Revert "specify language flag when source LANGUAGE property is set"

Revert commit 74b1c9fc8e (Explicitly specify language flag when source
LANGUAGE property is set, 2020-06-01, v3.19.0-rc1~722^2) and the lookup
tables from its two immediate ancestors.  The purpose of that change was
to convert an explicit `LANGUAGE` source file property into an explicit
language specification compiler flag like `-x c`.  This seems reasonable
since the property is documented as meaning "indicate what programming
language the source file is".  It is also needed to help compilers deal
with non-standard source file extensions they don't recognize.

However, some projects have been setting `LANGUAGE C` on `.S` assembler
source files to mean "use the C compiler".  Passing `-x c` for them
breaks the build because the `.S` sources are not written in C.  These
projects should be updated to use `enable_language(ASM)`, for which
CMake often chooses the C compiler as the assembler when using
toolchains that support it (which would have to be the case for projects
using the approach).

Revert the change for now to preserve the old behavior for such projects.
We can re-introduce it with a policy in a future version of CMake.

Fixes: #21469
Issue: #14516, #20716

Help/release/3.19.rst

@@ -330,3 +330,19 @@ Other Changes
 * If ``CUDA`` compiler detection fails with user-specified
   :variable:`CMAKE_CUDA_ARCHITECTURES` or
   :variable:`CMAKE_CUDA_HOST_COMPILER`, an error is raised.
+
+Updates
+=======
+
+Changes made since CMake 3.19.0 include the following.
+
+3.19.1
+------
+
+* CMake 3.19.0 compiles source files with the :prop_sf:`LANGUAGE`
+  property by passing an explicit language flag such as ``-x c``.
+  This is consistent with the property's documented meaning that
+  the source file is written in the specified language.  However,
+  it can break projects that were using the property only to
+  cause the specified language's compiler to be used.  This has
+  been reverted to restore behavior from CMake 3.18 and below.

Modules/Compiler/AppleClang-C.cmake

@@ -1,8 +1,6 @@
 include(Compiler/Clang)
 __compiler_clang(C)
 
-set(CMAKE_C_COMPILE_OPTIONS_EXPLICIT_LANGUAGE -x c)
-
 if(NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 4.0)
   set(CMAKE_C90_STANDARD_COMPILE_OPTION "-std=c90")
   set(CMAKE_C90_EXTENSION_COMPILE_OPTION "-std=gnu90")

Modules/Compiler/AppleClang-CXX.cmake

@@ -1,8 +1,6 @@
 include(Compiler/Clang)
 __compiler_clang(CXX)
 
-set(CMAKE_CXX_COMPILE_OPTIONS_EXPLICIT_LANGUAGE -x c++)
-
 if(NOT "x${CMAKE_CXX_SIMULATE_ID}" STREQUAL "xMSVC")
   set(CMAKE_CXX_COMPILE_OPTIONS_VISIBILITY_INLINES_HIDDEN "-fvisibility-inlines-hidden")
 endif()

Modules/Compiler/Clang-C.cmake

@@ -8,8 +8,6 @@ endif()
 
 if("x${CMAKE_C_COMPILER_FRONTEND_VARIANT}" STREQUAL "xMSVC")
   set(CMAKE_C_CLANG_TIDY_DRIVER_MODE "cl")
-elseif("x${CMAKE_CXX_COMPILER_FRONTEND_VARIANT}" STREQUAL "xGNU")
-  set(CMAKE_C_COMPILE_OPTIONS_EXPLICIT_LANGUAGE -x c)
 endif()
 
 if(NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 3.4)

Modules/Compiler/Clang-CXX.cmake

@@ -2,9 +2,7 @@ include(Compiler/Clang)
 __compiler_clang(CXX)
 __compiler_clang_cxx_standards(CXX)
 
-
 if("x${CMAKE_CXX_COMPILER_FRONTEND_VARIANT}" STREQUAL "xGNU")
-  set(CMAKE_CXX_COMPILE_OPTIONS_EXPLICIT_LANGUAGE -x c++)
   set(CMAKE_CXX_COMPILE_OPTIONS_VISIBILITY_INLINES_HIDDEN "-fvisibility-inlines-hidden")
 endif()
 

Modules/Compiler/GNU-C.cmake

@@ -1,8 +1,6 @@
 include(Compiler/GNU)
 __compiler_gnu(C)
 
-set(CMAKE_C_COMPILE_OPTIONS_EXPLICIT_LANGUAGE -x c)
-
 if (NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 4.5)
   set(CMAKE_C90_STANDARD_COMPILE_OPTION "-std=c90")
   set(CMAKE_C90_EXTENSION_COMPILE_OPTION "-std=gnu90")

Modules/Compiler/GNU-CXX.cmake

@@ -1,8 +1,6 @@
 include(Compiler/GNU)
 __compiler_gnu(CXX)
 
-set(CMAKE_CXX_COMPILE_OPTIONS_EXPLICIT_LANGUAGE -x c++)
-
 if (WIN32)
   if(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.6)
     set(CMAKE_CXX_COMPILE_OPTIONS_VISIBILITY_INLINES_HIDDEN "-fno-keep-inline-dllexport")

Modules/Compiler/Intel-C.cmake

@@ -28,8 +28,6 @@ if("x${CMAKE_C_SIMULATE_ID}" STREQUAL "xMSVC")
 
 else()
 
-  set(CMAKE_C_COMPILE_OPTIONS_EXPLICIT_LANGUAGE -x c)
-
   if (NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 15.0.0)
     set(CMAKE_C11_STANDARD_COMPILE_OPTION "-std=c11")
     set(CMAKE_C11_EXTENSION_COMPILE_OPTION "-std=gnu11")

Modules/Compiler/Intel-CXX.cmake

@@ -42,8 +42,6 @@ if("x${CMAKE_CXX_SIMULATE_ID}" STREQUAL "xMSVC")
 
 else()
 
-  set(CMAKE_CXX_COMPILE_OPTIONS_EXPLICIT_LANGUAGE -x c++)
-
   if (NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 19.0.0)
     set(CMAKE_CXX20_STANDARD_COMPILE_OPTION "-std=c++20")
     set(CMAKE_CXX20_EXTENSION_COMPILE_OPTION "-std=gnu++20")

Modules/Compiler/XL-C.cmake

@@ -6,8 +6,6 @@ string(APPEND CMAKE_C_FLAGS_MINSIZEREL_INIT " -DNDEBUG")
 # -qthreaded = Ensures that all optimizations will be thread-safe
 string(APPEND CMAKE_C_FLAGS_INIT " -qthreaded")
 
-set(CMAKE_C_COMPILE_OPTIONS_EXPLICIT_LANGUAGE -qsourcetype=c)
-
 if (CMAKE_C_COMPILER_VERSION VERSION_GREATER_EQUAL 10.1)
   set(CMAKE_C90_STANDARD_COMPILE_OPTION "-qlanglvl=stdc89")
   set(CMAKE_C90_EXTENSION_COMPILE_OPTION "-qlanglvl=extc89")

Modules/Compiler/XL-CXX.cmake

@@ -6,8 +6,6 @@ string(APPEND CMAKE_CXX_FLAGS_MINSIZEREL_INIT " -DNDEBUG")
 # -qthreaded = Ensures that all optimizations will be thread-safe
 string(APPEND CMAKE_CXX_FLAGS_INIT " -qthreaded")
 
-set(CMAKE_CXX_COMPILE_OPTIONS_EXPLICIT_LANGUAGE -+)
-
 if (CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 10.1)
   if(CMAKE_SYSTEM MATCHES "Linux")
     set(CMAKE_CXX98_STANDARD_COMPILE_OPTION "")
@@ -34,3 +32,6 @@ if (CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 10.1)
 endif ()
 
 __compiler_check_default_language_standard(CXX 10.1 98)
+
+set(CMAKE_CXX_COMPILE_OBJECT
+  "<CMAKE_CXX_COMPILER> -+ <DEFINES> <INCLUDES> <FLAGS> -o <OBJECT> -c <SOURCE>")

Modules/Compiler/XLClang-C.cmake

@@ -1,8 +1,6 @@
 include(Compiler/XLClang)
 __compiler_xlclang(C)
 
-set(CMAKE_C_COMPILE_OPTIONS_EXPLICIT_LANGUAGE -x c)
-
 if (CMAKE_C_COMPILER_VERSION VERSION_GREATER_EQUAL 13.1.1)
   set(CMAKE_C90_STANDARD_COMPILE_OPTION  "-std=c89")
   set(CMAKE_C90_EXTENSION_COMPILE_OPTION "-std=gnu89")

Modules/Compiler/XLClang-CXX.cmake

@@ -1,8 +1,6 @@
 include(Compiler/XLClang)
 __compiler_xlclang(CXX)
 
-set(CMAKE_CXX_COMPILE_OPTIONS_EXPLICIT_LANGUAGE -x c++)
-
 if (CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 13.1.1)
   set(CMAKE_CXX98_STANDARD_COMPILE_OPTION  "")
   set(CMAKE_CXX98_EXTENSION_COMPILE_OPTION "")

Source/cmExtraSublimeTextGenerator.cxx

@@ -349,13 +349,6 @@ std::string cmExtraSublimeTextGenerator::ComputeFlagsForObject(
   if (language.empty()) {
     language = "C";
   }
-
-  // explicitly add the explicit language flag before any other flag
-  // this way backwards compatibility with user flags is maintained
-  if (source->GetProperty("LANGUAGE")) {
-    lg->AppendFeatureOptions(flags, language, "EXPLICIT_LANGUAGE");
-  }
-
   std::string const& config =
     lg->GetMakefile()->GetSafeDefinition("CMAKE_BUILD_TYPE");
 

Source/cmGlobalXCodeGenerator.cxx

@@ -937,14 +937,6 @@ cmXCodeObject* cmGlobalXCodeGenerator::CreateXCodeSourceFile(
     default:
       break;
   }
-
-  // explicitly add the explicit language flag before any other flag
-  // this way backwards compatibility with user flags is maintained
-  if (sf->GetProperty("LANGUAGE")) {
-    this->CurrentLocalGenerator->AppendFeatureOptions(flags, lang,
-                                                      "EXPLICIT_LANGUAGE");
-  }
-
   const std::string COMPILE_FLAGS("COMPILE_FLAGS");
   if (cmProp cflags = sf->GetProperty(COMPILE_FLAGS)) {
     lg->AppendFlags(flags, genexInterpreter.Evaluate(*cflags, COMPILE_FLAGS));

Source/cmMakefileTargetGenerator.cxx

@@ -573,13 +573,6 @@ void cmMakefileTargetGenerator::WriteObjectRuleFiles(
   // Build the set of compiler flags.
   std::string flags;
 
-  // explicitly add the explicit language flag before any other flag
-  // this way backwards compatibility with user flags is maintained
-  if (source.GetProperty("LANGUAGE")) {
-    this->LocalGenerator->AppendFeatureOptions(flags, lang,
-                                               "EXPLICIT_LANGUAGE");
-  }
-
   // Add language-specific flags.
   std::string langFlags = cmStrCat("$(", lang, "_FLAGS", filterArch, ")");
   this->LocalGenerator->AppendFlags(flags, langFlags);

Source/cmNinjaTargetGenerator.cxx

@@ -190,16 +190,7 @@ std::string cmNinjaTargetGenerator::ComputeFlagsForObject(
     }
   }
 
-  std::string flags;
-  // explicitly add the explicit language flag before any other flag
-  // this way backwards compatibility with user flags is maintained
-  if (source->GetProperty("LANGUAGE")) {
-    this->LocalGenerator->AppendFeatureOptions(flags, language,
-                                               "EXPLICIT_LANGUAGE");
-    flags += " ";
-  }
-
-  flags += this->GetFlags(language, config, filterArch);
+  std::string flags = this->GetFlags(language, config, filterArch);
 
   // Add Fortran format flags.
   if (language == "Fortran") {

Tests/SetLang/CMakeLists.txt

@@ -15,10 +15,3 @@ if(CMAKE_GENERATOR MATCHES "^Visual Studio" AND "x${CMAKE_C_COMPILER_ID}" STREQU
   add_library(stay stay_c.c stay_cxx.cxx)
   set_property(TARGET stay PROPERTY COMPILE_OPTIONS "-TP")
 endif()
-
-if((CMAKE_C_COMPILER_ID MATCHES "(GNU|Clang|MSVC|Borland|Embarcadero|Intel|TI|XL)"))
-  add_library(zoom zoom.zzz)
-  set_source_files_properties(zoom.zzz PROPERTIES LANGUAGE CXX)
-  target_link_libraries(SetLang zoom)
-  target_compile_definitions(SetLang PRIVATE WITH_ZOOM)
-endif()

Tests/SetLang/bar.c

@@ -1,22 +1,10 @@
 #include <stdio.h>
 
 int foo();
-
-#ifdef WITH_ZOOM
-int zoom();
-#endif
-
 class A
 {
 public:
-  A()
-  {
-    this->i = foo();
-#ifdef WITH_ZOOM
-    i += zoom();
-    i -= zoom();
-#endif
-  }
+  A() { this->i = foo(); }
   int i;
 };
 

Tests/SetLang/zoom.zzz

@@ -1,7 +0,0 @@
-int zoom()
-{
-  int r = 10;
-  r++;
-  int ret = r + 10;
-  return ret;
-}