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

12 files changed, 3 insertions, 24 deletions

diff --git a/Modules/Compiler/AppleClang-C.cmake b/Modules/Compiler/AppleClang-C.cmake
index 26a4bbd..2794f52 100644
--- a/Modules/Compiler/AppleClang-C.cmake
+++ b/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")
diff --git a/Modules/Compiler/AppleClang-CXX.cmake b/Modules/Compiler/AppleClang-CXX.cmake
index 611c674..15edc21 100644
--- a/Modules/Compiler/AppleClang-CXX.cmake
+++ b/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()
diff --git a/Modules/Compiler/Clang-C.cmake b/Modules/Compiler/Clang-C.cmake
index fb6ffa7..7c4a263 100644
--- a/Modules/Compiler/Clang-C.cmake
+++ b/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)
diff --git a/Modules/Compiler/Clang-CXX.cmake b/Modules/Compiler/Clang-CXX.cmake
index 311d2b0..789e991 100644
--- a/Modules/Compiler/Clang-CXX.cmake
+++ b/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()
 
diff --git a/Modules/Compiler/GNU-C.cmake b/Modules/Compiler/GNU-C.cmake
index 8105a77..ca286b3 100644
--- a/Modules/Compiler/GNU-C.cmake
+++ b/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")
diff --git a/Modules/Compiler/GNU-CXX.cmake b/Modules/Compiler/GNU-CXX.cmake
index 59ec056..fcaaeab 100644
--- a/Modules/Compiler/GNU-CXX.cmake
+++ b/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")
diff --git a/Modules/Compiler/Intel-C.cmake b/Modules/Compiler/Intel-C.cmake
index 322f63d..ec3bfd8 100644
--- a/Modules/Compiler/Intel-C.cmake
+++ b/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")
diff --git a/Modules/Compiler/Intel-CXX.cmake b/Modules/Compiler/Intel-CXX.cmake
index 42adfd1..b71b946 100644
--- a/Modules/Compiler/Intel-CXX.cmake
+++ b/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")
diff --git a/Modules/Compiler/XL-C.cmake b/Modules/Compiler/XL-C.cmake
index 78c44d5..2077bda 100644
--- a/Modules/Compiler/XL-C.cmake
+++ b/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")
diff --git a/Modules/Compiler/XL-CXX.cmake b/Modules/Compiler/XL-CXX.cmake
index 3b911f3..41e3e11 100644
--- a/Modules/Compiler/XL-CXX.cmake
+++ b/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>")
diff --git a/Modules/Compiler/XLClang-C.cmake b/Modules/Compiler/XLClang-C.cmake
index 1668a4d..54c18a6 100644
--- a/Modules/Compiler/XLClang-C.cmake
+++ b/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")
diff --git a/Modules/Compiler/XLClang-CXX.cmake b/Modules/Compiler/XLClang-CXX.cmake
index 02638c7..9ea3d7c 100644
--- a/Modules/Compiler/XLClang-CXX.cmake
+++ b/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 "")