convinient way to use it that
-I know is making a "user config" and passing it to b2 using the --user-config
-parameter.
-'''
+# Hate speech
+# -----------
+#
+# Is there a person who doesn't hate Boost.Build?  I'm not sure, I'm definitely
+# _not_ one of these people.  Maybe it's the lack of adoption (meaning that
+# learning it is useless outside of Boost), maybe it's the incomprehensible
+# syntax.  Maybe it's the absolutely insane compiler-specific configuration
+# files (tools/build/src/tools/*.jam), which are impossible to figure out.
+# Maybe it's the fact that the implementation switched from C to C++ while some
+# half-baked Python implementation has been there since at least 2015 (see the
+# marvelous memo "Status: mostly ported." at the top of tools/build/src/build_system.py).
+#
+# What I hate the most though is how its various subtle, implicit and invisible
+# decision-making heuristics changed thoughout the release history of Boost.
+# You have a config and a compiler that will happily build version 1.65.0?
+# Great!  Want to use the same config and the same compiler to build version
+# 1.72.0?  Well, too fucking bad, it doesn't work anymore.  This I really do
+# hate the most.
+#
+# Three kinds of toolsets
+# -----------------------
+#
+# b2 accepts the toolset= parameter.  What about building b2 itself though?
+# Well, this is what the bootstrap.{sh,bat} scripts do.  They also accept a
+# toolset argument, but it is _completely_ different to that of b2.  That's
+# sort of OK, since e.g. cross-compiling b2 is something we rarely want to do
+# (and hence there must typically be a native toolset available).
+#
+# bootstrap.sh and bootstrap.bat are completely different (of course!), and
+# accept different arguments for their toolset parameters.
+#
+# Config file insanity
+# --------------------
+#
+# Say, we're building Boost on Windows using the GCC from a MinGW-w64
+# distribution.  We can pass toolset=gcc and all the required flags on the
+# command line no problem.  What if we want to make a user configuration file
+# so that 1) the command line is less polluted, and 2) it can possibly be
+# shared?  Well, if we put
+#
+#     using gcc : : : <name>value... ;
+#
+# there, Boost 1.65.0 will happily build everything, while Boost 1.72.0 will
+# complain about "duplicate initialization of gcc".  This is because when we
+# ran `bootstrap.bat gcc` earlier, it wrote `using gcc ;` in project-config.jam.
+# And while Boost 1.65.0 detects that toolset=gcc means we're going to use the
+# MinGW GCC, and magically turns toolset=gcc to toolset=gcc-mingw, Boost 1.72.0
+# does no such thing, and chokes on the "duplicate" GCC declaration.
+#
+# We also cannot put
+#
+#     using gcc : custom : : <options> ;
+#
+# without the executable path, since Boost insists that `g++ -dumpversion` must
+# equal to "custom" (which makes total sense, lol).  So we have to force it,
+# and do provide the path.
+#
+# Windows & Clang
+# ---------------
+#
+# Building Boost using Clang on Windows is a sad story.  As of 2020, there're
+# three main ways to install the native Clang toolchain on Windows:
+#
+#   * download the installer from llvm.org (`choco install llvm` does this)
+#     a.k.a. the upstream,
+#   * install it as part of a MSYS2 installation (`pacman -S mingw-w64-x86_64-clang`),
+#   * install as part of a Visual Studio installation.
+#
+# Using the latter method, you can switch a project to use the LLVM toolset
+# using Visual Studio, but that's stupid.  The former two, on the other hand,
+# give us the the required clang/clang++/clang-cl executables, so everything
+# seems to be fine.
+#
+# Except it's not fine.  Let's start with the fact that prior to 1.66.0,
+# toolset=clang is completely broken on Windows.  It's just an alias for
+# clang-linux, and it's hardcoded to require the ar & ranlib executables to
+# create static libraries.  Which is fine on Linux, since, and I'm quoting the
+# source, "ar is always available".  But it's not fine on Windows, since
+# ar/ranlib are not, in fact, available there by default.  Sure, you can
+# install some kind of MinGW toolchain, and it might even work, but what the
+# hell, honestly?
+#
+# Luckily, both the upstream distribution and the MSYS2 mingw-w64-x86_64-llvm
+# package come with the llvm-ar and llvm-ranlib utilities.  So we can put
+# something like this in the config:
+#
+#     using clang : custom : clang++.exe : <archiver>llvm-ar <ranlib>llvm-ranlib.exe ;
+#
+# and later call
+#
+#     b2 toolset=clang-custom --user-config=path/to/config.jam ...
+#
+# But, as I mentioned, prior to 1.66.0, toolset=clang is _hardcoded_ to use ar
+# & ranlib, these exact utility names.  So either get them as part of some
+# MinGW distribution or build Boost using another toolset.
+#
+# Now, it's all fine, but building stuff on Windows adds another thing into the
+# equation: debug runtimes.  When you build Boost using MSVC, for example, it
+# picks one of the appropriate /MT[d] or /MD[d] flags to build the Boost
+# libraries with.  Emulating these flags with toolset=clang is complicated and
+# inconvenient.  Luckily, there's the clang-cl.exe executable, which aims to
+# provide command line interface compatible with that of cl.exe.
+#
+# Boost.Build even supports toolset=clang-win, which should use clang-cl.exe.
+# But alas, it's completely broken prior to 1.69.0.  It just doesn't work at
+# all.  So, if you want to build w/ clang-cl.exe, either use Boost 1.69.0 or
+# later, or build using another toolset.
+#
+# Cygwin & Clang
+# --------------
+#
+# Now, a few words about Clang on Cygwin.  When building 1.65.0, I encountered
+# the following error:
+#
+#     /usr/include/w32api/synchapi.h:127:26: error: conflicting types for 'Sleep'
+#       WINBASEAPI VOID WINAPI Sleep (DWORD dwMilliseconds);
+#                              ^
+#     ./boost/smart_ptr/detail/yield_k.hpp:64:29: note: previous declaration is here
+#       extern "C" void __stdcall Sleep( unsigned long ms );
+#                                 ^
+#
+# GCC doesn't emit an error here because /usr/include is in a pre-configured
+# "system" include directories list, and the declaration there take precedence,
+# I guess?  The root of the problem BTW is that sizeof(unsigned long) is
+#
+#   * 4 for MSVC and MinGW-born GCCs,
+#   * 8 for Clang (and, strangely, Cygwin GCC; why don't we get runtime
+#     errors?).
+#
+# The fix is to add `define=BOOST_USE_WINDOWS_H`.  I don't even know what's the
+# point of not having it as a default.
 
 import abc
 from contextlib import contextmanager
 import logging
+import os.path
+import shutil
 
 import project.mingw
+import project.os
+from project.toolchain import ToolchainType
 from project.utils import temp_file
 
 
+class BootstrapToolchain(abc.ABC):
+    @abc.abstractmethod
+    def get_bootstrap_bat_args(self):
+        pass
+
+    @abc.abstractmethod
+    def get_bootstrap_sh_args(self):
+        pass
+
+    @staticmethod
+    def detect(hint):
+        if hint is ToolchainType.AUTO:
+            return BootstrapAuto()
+        if hint is ToolchainType.MSVC:
+            return BootstrapMSVC()
+        if hint is ToolchainType.GCC:
+            return BootstrapGCC()
+        if hint is ToolchainType.MINGW:
+            return BootstrapMinGW()
+        if hint is ToolchainType.CLANG:
+            return BootstrapClang()
+        if hint is ToolchainType.CLANG_CL:
+            return BootstrapClangCL()
+        raise NotImplementedError(f'unrecognized toolset: {hint}')
+
+
+class BootstrapAuto(BootstrapToolchain):
+    # Let Boost.Build do the detection.  Most commonly it means GCC on
+    # Linux-likes and MSVC on Windows.
+
+    def get_bootstrap_bat_args(self):
+        return []
+
+    def get_bootstrap_sh_args(self):
+        return []
+
+
+class BootstrapMSVC(BootstrapAuto):
+    # bootstrap.bat picks up MSVC by default.
+    pass
+
+
+class BootstrapGCC(BootstrapToolchain):
+    def get_bootstrap_bat_args(self):
+        return ['gcc']
+
+    def get_bootstrap_sh_args(self):
+        return ['--with-toolset=gcc']
+
+
+def _gcc_or_auto():
+    if shutil.which('gcc') is not None:
+        return ['gcc']
+    return []
+
+
+class BootstrapMinGW(BootstrapToolchain):
+    def get_bootstrap_bat_args(self):
+        # On Windows, prefer GCC if it's available.
+        return _gcc_or_auto()
+
+    def get_bootstrap_sh_args(self):
+        return []
+
+
+class BootstrapClang(BootstrapToolchain):
+    def get_bootstrap_bat_args(self):
+        # As of 1.74.0, bootstrap.bat isn't really aware of Clang, so try GCC,
+        # then auto-detect.
+        return _gcc_or_auto()
+
+    def get_bootstrap_sh_args(self):
+        # bootstrap.sh, on the other hand, is very much aware of Clang, and
+        # it can build b2 using this compiler.
+        return ['--with-toolset=clang']
+
+
+class BootstrapClangCL(BootstrapClang):
+    # There's no point in building b2 using clang-cl; clang though, presumably
+    # installed alongside clang-cl, should still be used if possible.
+    pass
+
+
 class Toolchain(abc.ABC):
     def __init__(self, platform):
         self.platform = platform
 
-    @abc.abstractmethod
     def get_b2_args(self):
-        pass
+        return [
+            # Always pass the address-model explicitly.
+            f'address-model={self.platform.get_address_model()}'
+        ]
 
     @staticmethod
     @contextmanager
-    def detect(platform, mingw=False):
-        if mingw:
+    def detect(hint, platform):
+        if hint is ToolchainType.AUTO:
+            yield Auto(platform)
+        elif hint is ToolchainType.MSVC:
+            yield MSVC(platform)
+        elif hint is ToolchainType.GCC:
+            with GCC.setup(platform) as toolchain:
+                yield toolchain
+        elif hint is ToolchainType.MINGW:
             with MinGW.setup(platform) as toolchain:
                 yield toolchain
+        elif hint is ToolchainType.CLANG:
+            with Clang.setup(platform) as toolchain:
+                yield toolchain
+        elif hint is ToolchainType.CLANG_CL:
+            yield ClangCL(platform)
         else:
-            yield Native(platform)
+            raise NotImplementedError(f'unrecognized toolset: {hint}')
 
-    @staticmethod
-    def _format_user_config(tag, compiler, **kwargs):
-        features = (f'<{k}>{v}' for k, v in kwargs.items())
-        features = ' '.join(features)
-        return f'using gcc : {tag} : {compiler} : {features} ;'
 
+class Auto(Toolchain):
+    # Let Boost.Build do the detection.  Most commonly it means GCC on
+    # Linux-likes and MSVC on Windows.
+    pass
 
-class Native(Toolchain):
+
+class MSVC(Auto):
     def get_b2_args(self):
-        return [f'address-model={self.platform.get_address_model()}']
+        return super().get_b2_args() + [
+            'toolset=msvc',
+        ]
+
 
+def _full_exe_name(exe):
+    if project.os.on_linux():
+        # There's no PATHEXT on Linux.
+        return exe
+    # b2 on Windows/Cygwin doesn't like it when the executable name doesn't
+    # include the extension.
+    dir_path = os.path.dirname(exe) or None
+    path = shutil.which(exe, path=dir_path)
+    if not path:
+        raise RuntimeError(f"executable '{exe}' could not be found")
+    if project.os.on_cygwin():
+        # On Cygwin, shutil.which('gcc') == '/usr/bin/gcc' and shutil.which('gcc.exe')
+        # == '/usr/bin/gcc.exe'; we want the latter version.  shutil.which('clang++')
+        # == '/usr/bin/clang++' is fine though, since it _is_ the complete path
+        # (clang++ is a symlink).
+        if os.path.exists(path) and os.path.exists(path + '.exe'):
+            path += '.exe'
+    if dir_path:
+        # If it was found in a specific directory, include the directory in the
+        # result.  shutil.which returns the executable name prefixed with the
+        # path argument.
+        return path
+    # If it was found in PATH, just return the basename (which includes the
+    # extension).
+    return os.path.basename(path)
 
-class MinGW(Toolchain):
-    TAG = 'custom'
 
-    def __init__(self, platform, config_path):
+class BoostBuildToolset:
+    CUSTOM = 'custom'
+
+    def __init__(self, compiler, path, options):
+        if not compiler:
+            raise RuntimeError('compiler type is required (like gcc, clang, etc.)')
+        self.compiler = compiler
+        self.version = BoostBuildToolset.CUSTOM
+        path = path or ''
+        path = path and _full_exe_name(path)
+        self.path = path
+        options = options or []
+        self.options = options
+
+    @property
+    def toolset_id(self):
+        if self.version:
+            return f'{self.compiler}-{self.version}'
+        return self.compiler
+
+    @property
+    def b2_arg(self):
+        return f'toolset={self.toolset_id}'
+
+    def _format_using_options(self):
+        return ''.join(f'\n    <{name}>{val}' for name, val in self.options)
+
+    def format_using(self):
+        version = self.version and f'{self.version} '
+        path = self.path and f'{self.path} '
+        return f'''using {self.compiler} : {version}: {path}:{self._format_using_options()}
+;'''
+
+
+class ConfigFile(Toolchain):
+    def __init__(self, platform, config_path, toolset):
         super().__init__(platform)
         self.config_path = config_path
-
-    def get_b2_args(self):
-        return [f'--user-config={self.config_path}', f'toolset=gcc-{MinGW.TAG}']
+        self.toolset = toolset
 
     @staticmethod
-    def _format_mingw_user_config(platform):
-        compiler = project.mingw.get_gxx(platform)
-        features = {
-            'target-os': 'windows',
-            'address-model': platform.get_address_model(),
-        }
-        return Toolchain._format_user_config(MinGW.TAG, compiler, **features)
+    @abc.abstractmethod
+    def get_toolset(platform):
+        pass
 
     @staticmethod
+    @abc.abstractmethod
+    def format_config(toolset):
+        pass
+
+    @classmethod
     @contextmanager
-    def setup(platform):
-        config = MinGW._format_mingw_user_config(platform)
+    def setup(cls, platform):
+        toolset = cls.get_toolset(platform)
+        config = cls.format_config(toolset)
         logging.info('Using user config:\n%s', config)
-        tmp = temp_file(config, mode='w', prefix='mingw_w64_', suffix='.jam')
+        tmp = temp_file(config, mode='w', prefix='user_config_', suffix='.jam')
         with tmp as path:
-            yield MinGW(platform, path)
+            yield cls(platform, path, toolset)
+
+    def get_b2_args(self):
+        # All the required options and the toolset definition should be in the
+        # user configuration file.
+        return super().get_b2_args() + [
+            f'--user-config={self.config_path}',
+            self.toolset.b2_arg,
+        ]
+
+
+class GCC(ConfigFile):
+    # Force GCC.  We don't care whether it's a native Linux GCC or a
+    # MinGW-flavoured GCC on Windows.
+    COMPILER = 'gcc'
+
+    @staticmethod
+    def get_options():
+        return [
+            # TODO: this is a petty attempt to get rid of build warnings in
+            # older Boost versions.  Revise and expand this list or remove it?
+            # warning: 'template<class> class std::auto_ptr' is deprecated
+            ('cxxflags', '-Wno-deprecated-declarations'),
+            # warning: unnecessary parentheses in declaration of 'assert_arg'
+            ('cxxflags', '-Wno-parentheses'),
+        ]
+
+    @staticmethod
+    def get_toolset(platform):
+        return BoostBuildToolset(GCC.COMPILER, 'g++', GCC.get_options())
+
+    @staticmethod
+    def format_config(toolset):
+        return toolset.format_using()
+
+
+class MinGW(GCC):
+    # It's important that Boost.Build is actually smart enough to detect the
+    # GCC prefix (like "x86_64-w64-mingw32" and prepend it to other tools like
+    # "ar").
+
+    @staticmethod
+    def get_toolset(platform):
+        path = project.mingw.get_gxx(platform)
+        return BoostBuildToolset(MinGW.COMPILER, path, MinGW.get_options())
+
+
+class Clang(ConfigFile):
+    COMPILER = 'clang'
+
+    @staticmethod
+    def get_toolset(platform):
+        options = [
+            ('cxxflags', '-DBOOST_USE_WINDOWS_H'),
+            # TODO: this is a petty attempt to get rid of build warnings in
+            # older Boost versions.  Revise and expand this list or remove it?
+            # warning: unused typedef 'boost_concept_check464' [-Wunused-local-typedef]
+            ('cxxflags', '-Wno-unused-local-typedef'),
+            # error: constant expression evaluates to -105 which cannot be narrowed to type 'boost::re_detail::cpp_regex_traits_implementation<char>::char_class_type' (aka 'unsigned int')
+            ('cxxflags', '-Wno-c++11-narrowing'),
+        ] + GCC.get_options()
+        if project.os.on_windows():
+            # Prefer LLVM binutils:
+            if shutil.which('llvm-ar') is not None:
+                options.append(('archiver', 'llvm-ar'))
+            if shutil.which('llvm-ranlib') is not None:
+                options.append(('ranlib', 'llvm-ranlib'))
+        return BoostBuildToolset(Clang.COMPILER, 'clang++', options)
+
+    @staticmethod
+    def format_config(toolset):
+        # To make clang.exe/clang++.exe work on Windows, some tweaks are
+        # required.  I borrowed them from CMake's Windows-Clang.cmake [1].
+        # Adding them globally to Boost.Build options is described in [2].
+        #
+        # [1]: https://github.com/Kitware/CMake/blob/v3.18.4/Modules/Platform/Windows-Clang.cmake
+        # [2]: https://stackoverflow.com/questions/2715106/how-to-create-a-new-variant-in-bjam
+        return f'''project : requirements
+    <target-os>windows:<define>_MT
+    <target-os>windows,<variant>debug:<define>_DEBUG
+    <target-os>windows,<runtime-link>static,<variant>debug:<cxxflags>"-Xclang -flto-visibility-public-std -Xclang --dependent-lib=libcmtd"
+    <target-os>windows,<runtime-link>static,<variant>release:<cxxflags>"-Xclang -flto-visibility-public-std -Xclang --dependent-lib=libcmt"
+    <target-os>windows,<runtime-link>shared,<variant>debug:<cxxflags>"-D_DLL -Xclang --dependent-lib=msvcrtd"
+    <target-os>windows,<runtime-link>shared,<variant>release:<cxxflags>"-D_DLL -Xclang --dependent-lib=msvcrt"
+;
+{toolset.format_using()}
+'''
+
+
+class ClangCL(Toolchain):
+    def get_b2_args(self):
+        return super().get_b2_args() + [
+            f'toolset=clang-win',
+            'define=BOOST_USE_WINDOWS_H',
+        ]
diff --git a/project/ci/boost.py b/project/ci/boost.py
index c17c9db..3da5c9b 100644
--- a/project/ci/boost.py
+++ b/project/ci/boost.py
@@ -7,8 +7,9 @@ import argparse
 import logging
 import sys
 
-from project.boost.download import DownloadParameters, download
 from project.boost.build import BuildParameters, build
+from project.boost.download import DownloadParameters, download
+from project.boost.toolchain import ToolchainType
 from project.linkage import Linkage
 
 
@@ -27,8 +28,10 @@ def _parse_args(dirs, argv=None):
     parser.add_argument('--runtime-link', metavar='LINKAGE',
                         type=Linkage.parse,
                         help='how the libraries link to the runtime')
-    parser.add_argument('--mingw', action='store_true',
-                        help='build using MinGW-w64')
+    parser.add_argument('--toolset', metavar='TOOLSET',
+                        type=ToolchainType.parse,
+                        help='toolset to use')
+
     parser.add_argument('b2_args', metavar='B2_ARG',
                         nargs='*', default=[],
                         help='additional b2 arguments, to be passed verbatim')
@@ -50,6 +53,6 @@ def build_ci(dirs, argv=None):
                              configurations=(dirs.get_configuration(),),
                              link=args.link,
                              runtime_link=args.runtime_link,
-                             mingw=args.mingw,
+                             toolset=args.toolset,
                              b2_args=args.b2_args)
     build(params)
diff --git a/project/ci/cmake.py b/project/ci/cmake.py
index 4a58749..df2b55a 100644
--- a/project/ci/cmake.py
+++ b/project/ci/cmake.py
@@ -8,6 +8,7 @@ import logging
 import sys
 
 from project.cmake.build import BuildParameters, build
+from project.toolchain import ToolchainType
 
 
 def _parse_args(dirs, argv=None):
@@ -23,8 +24,9 @@ def _parse_args(dirs, argv=None):
                         help='install directory')
     parser.add_argument('--boost', metavar='DIR', dest='boost_dir',
                         help='set Boost directory path')
-    parser.add_argument('--mingw', action='store_true',
-                        help='build using MinGW-w64')
+    parser.add_argument('--toolset', metavar='TOOLSET',
+                        type=ToolchainType.parse,
+                        help=f'toolset to use')
     parser.add_argument('cmake_args', nargs='*', metavar='CMAKE_ARG', default=[],
                         help='additional CMake arguments, to be passed verbatim')
     return parser.parse_args(argv)
@@ -39,6 +41,6 @@ def build_ci(dirs, argv=None):
                              platform=dirs.get_platform(),
                              configuration=dirs.get_configuration(),
                              boost_dir=args.boost_dir or dirs.get_boost_dir(),
-                             mingw=args.mingw,
+                             toolset=args.toolset,
                              cmake_args=dirs.get_cmake_args() + args.cmake_args)
     build(params)
diff --git a/project/cmake/build.py b/project/cmake/build.py
index e683eff..6bc7772 100644
--- a/project/cmake/build.py
+++ b/project/cmake/build.py
@@ -30,10 +30,13 @@ import tempfile
 from project.cmake.toolchain import Toolchain
 from project.configuration import Configuration
 from project.platform import Platform
+from project.toolchain import ToolchainType
 from project.utils import normalize_path, run, setup_logging
 
 
+DEFAULT_PLATFORM = Platform.native()
 DEFAULT_CONFIGURATION = Configuration.DEBUG
+DEFAULT_TOOLSET = ToolchainType.AUTO
 
 
 def run_cmake(cmake_args):
@@ -41,15 +44,18 @@ def run_cmake(cmake_args):
 
 
 class GenerationPhase:
-    def __init__(self, src_dir, build_dir, install_dir=None,
-                 platform=None, configuration=DEFAULT_CONFIGURATION,
-                 boost_dir=None, mingw=False, cmake_args=None):
+    def __init__(self, src_dir, build_dir, install_dir=None, platform=None,
+                 configuration=None, boost_dir=None, toolset=None,
+                 cmake_args=None):
         src_dir = normalize_path(src_dir)
         build_dir = normalize_path(build_dir)
         if install_dir is not None:
             install_dir = normalize_path(install_dir)
+        platform = platform or DEFAULT_PLATFORM
+        configuration = configuration or DEFAULT_CONFIGURATION
         if boost_dir is not None:
             boost_dir = normalize_path(boost_dir)
+        toolset = toolset or DEFAULT_TOOLSET
         cmake_args = cmake_args or []
 
         self.src_dir = src_dir
@@ -58,7 +64,7 @@ class GenerationPhase:
         self.platform = platform
         self.configuration = configuration
         self.boost_dir = boost_dir
-        self.mingw = mingw
+        self.toolset = toolset
         self.cmake_args = cmake_args
 
     def _cmake_args(self, toolchain):
@@ -87,44 +93,47 @@ class GenerationPhase:
             platform = Platform.native()
         return os.path.join(boost_dir, 'stage', str(platform), str(configuration), 'lib')
 
-    def run(self):
-        with Toolchain.detect(self.platform, self.build_dir, mingw=self.mingw) as toolchain:
-            run_cmake(self._cmake_args(toolchain))
+    def run(self, toolchain):
+        run_cmake(self._cmake_args(toolchain))
 
 
 class BuildPhase:
-    def __init__(self, build_dir, install_dir=None,
-                 configuration=DEFAULT_CONFIGURATION):
+    def __init__(self, build_dir, install_dir=None, configuration=None):
 
         build_dir = normalize_path(build_dir)
+        configuration = configuration or DEFAULT_CONFIGURATION
 
         self.build_dir = build_dir
         self.install_dir = install_dir
         self.configuration = configuration
 
-    def _cmake_args(self):
+    def _cmake_args(self, toolchain):
         result = ['--build', self.build_dir]
         result += ['--config', str(self.configuration)]
         if self.install_dir is not None:
             result += ['--target', 'install']
+        result += ['--'] + toolchain.get_build_args()
         return result
 
-    def run(self):
-        run_cmake(self._cmake_args())
+    def run(self, toolchain):
+        run_cmake(self._cmake_args(toolchain))
 
 
 class BuildParameters:
     def __init__(self, src_dir, build_dir=None, install_dir=None,
-                 platform=None, configuration=DEFAULT_CONFIGURATION,
-                 boost_dir=None, mingw=False, cmake_args=None):
+                 platform=None, configuration=None, boost_dir=None,
+                 toolset=None, cmake_args=None):
 
         src_dir = normalize_path(src_dir)
         if build_dir is not None:
             build_dir = normalize_path(build_dir)
         if install_dir is not None:
             install_dir = normalize_path(install_dir)
+        platform = platform or DEFAULT_PLATFORM
+        configuration = configuration or DEFAULT_CONFIGURATION
         if boost_dir is not None:
             boost_dir = normalize_path(boost_dir)
+        toolset = toolset or DEFAULT_TOOLSET
         cmake_args = cmake_args or []
 
         self.src_dir = src_dir
@@ -133,7 +142,7 @@ class BuildParameters:
         self.platform = platform
         self.configuration = configuration
         self.boost_dir = boost_dir
-        self.mingw = mingw
+        self.toolset = toolset
         self.cmake_args = cmake_args
 
     @staticmethod
@@ -158,17 +167,19 @@ class BuildParameters:
 
 def build(params):
     with params.create_build_dir() as build_dir:
+        toolchain = Toolchain.detect(params.toolset, params.platform, build_dir)
+
         gen_phase = GenerationPhase(params.src_dir, build_dir,
                                     install_dir=params.install_dir,
                                     platform=params.platform,
                                     configuration=params.configuration,
                                     boost_dir=params.boost_dir,
-                                    mingw=params.mingw,
+                                    toolset=params.toolset,
                                     cmake_args=params.cmake_args)
-        gen_phase.run()
+        gen_phase.run(toolchain)
         build_phase = BuildPhase(build_dir, install_dir=params.install_dir,
                                  configuration=params.configuration)
-        build_phase.run()
+        build_phase.run(toolchain)
 
 
 def _parse_args(argv=None):
@@ -201,8 +212,10 @@ def _parse_args(argv=None):
                         type=normalize_path,
                         help='set Boost directory path')
 
-    parser.add_argument('--mingw', action='store_true',
-                        help='build using MinGW-w64')
+    toolset_options = '/'.join(map(str, ToolchainType.all()))
+    parser.add_argument('--toolset', metavar='TOOLSET',
+                        type=ToolchainType.parse, default=ToolchainType.AUTO,
+                        help=f'toolset to use ({toolset_options})')
 
     parser.add_argument('src_dir', metavar='DIR',
                         type=normalize_path,
diff --git a/project/cmake/toolchain.py b/project/cmake/toolchain.py
index 073cd1b..7c96628 100644
--- a/project/cmake/toolchain.py
+++ b/project/cmake/toolchain.py
@@ -3,13 +3,127 @@
 # For details, see https://github.com/egor-tensin/cmake-common.
 # Distributed under the MIT License.
 
+# Default generator
+# -----------------
+#
+# As of CMake 3.18, the default generator (unless set explicitly) is:
+#   * the newest Visual Studio or "NMake Makefiles" on Windows,
+#   * "Unix Makefiles" otherwise.
+# This is regardless of whether any executables like gcc, cl or make are
+# available [1].
+#
+# Makefile generators
+# -------------------
+#
+# CMake has a number of "... Makefiles" generators.  "Unix Makefiles" uses
+# gmake/make/smake, whichever is found first, and cc/c++ for compiler
+# detection [2].  "MinGW Makefiles" looks for mingw32-make.exe in a number of
+# well-known locations, uses gcc/g++ directly, and is aware of windres [3].  In
+# addition, "Unix Makefiles" uses /bin/sh as the SHELL value in the Makefile,
+# while the MinGW version uses cmd.exe.  I don't think it matters on Windows
+# though, since the non-existent /bin/sh is ignored anyway [4].  "NMake
+# Makefiles" is similar, except it defaults to using cl [5].
+#
+# It's important to _not_ use the -A parameter with any of the Makefile
+# generators - it's an error.  This goes for "NMake Makefiles" also.  "NMake
+# Makefiles" doesn't attempt to search for installed Visual Studio compilers,
+# you need to use it from one of the Visual Studio-provided shells.
+#
+# Visual Studio generators
+# ------------------------
+#
+# These are special.  They ignore the CMAKE_<LANG>_COMPILER parameters and use
+# cl by default [9].  They support specifying the toolset to use via the -T
+# parameter (the "Platform Toolset" value in the project's properties) since
+# 3.18 [10].  The toolset list varies between Visual Studio versions, and I'm
+# too lazy to learn exactly which version supports which toolsets.
+#
+# cmake --build uses msbuild with Visual Studio generators.  You can pass the
+# path to a different cl.exe by doing something like
+#
+#     msbuild ... /p:CLToolExe=another-cl.exe /p:CLToolPath=C:\parent\dir
+#
+# It's important that the generators for Visual Studio 2017 or older use Win32
+# Win32 as the default platform [12].  Because of that, we need to pass the -A
+# parameter.
+#
+# mingw32-make vs make
+# --------------------
+#
+# No idea what the actual differences are.  The explanation in the FAQ [6]
+# about how GNU make "is lacking in some functionality and has modified
+# functionality due to the lack of POSIX on Win32" isn't terribly helpful.
+#
+# It's important that you can install either on Windows (`choco install make`
+# for GNU make and `choco install mingw` to install a MinGW-w64 distribution
+# with mingw32-make.exe included).  Personally, I don't see any difference
+# between using either make.exe or mingw32-make.exe w/ CMake on Windows.  But,
+# since MinGW-w64 distributions do include mingw32-make.exe and not make.exe,
+# we'll try to detect that.
+#
+# Cross-compilation
+# -----------------
+#
+# If you want to e.g. build x86 binary on x64 and vice versa, the easiest way
+# seems to be to make a CMake "toolchain file", which initializes the proper
+# compiler flags (like -m64/-m32, etc.).  Such file could look like this:
+#
+#     set(CMAKE_C_COMPILER   gcc)
+#     set(CMAKE_C_FLAGS      -m32)
+#     set(CMAKE_CXX_COMPILER g++)
+#     set(CMAKE_CXX_FLAGS    -m32)
+#
+# You can then pass the path to it using the CMAKE_TOOLCHAIN_FILE parameter.
+#
+# If you use the Visual Studio generators, just use the -A parameter, like "-A
+# Win32".
+#
+# As a side note, if you want to cross-compile between x86 and x64 using GCC on
+# Ubuntu, you need to install the gcc-multilib package.
+#
+# Windows & Clang
+# ---------------
+#
+# Using Clang on Windows is no easy task, of course.  Prior to 3.15, there was
+# no support for building things using the clang++.exe executable, only
+# clang-cl.exe was supported [7].  If you specified -DCMAKE_CXX_COMPILER=clang++,
+# CMake would stil pass MSVC-style command line options to the compiler (like
+# /MD, /nologo, etc.), which clang++ doesn't like [8].
+#
+# So, in summary, you can only use clang++ since 3.15.  clang-cl doesn't work
+# with Visual Studio generators unless you specify the proper toolset using the
+# -T parameter.  You can set the ClToolExe property using msbuild, but while
+# that might work in practice, clang-cl.exe needs to map some unsupported
+# options for everything to work properly.  For an example of how this is done,
+# see the LLVM.Cpp.Common.* files at [11].
+#
+# I recommend using Clang (either clang-cl or clang++ since 3.15) using the
+# "NMake Makefiles" generator.
+#
+# References
+# ----------
+#
+# [1]: cmake::EvaluateDefaultGlobalGenerator
+#      https://github.com/Kitware/CMake/blob/v3.18.4/Source/cmake.cxx
+# [2]: https://github.com/Kitware/CMake/blob/v3.18.4/Source/cmGlobalUnixMakefileGenerator3.cxx
+# [3]: https://github.com/Kitware/CMake/blob/v3.18.4/Source/cmGlobalMinGWMakefileGenerator.cxx
+# [4]: https://www.gnu.org/software/make/manual/html_node/Choosing-the-Shell.html
+# [5]: https://github.com/Kitware/CMake/blob/v3.18.4/Source/cmGlobalNMakeMakefileGenerator.cxx
+# [6]: http://mingw.org/wiki/FAQ
+# [7]: https://cmake.org/cmake/help/v3.15/release/3.15.html#compilers
+# [8]: https://github.com/Kitware/CMake/blob/v3.14.7/Modules/Platform/Windows-Clang.cmake
+# [9]: https://gitlab.kitware.com/cmake/cmake/-/issues/19174
+# [10]: https://cmake.org/cmake/help/v3.8/release/3.8.html
+# [11]: https://github.com/llvm/llvm-project/tree/e408935bb5339e20035d84307c666fbdd15e99e0/llvm/tools/msbuild
+# [12]: https://cmake.org/cmake/help/v3.18/generator/Visual%20Studio%2015%202017.html
+
 import abc
-from contextlib import contextmanager
 import os.path
+import shutil
 
 import project.mingw
-from project.platform import Platform
 from project.os import on_windows
+from project.toolchain import ToolchainType
 
 
 class Toolchain(abc.ABC):
@@ -17,89 +131,164 @@ class Toolchain(abc.ABC):
     def get_cmake_args(self):
         pass
 
-    @staticmethod
-    @contextmanager
-    def detect(platform, build_dir, mingw=False):
-        if mingw:
-            with MinGW.setup(platform, build_dir) as toolchain:
-                yield toolchain
-                return
-
-        if on_windows():
-            # MSVC is assumed.
-            if platform is None:
-                yield Native()
-                return
-            yield MSVC(platform)
-            return
+    @abc.abstractmethod
+    def get_build_args(self):
+        pass
 
-        with GCC.setup(platform, build_dir) as toolchain:
-            yield toolchain
-            return
+    @staticmethod
+    def detect(hint, platform, build_dir):
+        if hint is ToolchainType.AUTO:
+            if on_windows():
+                # We need to specify the -A parameter.  This might break if
+                # none of the Visual Studio generators are available, but the
+                # NMake one is, although I don't know how this can be possible
+                # normally.
+                hint = ToolchainType.MSVC
+            else:
+                # Same thing for the -m32/-m64 flags.
+                hint = ToolchainType.GCC
+        if hint is ToolchainType.MSVC:
+            return MSVC(platform)
+        if hint is ToolchainType.GCC:
+            return GCC.setup(platform, build_dir)
+        if hint is ToolchainType.MINGW:
+            return MinGW.setup(platform, build_dir)
+        if hint is ToolchainType.CLANG:
+            return Clang.setup(platform, build_dir)
+        if hint is ToolchainType.CLANG_CL:
+            return ClangCL.setup(platform, build_dir)
+        raise NotImplementedError(f'unrecognized toolset: {hint}')
 
 
-class Native(Toolchain):
+class Auto(Toolchain):
     def get_cmake_args(self):
         return []
 
+    def get_build_args(self):
+        return []
 
-class MSVC(Toolchain):
+
+class MSVC(Auto):
     def __init__(self, platform):
         self.platform = platform
 
     def get_cmake_args(self):
+        # This doesn't actually specify the generator of course, but I don't
+        # want to implement VS detection logic.
         return ['-A', self.platform.get_cmake_arch()]
 
+    def get_build_args(self):
+        return ['/m']
+
 
-class File(Toolchain):
+class Makefile(Toolchain):
     def __init__(self, path):
         self.path = path
 
     @staticmethod
-    def _get_path(build_dir):
+    def _get_config_path(build_dir):
         return os.path.join(build_dir, 'custom_toolchain.cmake')
 
+    def _get_makefile_generator(self):
+        if on_windows():
+            if shutil.which('mingw32-make'):
+                return 'MinGW Makefiles'
+            return 'Unix Makefiles'
+        # On Linux/Cygwin, make all the way:
+        return 'Unix Makefiles'
+
+    @classmethod
+    def write_config(cls, build_dir, contents):
+        path = Makefile._get_config_path(build_dir)
+        with open(path, mode='w') as file:
+            file.write(contents)
+        return cls(path)
+
     def get_cmake_args(self):
-        return ['-D', f'CMAKE_TOOLCHAIN_FILE={self.path}']
+        return [
+            '-D', f'CMAKE_TOOLCHAIN_FILE={self.path}',
+            # The Visual Studio generator is the default on Windows, override
+            # it:
+            '-G', self._get_makefile_generator(),
+        ]
+
+    def get_build_args(self):
+        return []
 
 
-class GCC(File):
+class GCC(Makefile):
     @staticmethod
     def _format(platform):
         return f'''
-set(CMAKE_C_COMPILER gcc)
-set(CMAKE_C_FLAGS -m{platform.get_address_model()})
-set(CMAKE_CXX_COMIPLER g++)
-set(CMAKE_CXX_FLAGS -m{platform.get_address_model()})
+set(CMAKE_C_COMPILER   gcc)
+set(CMAKE_C_FLAGS      -m{platform.get_address_model()})
+set(CMAKE_CXX_COMPILER g++)
+set(CMAKE_CXX_FLAGS    -m{platform.get_address_model()})
 '''
 
     @staticmethod
-    @contextmanager
     def setup(platform, build_dir):
-        if platform is None:
-            yield Native()
-            return
-        path = File._get_path(build_dir)
-        with open(path, mode='w') as file:
-            file.write(GCC._format(platform))
-        yield GCC(path)
+        return GCC.write_config(build_dir, GCC._format(platform))
 
 
-class MinGW(File):
+class MinGW(Makefile):
     @staticmethod
     def _format(platform):
         return f'''
 set(CMAKE_C_COMPILER   {project.mingw.get_gcc(platform)})
 set(CMAKE_CXX_COMPILER {project.mingw.get_gxx(platform)})
-set(CMAKE_RC_COMILER   {project.mingw.get_windres(platform)})
+set(CMAKE_AR           {project.mingw.get_ar(platform)})
+set(CMAKE_RANLIB       {project.mingw.get_ranlib(platform)})
+set(CMAKE_RC_COMPILER  {project.mingw.get_windres(platform)})
 set(CMAKE_SYSTEM_NAME  Windows)
 '''
 
     @staticmethod
-    @contextmanager
     def setup(platform, build_dir):
-        platform = platform or Platform.native()
-        path = File._get_path(build_dir)
-        with open(path, mode='w') as file:
-            file.write(MinGW._format(platform))
-        yield MinGW(path)
+        return MinGW.write_config(build_dir, MinGW._format(platform))
+
+
+class Clang(Makefile):
+    @staticmethod
+    def _format(platform):
+        return f'''
+if(CMAKE_VERSION VERSION_LESS "3.15" AND WIN32)
+    set(CMAKE_C_COMPILER   clang-cl)
+    set(CMAKE_CXX_COMPILER clang-cl)
+    set(CMAKE_C_FLAGS      -m{platform.get_address_model()})
+    set(CMAKE_CXX_FLAGS    -m{platform.get_address_model()})
+else()
+    set(CMAKE_C_COMPILER   clang)
+    set(CMAKE_CXX_COMPILER clang++)
+    set(CMAKE_C_FLAGS      -m{platform.get_address_model()})
+    set(CMAKE_CXX_FLAGS    -m{platform.get_address_model()})
+endif()
+'''
+
+    def _get_makefile_generator(self):
+        if on_windows():
+            # MinGW utilities like make might be unavailable, but NMake can
+            # very much be there.
+            if shutil.which('nmake'):
+                return 'NMake Makefiles'
+        return super()._get_makefile_generator()
+
+    @staticmethod
+    def setup(platform, build_dir):
+        return Clang.write_config(build_dir, Clang._format(platform))
+
+
+class ClangCL(Clang):
+    @staticmethod
+    def _format(platform):
+        return f'''
+set(CMAKE_C_COMPILER   clang-cl)
+set(CMAKE_CXX_COMPILER clang-cl)
+set(CMAKE_C_FLAGS      -m{platform.get_address_model()})
+set(CMAKE_CXX_FLAGS    -m{platform.get_address_model()})
+set(CMAKE_SYSTEM_NAME  Windows)
+'''
+
+    @staticmethod
+    def setup(platform, build_dir):
+        return ClangCL.write_config(build_dir, ClangCL._format(platform))
diff --git a/project/mingw.py b/project/mingw.py
index 1e136cd..731cee9 100644
--- a/project/mingw.py
+++ b/project/mingw.py
@@ -3,8 +3,6 @@
 # For details, see https://github.com/egor-tensin/cmake-common.
 # Distributed under the MIT License.
 
-from project.os import on_windows_like
-
 
 def _get_compiler_prefix(platform):
     target_arch = platform.get_address_model()
@@ -17,11 +15,7 @@ def _get_compiler_prefix(platform):
 
 def _get(platform, what):
     prefix = _get_compiler_prefix(platform)
-    ext = ''
-    if on_windows_like():
-        # Boost.Build wants the .exe extension at the end on Cygwin.
-        ext = '.exe'
-    path = f'{prefix}-w64-mingw32-{what}{ext}'
+    path = f'{prefix}-w64-mingw32-{what}'
     return path
 
 
@@ -33,5 +27,13 @@ def get_gxx(platform):
     return _get(platform, 'g++')
 
 
+def get_ar(platform):
+    return _get(platform, 'gcc-ar')
+
+
+def get_ranlib(platform):
+    return _get(platform, 'gcc-ranlib')
+
+
 def get_windres(platform):
     return _get(platform, 'windres')
diff --git a/project/toolchain.py b/project/toolchain.py
new file mode 100644
index 0000000..d931c6b
--- /dev/null
+++ b/project/toolchain.py
@@ -0,0 +1,45 @@
+# Copyright (c) 2020 Egor Tensin <Egor.Tensin@gmail.com>
+# This file is part of the "cmake-common" project.
+# For details, see https://github.com/egor-tensin/cmake-common.
+# Distributed under the MIT License.
+
+'''Supported platform/build system/compiler combinations include, but are not
+limited to:
+
+* Linux / make / Clang,
+* Linux / make / GCC,
+* Linux / make / MinGW-w64,
+* Windows / make / Clang (clang.exe & clang++.exe),
+* Windows / make / Clang (clang-cl.exe, Boost 1.69.0 or higher only),
+* Windows / make / MinGW-w64,
+* Windows / msbuild / MSVC,
+* Cygwin / make / Clang,
+* Cygwin / make / GCC,
+* Cygwin / make / MinGW-w64.
+'''
+
+import argparse
+from enum import Enum
+
+
+class ToolchainType(Enum):
+    AUTO = 'auto'   # This most commonly means GCC on Linux and MSVC on Windows.
+    MSVC = 'msvc'   # Force MSVC.
+    GCC = 'gcc'     # Force GCC.
+    MINGW = 'mingw' # As in MinGW-w64; GCC with the PLATFORM-w64-mingw32 prefix.
+    CLANG = 'clang'
+    CLANG_CL = 'clang-cl'
+
+    def __str__(self):
+        return self.value
+
+    @staticmethod
+    def all():
+        return tuple(ToolchainType)
+
+    @staticmethod
+    def parse(s):
+        try:
+            return ToolchainType(s)
+        except ValueError:
+            raise argparse.ArgumentTypeError(f'invalid toolset: {s}')