Genex: Enable use of COMPILE_LANGUAGE for compile options.

Follow-ups will allow the use of the generator expression
for compile definitions and include directories for non-IDE
generators.

1 files changed, 30 insertions, 0 deletions

diff --git a/Help/manual/cmake-generator-expressions.7.rst b/Help/manual/cmake-generator-expressions.7.rst
index c47a7c4..b1b1102 100644
--- a/Help/manual/cmake-generator-expressions.7.rst
+++ b/Help/manual/cmake-generator-expressions.7.rst
@@ -93,6 +93,32 @@ Available logical expressions are:
   for the 'head' target, an error is reported.  See the
   :manual:`cmake-compile-features(7)` manual for information on
   compile features.
+``$<COMPILE_LANGUAGE:lang>``
+  ``1`` when the language used for compilation unit matches ``lang``,
+  otherwise ``0``.  This expression used to specify compile options for
+  source files of a particular language in a target. For example, to specify
+  the use of the ``-fno-exceptions`` compile option (compiler id checks
+  elided):
+
+  .. code-block:: cmake
+
+    add_executable(myapp main.cpp foo.c bar.cpp)
+    target_compile_options(myapp
+      PRIVATE $<$<COMPILE_LANGUAGE:CXX>:-fno-exceptions>
+    )
+
+  This generator expression has limited use because it is not possible to
+  use it with the Visual Studio generators.  Portable buildsystems would
+  not use this expression, and would create separate libraries for each
+  source file language instead:
+
+  .. code-block:: cmake
+
+    add_library(myapp_c foo.c)
+    add_library(myapp_cxx foo.c)
+    target_compile_options(myapp_cxx PUBLIC -fno-exceptions)
+    add_executable(myapp main.cpp)
+    target_link_libraries(myapp myapp_c myapp_cxx)
 
 Informational Expressions
 =========================
@@ -174,6 +200,10 @@ Available informational expressions are:
 ``$<INSTALL_PREFIX>``
   Content of the install prefix when the target is exported via
   :command:`install(EXPORT)` and empty otherwise.
+``$<COMPILE_LANGUAGE>``
+  The compile language of source files when evaluating compile options. See
+  the unary version for notes about portability of this generator
+  expression.
 
 Output Expressions
 ==================