Treat '.ixx' and '.cppm' files as C++ sources

These extensions are used by convention for C++ modules and interface
units with MSVC.

Help/release/dev/cxx-module-extensions.rst

@@ -0,0 +1,4 @@
+cxx-module-extensions
+---------------------
+
+* Source file extensions ``.ixx`` and ``.cppm`` are now treated as C++.

Modules/CMakeCXXCompiler.cmake.in

@@ -44,7 +44,7 @@ if(CMAKE_COMPILER_IS_MINGW)
   set(MINGW 1)
 endif()
 set(CMAKE_CXX_COMPILER_ID_RUN 1)
-set(CMAKE_CXX_SOURCE_FILE_EXTENSIONS C;M;c++;cc;cpp;cxx;m;mm;mpp;CPP)
+set(CMAKE_CXX_SOURCE_FILE_EXTENSIONS C;M;c++;cc;cpp;cxx;m;mm;mpp;CPP;ixx;cppm)
 set(CMAKE_CXX_IGNORE_EXTENSIONS inl;h;hpp;HPP;H;o;O;obj;OBJ;def;DEF;rc;RC)
 
 foreach (lang C OBJC OBJCXX)

Source/cmMakefile.cxx

@@ -1222,7 +1222,7 @@ void cmMakefile::AddCustomCommandOldStyle(
 
   // Each output must get its own copy of this rule.
   cmsys::RegularExpression sourceFiles(
-    "\\.(C|M|c|c\\+\\+|cc|cpp|cxx|mpp|cu|m|mm|"
+    "\\.(C|M|c|c\\+\\+|cc|cpp|cxx|mpp|ixx|cppm|cu|m|mm|"
     "rc|def|r|odl|idl|hpj|bat|h|h\\+\\+|"
     "hm|hpp|hxx|in|txx|inl)$");
 

Source/cmSourceFile.h

@@ -175,7 +175,7 @@ private:
 #define CM_HEADER_REGEX "\\.(h|hh|h\\+\\+|hm|hpp|hxx|in|txx|inl)$"
 
 #define CM_SOURCE_REGEX                                                       \
-  "\\.(C|F|M|c|c\\+\\+|cc|cpp|mpp|cxx|cu|f|f90|for|fpp|ftn|m|mm|"             \
+  "\\.(C|F|M|c|c\\+\\+|cc|cpp|mpp|cxx|ixx|cppm|cu|f|f90|for|fpp|ftn|m|mm|"    \
   "rc|def|r|odl|idl|hpj|bat)$"
 
 #define CM_PCH_REGEX "cmake_pch(_[^.]+)?\\.(h|hxx)$"

Source/cmake.cxx

@@ -209,9 +209,9 @@ cmake::cmake(Role role, cmState::Mode mode)
     };
 
     // The "c" extension MUST precede the "C" extension.
-    setupExts(
-      this->CLikeSourceFileExtensions,
-      { "c", "C", "c++", "cc", "cpp", "cxx", "cu", "mpp", "m", "M", "mm" });
+    setupExts(this->CLikeSourceFileExtensions,
+              { "c", "C", "c++", "cc", "cpp", "cxx", "cu", "mpp", "m", "M",
+                "mm", "ixx", "cppm" });
     setupExts(this->HeaderFileExtensions,
               { "h", "hh", "h++", "hm", "hpp", "hxx", "in", "txx" });
     setupExts(this->CudaFileExtensions, { "cu" });