Compile with explicit language flag when source LANGUAGE property is set

This change was originally made by commit 74b1c9fc8e (Explicitly specify
language flag when source LANGUAGE property is set, 2020-06-01,
v3.19.0-rc1~722^2), but it was reverted by commit 30aa715fac (Revert
"specify language flag when source LANGUAGE property is set",
2020-11-19) to restore compatibility with pre-3.19 behavior.

Implement the change again, but add policy CMP0119 to make this change
while preserving compatibility with existing projects.

Note that the `Compiler/{Clang,Intel,MSVC}-CXX` modules do not need to
specify `-TP` for their MSVC-like variants because we already use the
flag in `CMAKE_CXX_COMPILE_OBJECT`.  Similarly for `Compiler/XL-CXX`
and `Platform/Windows-Embarcadero`.

Note also that this does not seem possible to implement for XL C.
Even with `-qsourcetype=c`, `xlc` complains about an unknown suffix:
`1501-218 (W) file /.../AltExtC.zzz contains an incorrect file suffix`.
It returns non-zero even with `-qsuppress=1501-218`.

Co-Author: Robert Maynard <robert.maynard@kitware.com>
Fixes: #14516, #20716

1 files changed, 36 insertions, 0 deletions

diff --git a/Help/policy/CMP0119.rst b/Help/policy/CMP0119.rst
new file mode 100644
index 0000000..61c8bdc
--- /dev/null
+++ b/Help/policy/CMP0119.rst
@@ -0,0 +1,36 @@
+CMP0119
+-------
+
+.. versionadded:: 3.20
+
+:prop_sf:`LANGUAGE` source file property explicitly compiles as specified
+language.
+
+The :prop_sf:`LANGUAGE` source file property is documented to mean that the
+source file is written in the specified language.  In CMake 3.19 and below,
+setting this property causes CMake to compile the source file using the
+compiler for the specified language.  However, it only passes an explicit
+flag to tell the compiler to treat the source as the specified language
+for MSVC-like, XL, and Embarcadero compilers for the ``CXX`` language.
+CMake 3.20 and above prefer to also explicitly tell the compiler to use
+the specified language using a flag such as ``-x c`` on all compilers
+for which such flags are known.
+
+This policy provides compatibility for projects that have not been updated
+to expect this behavior.  For example, some projects were setting the
+``LANGUAGE`` property to ``C`` on assembly-language ``.S`` source files
+in order to compile them using the C compiler.  Such projects should be
+updated to use ``enable_language(ASM)``, for which CMake will often choose
+the C compiler as the assembler on relevant platforms anyway.
+
+The ``OLD`` behavior for this policy is to interpret the ``LANGUAGE <LANG>``
+property using its undocumented meaning to "use the ``<LANG>`` compiler".
+The ``NEW`` behavior for this policy is to interpret the ``LANGUAGE <LANG>``
+property using its documented meaning to "compile as a ``<LANG>`` source".
+
+This policy was introduced in CMake version 3.20.  Use the
+:command:`cmake_policy` command to set it to ``OLD`` or ``NEW`` explicitly.
+Unlike many policies, CMake version |release| does *not* warn
+when this policy is not set and simply uses ``OLD`` behavior.
+
+.. include:: DEPRECATED.txt