Fix CMAKE_C[XX]_IMPLICIT_INCLUDE_DIRECTORIES

This fixes CMAKE_C_IMPLICIT_INCLUDE_DIRECTORIES for the case when
we don't use compiler-auto-detection. i.e.
`-DEMSCRIPTEN_FORCE_COMPILERS=OFF` which happens to be the default
today.

As a followup we should consider completely removing
`EMSCRIPTEN_FORCE_COMPILERS`.

Fixes: emscripten-core#23444

cmake/Modules/Platform/Emscripten.cmake

@@ -110,6 +110,16 @@ if (NOT EMSCRIPTEN_VERSION)
   set(EMSCRIPTEN_VERSION "${CMAKE_MATCH_1}")
 endif()
 
+execute_process(COMMAND "${EMSCRIPTEN_ROOT_PATH}/em-config${EMCC_SUFFIX}" "CACHE"
+  RESULT_VARIABLE _emcache_result
+  OUTPUT_VARIABLE _emcache_output
+  OUTPUT_STRIP_TRAILING_WHITESPACE)
+if (NOT _emcache_result EQUAL 0)
+  message(FATAL_ERROR "Failed to find emscripten cache directory with command \"'${EMSCRIPTEN_ROOT_PATH}/em-config${EMCC_SUFFIX}' CACHE\"! Process returned with error code ${_emcache_result}.")
+endif()
+file(TO_CMAKE_PATH "${_emcache_output}" _emcache_output)
+set(EMSCRIPTEN_SYSROOT "${_emcache_output}/sysroot")
+
 # Don't allow CMake to autodetect the compiler, since this is quite slow with
 # Emscripten.
 # Pass -DEMSCRIPTEN_FORCE_COMPILERS=OFF to disable (sensible mostly only for
@@ -159,6 +169,13 @@ if (EMSCRIPTEN_FORCE_COMPILERS)
   set(CMAKE_C_PLATFORM_ID "emscripten")
   set(CMAKE_CXX_PLATFORM_ID "emscripten")
 
+  if (NOT DEFINED CMAKE_C_IMPLICIT_INCLUDE_DIRECTORIES)
+    set(CMAKE_C_IMPLICIT_INCLUDE_DIRECTORIES "${EMSCRIPTEN_SYSROOT}/include")
+  endif()
+  if (NOT DEFINED CMAKE_CXX_IMPLICIT_INCLUDE_DIRECTORIES)
+    set(CMAKE_CXX_IMPLICIT_INCLUDE_DIRECTORIES "${EMSCRIPTEN_SYSROOT}/include;${EMSCRIPTEN_SYSROOT}/include/c++/v1")
+  endif()
+
   if ("${CMAKE_VERSION}" VERSION_LESS "3.8")
     set(CMAKE_C_COMPILE_FEATURES "c_function_prototypes;c_restrict;c_variadic_macros;c_static_assert")
     set(CMAKE_C90_COMPILE_FEATURES "c_function_prototypes")
@@ -201,16 +218,6 @@ if (EMSCRIPTEN_FORCE_COMPILERS)
   endif()
 endif()
 
-execute_process(COMMAND "${EMSCRIPTEN_ROOT_PATH}/em-config${EMCC_SUFFIX}" "CACHE"
-  RESULT_VARIABLE _emcache_result
-  OUTPUT_VARIABLE _emcache_output
-  OUTPUT_STRIP_TRAILING_WHITESPACE)
-if (NOT _emcache_result EQUAL 0)
-  message(FATAL_ERROR "Failed to find emscripten cache directory with command \"'${EMSCRIPTEN_ROOT_PATH}/em-config${EMCC_SUFFIX}' CACHE\"! Process returned with error code ${_emcache_result}.")
-endif()
-file(TO_CMAKE_PATH "${_emcache_output}" _emcache_output)
-set(EMSCRIPTEN_SYSROOT "${_emcache_output}/sysroot")
-
 list(APPEND CMAKE_FIND_ROOT_PATH "${EMSCRIPTEN_SYSROOT}")
 list(APPEND CMAKE_SYSTEM_PREFIX_PATH /)
 
@@ -366,11 +373,3 @@ endif()
 # complain about unused CMake variable.
 if (CMAKE_CROSSCOMPILING_EMULATOR)
 endif()
-
-# TODO: CMake appends <sysroot>/usr/include to implicit includes; switching to use usr/include will make this redundant.
-if (NOT DEFINED CMAKE_C_IMPLICIT_INCLUDE_DIRECTORIES)
-  set(CMAKE_C_IMPLICIT_INCLUDE_DIRECTORIES "${EMSCRIPTEN_SYSROOT}/include")
-endif()
-if (NOT DEFINED CMAKE_CXX_IMPLICIT_INCLUDE_DIRECTORIES)
-  set(CMAKE_CXX_IMPLICIT_INCLUDE_DIRECTORIES "${EMSCRIPTEN_SYSROOT}/include")
-endif()

test/cmake/static_lib/CMakeLists.txt

@@ -2,6 +2,11 @@ cmake_minimum_required(VERSION 3.0)
 
 project(static_library)
 
+if (CMAKE_EXPORT_COMPILE_COMMANDS)
+  message(STATUS "CMAKE_CXX_IMPLICIT_INCLUDE_DIRECTORIES -> ${CMAKE_CXX_IMPLICIT_INCLUDE_DIRECTORIES}")
+  set(CMAKE_CXX_STANDARD_INCLUDE_DIRECTORIES ${CMAKE_CXX_IMPLICIT_INCLUDE_DIRECTORIES})
+endif()
+
 # Test mode that checks that it's possible to override the file suffix
 # (independent of whether ar archive or LLVM bitcode file is created)
 # Note that "-DCMAKE_STATIC_LIBRARY_SUFFIX=foo" cannot be passed to CMake from

test/test_other.py

@@ -1020,6 +1020,25 @@ def test_cmake_bitcode_static_libraries(self):
     err = self.expect_fail([EMCMAKE, 'cmake', test_file('cmake/static_lib'), '-DEMSCRIPTEN_GENERATE_BITCODE_STATIC_LIBRARIES=ON'])
     self.assertContained('EMSCRIPTEN_GENERATE_BITCODE_STATIC_LIBRARIES is not compatible with the', err)
 
+  @crossplatform
+  @parameterized({
+    '': ([],),
+    'noforce': (['-DEMSCRIPTEN_FORCE_COMPILERS=OFF'],),
+  })
+  def test_cmake_compile_commands(self, args):
+    self.run_process([EMCMAKE, 'cmake', test_file('cmake/static_lib'), '-DCMAKE_EXPORT_COMPILE_COMMANDS=ON'] + args)
+    self.assertExists('compile_commands.json')
+    compile_commands = json.load(open('compile_commands.json'))
+    command = compile_commands[0]['command']
+    # Sometimes cmake puts the include dirs in an RSP file
+    rsp = [p for p in command.split() if 'includes_CXX.rsp' in p]
+    if rsp:
+      command = read_file(rsp[0][1:])
+    include_dir = utils.normalize_path(cache.get_sysroot_dir('include'))
+    include_dir_cxx = utils.normalize_path(cache.get_sysroot_dir('include/c++/v1'))
+    self.assertContained(include_dir, command)
+    self.assertContained(include_dir_cxx, command)
+
   @parameterized({
     '': ['0'],
     'suffix': ['1'],