MSVC: Add abstraction for runtime library selection

Replace our hard-coded defaults for `/MD` and `/MDd` with a first-class
abstraction to select the runtime library from an enumeration of logical
names.  We've long hesitated to do this because the idea of "runtime
library selection" touches on related concepts on several platforms.
Avoid that scope creep by simply defining an abstraction that applies
only when targeting the MSVC ABI on Windows.

Removing the old default flags requires a policy because existing
projects may rely on string processing to edit them and choose a runtime
library under the old behavior.  Add policy CMP0091 to provide
compatibility.

Fixes: #19108

9 files changed, 126 insertions, 0 deletions

diff --git a/Help/command/try_compile.rst b/Help/command/try_compile.rst
index ca8fc77..0bc2ca3 100644
--- a/Help/command/try_compile.rst
+++ b/Help/command/try_compile.rst
@@ -135,6 +135,7 @@ default values:
 * :variable:`CMAKE_ENABLE_EXPORTS`
 * :variable:`CMAKE_LINK_SEARCH_START_STATIC`
 * :variable:`CMAKE_LINK_SEARCH_END_STATIC`
+* :variable:`CMAKE_MSVC_RUNTIME_LIBRARY`
 * :variable:`CMAKE_POSITION_INDEPENDENT_CODE`
 
 If :policy:`CMP0056` is set to ``NEW``, then
diff --git a/Help/manual/cmake-policies.7.rst b/Help/manual/cmake-policies.7.rst
index e89ea3da..043fb5c 100644
--- a/Help/manual/cmake-policies.7.rst
+++ b/Help/manual/cmake-policies.7.rst
@@ -57,6 +57,7 @@ Policies Introduced by CMake 3.15
 .. toctree::
    :maxdepth: 1
 
+   CMP0091: MSVC runtime library flags are selected by an abstraction. </policy/CMP0091>
    CMP0090: export(PACKAGE) does not populate package registry by default. </policy/CMP0090>
    CMP0089: Compiler id for IBM Clang-based XL compilers is now XLClang. </policy/CMP0089>
 
diff --git a/Help/manual/cmake-properties.7.rst b/Help/manual/cmake-properties.7.rst
index 4d4b9ff..3da9144 100644
--- a/Help/manual/cmake-properties.7.rst
+++ b/Help/manual/cmake-properties.7.rst
@@ -280,6 +280,7 @@ Properties on Targets
    /prop_tgt/MACOSX_RPATH
    /prop_tgt/MANUALLY_ADDED_DEPENDENCIES
    /prop_tgt/MAP_IMPORTED_CONFIG_CONFIG
+   /prop_tgt/MSVC_RUNTIME_LIBRARY
    /prop_tgt/NAME
    /prop_tgt/NO_SONAME
    /prop_tgt/NO_SYSTEM_FROM_IMPORTED
diff --git a/Help/manual/cmake-variables.7.rst b/Help/manual/cmake-variables.7.rst
index 18dd9d7..22e8add 100644
--- a/Help/manual/cmake-variables.7.rst
+++ b/Help/manual/cmake-variables.7.rst
@@ -386,6 +386,7 @@ Variables that Control the Build
    /variable/CMAKE_MODULE_LINKER_FLAGS_CONFIG_INIT
    /variable/CMAKE_MODULE_LINKER_FLAGS_INIT
    /variable/CMAKE_MSVCIDE_RUN_PATH
+   /variable/CMAKE_MSVC_RUNTIME_LIBRARY
    /variable/CMAKE_NINJA_OUTPUT_PATH_PREFIX
    /variable/CMAKE_NO_BUILTIN_CHRPATH
    /variable/CMAKE_NO_SYSTEM_FROM_IMPORTED
diff --git a/Help/policy/CMP0091.rst b/Help/policy/CMP0091.rst
new file mode 100644
index 0000000..5b7c4e3
--- /dev/null
+++ b/Help/policy/CMP0091.rst
@@ -0,0 +1,47 @@
+CMP0091
+-------
+
+MSVC runtime library flags are selected by an abstraction.
+
+Compilers targeting the MSVC ABI have flags to select the MSVC runtime library.
+Runtime library selection typically varies with build configuration because
+there is a separate runtime library for Debug builds.
+
+In CMake 3.14 and below, MSVC runtime library selection flags are added to
+the default :variable:`CMAKE_<LANG>_FLAGS_<CONFIG>` cache entries by CMake
+automatically.  This allows users to edit their cache entries to adjust the
+flags.  However, the presence of such default flags is problematic for
+projects that want to choose a different runtime library programmatically.
+In particular, it requires string editing of the
+:variable:`CMAKE_<LANG>_FLAGS_<CONFIG>` variables with knowledge of the
+CMake builtin defaults so they can be replaced.
+
+CMake 3.15 and above prefer to leave the MSVC runtime library selection flags
+out of the default :variable:`CMAKE_<LANG>_FLAGS_<CONFIG>` values and instead
+offer a first-class abstraction.  The :variable:`CMAKE_MSVC_RUNTIME_LIBRARY`
+variable and :prop_tgt:`MSVC_RUNTIME_LIBRARY` target property may be set to
+select the MSVC runtime library.
+
+This policy provides compatibility with projects that have not been updated
+to be aware of the abstraction.  The policy setting takes effect as of the
+first :command:`project` or :command:`enable_language` command that enables
+a language whose compiler targets the MSVC ABI.
+
+.. note::
+
+  Once the policy has taken effect at the top of a project, that choice
+  must be used throughout the tree.  In projects that have nested projects
+  in subdirectories, be sure to convert everything together.
+
+The ``OLD`` behavior for this policy is to place MSVC runtime library
+flags in the default :variable:`CMAKE_<LANG>_FLAGS_<CONFIG>` cache
+entries and ignore the :variable:`CMAKE_MSVC_RUNTIME_LIBRARY` abstraction.
+The ``NEW`` behavior for this policy is to *not* place MSVC runtime
+library flags in the default cache entries and use the abstraction instead.
+
+This policy was introduced in CMake version 3.15.  Use the
+:command:`cmake_policy` command to set it to ``OLD`` or ``NEW`` explicitly.
+Unlike many policies, CMake version |release| does *not* warn
+when this policy is not set and simply uses ``OLD`` behavior.
+
+.. include:: DEPRECATED.txt
diff --git a/Help/prop_tgt/MSVC_RUNTIME_LIBRARY-VALUES.txt b/Help/prop_tgt/MSVC_RUNTIME_LIBRARY-VALUES.txt
new file mode 100644
index 0000000..2bf71a9
--- /dev/null
+++ b/Help/prop_tgt/MSVC_RUNTIME_LIBRARY-VALUES.txt
@@ -0,0 +1,15 @@
+``MultiThreaded``
+  Compile with ``-MT`` or equivalent flag(s) to use a multi-threaded
+  statically-linked runtime library.
+``MultiThreadedDLL``
+  Compile with ``-MD`` or equivalent flag(s) to use a multi-threaded
+  dynamically-linked runtime library.
+``MultiThreadedDebug``
+  Compile with ``-MTd`` or equivalent flag(s) to use a multi-threaded
+  statically-linked runtime library.
+``MultiThreadedDebugDLL``
+  Compile with ``-MDd`` or equivalent flag(s) to use a multi-threaded
+  dynamically-linked runtime library.
+
+The value is ignored on non-MSVC compilers but an unsupported value will
+be rejected as an error when using a compiler targeting the MSVC ABI.
diff --git a/Help/prop_tgt/MSVC_RUNTIME_LIBRARY.rst b/Help/prop_tgt/MSVC_RUNTIME_LIBRARY.rst
new file mode 100644
index 0000000..1e3f5e9
--- /dev/null
+++ b/Help/prop_tgt/MSVC_RUNTIME_LIBRARY.rst
@@ -0,0 +1,26 @@
+MSVC_RUNTIME_LIBRARY
+--------------------
+
+Select the MSVC runtime library for use by compilers targeting the MSVC ABI.
+
+The allowed values are:
+
+.. include:: MSVC_RUNTIME_LIBRARY-VALUES.txt
+
+Use :manual:`generator expressions <cmake-generator-expressions(7)>` to
+support per-configuration specification.  For example, the code:
+
+.. code-block:: cmake
+
+  add_executable(foo foo.c)
+  set_property(TARGET foo PROPERTY
+    MSVC_RUNTIME_LIBRARY "MultiThreaded$<$<CONFIG:Debug>:Debug>")
+
+selects for the target ``foo`` a multi-threaded statically-linked runtime
+library with or without debug information depending on the configuration.
+
+.. note::
+
+  This property has effect only when policy :policy:`CMP0091` is set to ``NEW``
+  prior to the first :command:`project` or :command:`enable_language` command
+  that enables a language using a compiler targeting the MSVC ABI.
diff --git a/Help/release/dev/msvc-runtime-library.rst b/Help/release/dev/msvc-runtime-library.rst
new file mode 100644
index 0000000..4dddac2
--- /dev/null
+++ b/Help/release/dev/msvc-runtime-library.rst
@@ -0,0 +1,7 @@
+msvc-runtime-library
+--------------------
+
+* The :variable:`CMAKE_MSVC_RUNTIME_LIBRARY` variable and
+  :prop_tgt:`MSVC_RUNTIME_LIBRARY` target property were introduced to
+  select the runtime library used by compilers targeting the MSVC ABI.
+  See policy :policy:`CMP0091`.
diff --git a/Help/variable/CMAKE_MSVC_RUNTIME_LIBRARY.rst b/Help/variable/CMAKE_MSVC_RUNTIME_LIBRARY.rst
new file mode 100644
index 0000000..6ed68c9
--- /dev/null
+++ b/Help/variable/CMAKE_MSVC_RUNTIME_LIBRARY.rst
@@ -0,0 +1,27 @@
+CMAKE_MSVC_RUNTIME_LIBRARY
+--------------------------
+
+Select the MSVC runtime library for use by compilers targeting the MSVC ABI.
+This variable is used to initialize the :prop_tgt:`MSVC_RUNTIME_LIBRARY`
+property on all targets as they are created.  It is also propagated by
+calls to the :command:`try_compile` command into the test project.
+
+The allowed values are:
+
+.. include:: ../prop_tgt/MSVC_RUNTIME_LIBRARY-VALUES.txt
+
+Use :manual:`generator expressions <cmake-generator-expressions(7)>` to
+support per-configuration specification.  For example, the code:
+
+.. code-block:: cmake
+
+  set(CMAKE_MSVC_RUNTIME_LIBRARY "MultiThreaded$<$<CONFIG:Debug>:Debug>")
+
+selects for all following targets a multi-threaded statically-linked runtime
+library with or without debug information depending on the configuration.
+
+.. note::
+
+  This variable has effect only when policy :policy:`CMP0091` is set to ``NEW``
+  prior to the first :command:`project` or :command:`enable_language` command
+  that enables a language using a compiler targeting the MSVC ABI.