Merge topic 'target-language-genex'

232a6883 Help: Add release notes for target-language-genex.
9e168941 File(GENERATE): Process genex evaluation files for each language.
b734fa44 Genex: Allow COMPILE_LANGUAGE when processing include directories.
0b945ea9 Genex: Allow COMPILE_LANGUAGE when processing compile definitions.
5c559f11 Genex: Enable use of COMPILE_LANGUAGE for compile options.
e387ce7d Genex: Add a COMPILE_LANGUAGE generator expression.
4a0128f4 VS6: Compute CMAKE_*_FLAGS and COMPILE_DEFINITIONS* only when needed

2 files changed, 53 insertions, 0 deletions

diff --git a/Help/manual/cmake-generator-expressions.7.rst b/Help/manual/cmake-generator-expressions.7.rst
index c47a7c4..d38cf7e 100644
--- a/Help/manual/cmake-generator-expressions.7.rst
+++ b/Help/manual/cmake-generator-expressions.7.rst
@@ -93,6 +93,46 @@ 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)
+
+  The ``Makefile`` and ``Ninja`` based generators can also use this
+  expression to specify compile-language specific compile definitions
+  and include directories:
+
+  .. code-block:: cmake
+
+    add_executable(myapp main.cpp foo.c bar.cpp)
+    target_compile_definitions(myapp
+      PRIVATE $<$<COMPILE_LANGUAGE:CXX>:COMPILING_CXX>
+    )
+    target_include_directories(myapp
+      PRIVATE $<$<COMPILE_LANGUAGE:CXX>:/opt/foo/cxx_headers>
+    )
 
 Informational Expressions
 =========================
@@ -174,6 +214,10 @@ 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
 ==================
diff --git a/Help/release/dev/target-language-genex.rst b/Help/release/dev/target-language-genex.rst
new file mode 100644
index 0000000..ed4cb5e
--- /dev/null
+++ b/Help/release/dev/target-language-genex.rst
@@ -0,0 +1,9 @@
+target-language-genex
+---------------------
+
+* A new ``COMPILE_LANGUAGE`` generator expression was introduced to
+  allow specification of compile options for target files based on the
+  :prop_sf:`LANGUAGE` of each source file.  Due to limitations of the
+  underlying native build tools, this feature has varying support across
+  generators.  See the :manual:`cmake-generator-expressions(7)` manual
+  for details.