Help: Document that toolchain files may set implicit link information

Document in `CMAKE_<LANG>_IMPLICIT_LINK_{LIBRARIES,DIRECTORIES}` how
their value is used and that a fallback value may be provided by a
toolchain file.

2 files changed, 33 insertions, 5 deletions

diff --git a/Help/variable/CMAKE_LANG_IMPLICIT_LINK_DIRECTORIES.rst b/Help/variable/CMAKE_LANG_IMPLICIT_LINK_DIRECTORIES.rst
index e9e04be..081c4da 100644
--- a/Help/variable/CMAKE_LANG_IMPLICIT_LINK_DIRECTORIES.rst
+++ b/Help/variable/CMAKE_LANG_IMPLICIT_LINK_DIRECTORIES.rst
@@ -6,8 +6,20 @@ Implicit linker search path detected for language ``<LANG>``.
 Compilers typically pass directories containing language runtime
 libraries and default library search paths when they invoke a linker.
 These paths are implicit linker search directories for the compiler's
-language.  CMake automatically detects these directories for each
-language and reports the results in this variable.
+language.  For each language enabled by the :command:`project` or
+:command:`enable_language` command, CMake automatically detects these
+directories and reports the results in this variable.
+
+When linking to a static library, CMake adds the implicit link directories
+from this variable for each language used in the static library (except
+the language whose compiler is used to drive linking).  In the case of an
+imported static library, the :prop_tgt:`IMPORTED_LINK_INTERFACE_LANGUAGES`
+target property lists the languages whose implicit link information is
+needed.  If any of the languages is not enabled, its value for the
+``CMAKE_<LANG>_IMPLICIT_LINK_DIRECTORIES`` variable may instead be provided
+by the project.  Or, a :variable:`toolchain file <CMAKE_TOOLCHAIN_FILE>`
+may set the variable to a value known for the specified toolchain.  It will
+either be overridden when the language is enabled, or used as a fallback.
 
 Some toolchains read implicit directories from an environment variable such as
 ``LIBRARY_PATH``.  If using such an environment variable, keep its value
@@ -18,3 +30,5 @@ If policy :policy:`CMP0060` is not set to ``NEW``, then when a library in one
 of these directories is given by full path to :command:`target_link_libraries`
 CMake will generate the ``-l<name>`` form on link lines for historical
 purposes.
+
+See also the :variable:`CMAKE_<LANG>_IMPLICIT_LINK_LIBRARIES` variable.
diff --git a/Help/variable/CMAKE_LANG_IMPLICIT_LINK_LIBRARIES.rst b/Help/variable/CMAKE_LANG_IMPLICIT_LINK_LIBRARIES.rst
index ec16477..0c25489 100644
--- a/Help/variable/CMAKE_LANG_IMPLICIT_LINK_LIBRARIES.rst
+++ b/Help/variable/CMAKE_LANG_IMPLICIT_LINK_LIBRARIES.rst
@@ -5,6 +5,20 @@ Implicit link libraries and flags detected for language ``<LANG>``.
 
 Compilers typically pass language runtime library names and other
 flags when they invoke a linker.  These flags are implicit link
-options for the compiler's language.  CMake automatically detects
-these libraries and flags for each language and reports the results in
-this variable.
+options for the compiler's language.  For each language enabled
+by the :command:`project` or :command:`enable_language` command,
+CMake automatically detects these libraries and flags and reports
+the results in this variable.
+
+When linking to a static library, CMake adds the implicit link libraries and
+flags from this variable for each language used in the static library (except
+the language whose compiler is used to drive linking).  In the case of an
+imported static library, the :prop_tgt:`IMPORTED_LINK_INTERFACE_LANGUAGES`
+target property lists the languages whose implicit link information is
+needed.  If any of the languages is not enabled, its value for the
+``CMAKE_<LANG>_IMPLICIT_LINK_LIBRARIES`` variable may instead be provided
+by the project.  Or, a :variable:`toolchain file <CMAKE_TOOLCHAIN_FILE>`
+may set the variable to a value known for the specified toolchain.  It will
+either be overridden when the language is enabled, or used as a fallback.
+
+See also the :variable:`CMAKE_<LANG>_IMPLICIT_LINK_DIRECTORIES` variable.