New in version 3.20.

LANGUAGE source file property explicitly compiles as specified language.

The 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 cmake_policy() command to set it to OLD or NEW explicitly. Unlike many policies, CMake version 3.21.7 does not warn when this policy is not set and simply uses OLD behavior.


The OLD behavior of a policy is deprecated by definition and may be removed in a future version of CMake.