cmake: add project language to cmakelists.txt

cmake: get language from Meson project if not specified as depedency(..., langugage: ...)

deps: add threads method:cmake

dependency('threads', method: 'cmake') is useful for cmake unit tests
or those who just want to find threads using cmake.

cmake: project(... Fortran) generally also requires C language

8 files changed, 101 insertions, 15 deletions

diff --git a/mesonbuild/dependencies/base.py b/mesonbuild/dependencies/base.py
index 8e7179a68..cbc9f65d1 100644
--- a/mesonbuild/dependencies/base.py
+++ b/mesonbuild/dependencies/base.py
@@ -1037,7 +1037,23 @@ class CMakeDependency(ExternalDependency):
         # one module
         return module
 
-    def __init__(self, name: str, environment: Environment, kwargs, language=None):
+    def __init__(self, name: str, environment: Environment, kwargs, language: str = None):
+        if language is None:
+            if kwargs.get('native', False):
+                if 'c' in environment.coredata.compilers.build.keys():
+                    language = 'c'
+                elif 'cpp' in environment.coredata.compilers.build.keys():
+                    language = 'cpp'
+                elif 'fortran' in environment.coredata.compilers.build.keys():
+                    language = 'fortran'
+            else:
+                if 'c' in environment.coredata.compilers.host.keys():
+                    language = 'c'
+                elif 'cpp' in environment.coredata.compilers.host.keys():
+                    language = 'cpp'
+                elif 'fortran' in environment.coredata.compilers.host.keys():
+                    language = 'fortran'
+
         super().__init__('cmake', environment, language, kwargs)
         self.name = name
         self.is_libtool = False
@@ -1460,9 +1476,40 @@ class CMakeDependency(ExternalDependency):
         build_dir = Path(self.cmake_root_dir) / 'cmake_{}'.format(self.name)
         build_dir.mkdir(parents=True, exist_ok=True)
 
-        # Copy the CMakeLists.txt
+        # Insert language parameters into the CMakeLists.txt and write new CMakeLists.txt
         src_cmake = Path(__file__).parent / 'data' / cmake_file
-        shutil.copyfile(str(src_cmake), str(build_dir / 'CMakeLists.txt'))  # str() is for Python 3.5
+        cmake_txt = src_cmake.read_text()
+
+        # In general, some Fortran CMake find_package() also require C language enabled,
+        # even if nothing from C is directly used. An easy Fortran example that fails
+        # without C language is
+        #   find_package(Threads)
+        # To make this general to
+        # any other language that might need this, we use a list for all
+        # languages and expand in the cmake Project(... LANGUAGES ...) statement.
+        if self.language is None:
+            cmake_language = ['NONE']
+        elif self.language == 'c':
+            cmake_language = ['C']
+        elif self.language == 'cpp':
+            cmake_language = ['CXX']
+        elif self.language == 'cs':
+            cmake_language = ['CSharp']
+        elif self.language == 'cuda':
+            cmake_language = ['CUDA']
+        elif self.language == 'fortran':
+            cmake_language = ['C', 'Fortran']
+        elif self.language == 'objc':
+            cmake_language = ['OBJC']
+        elif self.language == 'objcpp':
+            cmake_language = ['OBJCXX']
+
+        cmake_txt = """
+cmake_minimum_required(VERSION ${{CMAKE_VERSION}})
+project(MesonTemp LANGUAGES {})
+""".format(' '.join(cmake_language)) + cmake_txt
+
+        (build_dir / 'CMakeLists.txt').write_text(cmake_txt)
 
         return str(build_dir)
 
diff --git a/mesonbuild/dependencies/data/CMakeLists.txt b/mesonbuild/dependencies/data/CMakeLists.txt
index 64f5b235c..b52a69a85 100644
--- a/mesonbuild/dependencies/data/CMakeLists.txt
+++ b/mesonbuild/dependencies/data/CMakeLists.txt
@@ -1,4 +1,8 @@
-cmake_minimum_required(VERSION ${CMAKE_MAJOR_VERSION}.${CMAKE_MINOR_VERSION}.${CMAKE_PATCH_VERSION} )
+# fail noisily if attempt to use this file without setting:
+# cmake_minimum_required(VERSION ${CMAKE_VERSION})
+# project(... LANGUAGES ...)
+
+cmake_policy(SET CMP0000 NEW)
 
 set(PACKAGE_FOUND FALSE)
 set(_packageName "${NAME}")
diff --git a/mesonbuild/dependencies/misc.py b/mesonbuild/dependencies/misc.py
index bfd450c4f..d773eb7ba 100644
--- a/mesonbuild/dependencies/misc.py
+++ b/mesonbuild/dependencies/misc.py
@@ -22,6 +22,7 @@ import sysconfig
 from .. import mlog
 from .. import mesonlib
 from ..environment import detect_cpu_family
+from ..mesonlib import listify
 
 from .base import (
     DependencyException, DependencyMethods, ExternalDependency,
@@ -109,15 +110,34 @@ class ThreadDependency(ExternalDependency):
     def __init__(self, environment, kwargs):
         super().__init__('threads', environment, None, kwargs)
         self.name = 'threads'
-        self.is_found = True
-        # Happens if you are using a language with threads
-        # concept without C, such as plain Cuda.
-        if self.clib_compiler is None:
-            self.compile_args = []
-            self.link_args = []
-        else:
-            self.compile_args = self.clib_compiler.thread_flags(environment)
-            self.link_args = self.clib_compiler.thread_link_flags(environment)
+        self.is_found = False
+        methods = listify(self.methods)
+        if DependencyMethods.AUTO in methods:
+            self.is_found = True
+            # Happens if you are using a language with threads
+            # concept without C, such as plain Cuda.
+            if self.clib_compiler is None:
+                self.compile_args = []
+                self.link_args = []
+            else:
+                self.compile_args = self.clib_compiler.thread_flags(environment)
+                self.link_args = self.clib_compiler.thread_link_flags(environment)
+            return
+
+        if DependencyMethods.CMAKE in methods:
+            # for unit tests and for those who simply want
+            # dependency('threads', method: 'cmake')
+            cmakedep = CMakeDependency('Threads', environment, kwargs)
+            if cmakedep.found():
+                self.compile_args = cmakedep.get_compile_args()
+                self.link_args = cmakedep.get_link_args()
+                self.version = cmakedep.get_version()
+                self.is_found = True
+                return
+
+    @staticmethod
+    def get_methods():
+        return [DependencyMethods.AUTO, DependencyMethods.CMAKE]
 
 
 class BlocksDependency(ExternalDependency):
diff --git a/mesonbuild/mesonlib.py b/mesonbuild/mesonlib.py
index 0d8ec3cff..5c3fc45cf 100644
--- a/mesonbuild/mesonlib.py
+++ b/mesonbuild/mesonlib.py
@@ -976,7 +976,9 @@ def replace_if_different(dst, dst_tmp):
     else:
         os.unlink(dst_tmp)
 
-def listify(item, flatten=True, unholder=False):
+def listify(item: typing.Any,
+            flatten: bool = True,
+            unholder: bool = False) -> typing.List[typing.Any]:
     '''
     Returns a list with all args embedded in a list if they are not a list.
     This function preserves order.
diff --git a/test cases/cmake/14 fortran threads/meson.build b/test cases/cmake/14 fortran threads/meson.build
new file mode 100644
index 000000000..f160ca806
--- /dev/null
+++ b/test cases/cmake/14 fortran threads/meson.build
@@ -0,0 +1,9 @@
+project('FortranThreads')
+
+if not add_languages('fortran', required: false)
+  error('MESON_SKIP_TEST: Fortran language not available.')
+endif
+
+# want to be sure that CMake can find dependencies where even if the
+# project isn't C, the C language is required to find the library.
+threads = dependency('threads', method: 'cmake', required: true)
diff --git a/test cases/frameworks/1 boost/meson.build b/test cases/frameworks/1 boost/meson.build
index 8f45dc7fe..4526c300c 100644
--- a/test cases/frameworks/1 boost/meson.build
+++ b/test cases/frameworks/1 boost/meson.build
@@ -1,3 +1,4 @@
+# this test requires the following on Ubuntu: libboost-{system,python,log,thread,test}-dev
 project('boosttest', 'cpp',
   default_options : ['cpp_std=c++11'])
 
diff --git a/test cases/frameworks/2 gtest/meson.build b/test cases/frameworks/2 gtest/meson.build
index 3f30215de..2d93b5294 100644
--- a/test cases/frameworks/2 gtest/meson.build
+++ b/test cases/frameworks/2 gtest/meson.build
@@ -1,3 +1,4 @@
+# on Ubuntu this test requires libgtest-dev
 project('gtest', 'cpp')
 
 gtest = dependency('gtest', main : true, required : false)
diff --git a/test cases/linuxlike/13 cmake dependency/meson.build b/test cases/linuxlike/13 cmake dependency/meson.build
index 682d6d84f..775355025 100644
--- a/test cases/linuxlike/13 cmake dependency/meson.build
+++ b/test cases/linuxlike/13 cmake dependency/meson.build
@@ -1,3 +1,5 @@
+# this test can ONLY be run successfully from run_project_test.py
+# due to use of setup_env.json
 project('external CMake dependency', 'c')
 
 if not find_program('cmake', required: false).found()
@@ -37,7 +39,7 @@ depf2 = dependency('ZLIB', required : false, method : 'cmake', modules : 'dfggh:
 assert(depf2.found() == false, 'Invalid CMake targets should fail')
 
 # Try to find cmMesonTestDep in a custom prefix
-
+# setup_env.json is used by run_project_tests.py:_run_test to point to ./cmake_pref_env/
 depPrefEnv = dependency('cmMesonTestDep', required : true, method : 'cmake')
 
 # Try to find a dependency with a custom CMake module