Green Hills Multi does not support COMPILE_LANGUAGE Generator Expressions
The Green Hills Multi generator is not supported for the COMPILE_LANGUAGE, COMPILE_LANG_AND_ID, LINK_LANGUAGE, and LINK_LANG_AND_ID generator expressions (not mentioned in the documentation at all). The Evaluate() methods in cmGeneratorExpressionNode.cxx contains a check for the generator name, and this check excludes GHS.
In practice, this is causing compatibility issues with the Conan's (the C++ dependency manager) CMakeDeps, which uses the COMPILE_LANGUAGE generator expressions to set COMPILE_OPTIONS for targets in the cmake files it generates.