cmTarget: add support for C++ module fileset types

C++ modules have two variants which are of importance to CMake:

  - `CXX_MODULES`: interface modules (those using `export module M;`,
    `export module M:part;`, or `module M:internal_part;`)
  - `CXX_MODULE_HEADER_UNITS`: importable header units

Creating C++ modules or partitions are *not* supported in any other
source listing. This is because the source files must be installed (so
their scope matters), but not part of usage requirements (what it means
for a module source to be injected into a consumer is not clear at this
moment). Due to the way `FILE_SET` works with scopes, they are a perfect
fit as long as `INTERFACE` is not allowed (which it is not).

15 files changed, 283 insertions, 20 deletions

diff --git a/Help/command/target_sources.rst b/Help/command/target_sources.rst
index 1ad6c37..0c4323c 100644
--- a/Help/command/target_sources.rst
+++ b/Help/command/target_sources.rst
@@ -75,9 +75,33 @@ File Sets
 Adds a file set to a target, or adds files to an existing file set. Targets
 have zero or more named file sets. Each file set has a name, a type, a scope of
 ``INTERFACE``, ``PUBLIC``, or ``PRIVATE``, one or more base directories, and
-files within those directories. The only acceptable type is ``HEADERS``. The
-optional default file sets are named after their type. The target may not be a
-custom target or :prop_tgt:`FRAMEWORK` target.
+files within those directories. The acceptable types include:
+
+``HEADERS``
+
+  Sources intended to be used via a language's ``#include`` mechanism.
+
+``CXX_MODULES``
+
+.. note ::
+
+  Experimental. Gated by ``CMAKE_EXPERIMENTAL_CXX_MODULE_CMAKE_API``
+
+  Sources which contain C++ interface module or partition units (i.e., those
+  using the ``export`` keyword). This file set type may not have an
+  ``INTERFACE`` scope.
+
+``CXX_MODULE_HEADER_UNITS``
+
+.. note ::
+
+  Experimental. Gated by ``CMAKE_EXPERIMENTAL_CXX_MODULE_CMAKE_API``
+
+  C++ header sources which may be imported by other C++ source code. This file
+  set type may not have an ``INTERFACE`` scope.
+
+The optional default file sets are named after their type. The target may not
+be a custom target or :prop_tgt:`FRAMEWORK` target.
 
 Files in a ``PRIVATE`` or ``PUBLIC`` file set are marked as source files for
 the purposes of IDE integration. Additionally, files in ``HEADERS`` file sets
@@ -93,16 +117,17 @@ Each ``target_sources(FILE_SET)`` entry starts with ``INTERFACE``, ``PUBLIC``, o
 
   The name of the file set to create or add to. It must contain only letters,
   numbers and underscores. Names starting with a capital letter are reserved
-  for built-in file sets predefined by CMake. The only predefined set name is
-  ``HEADERS``. All other set names must not start with a capital letter or
+  for built-in file sets predefined by CMake. The only predefined set names
+  are those matching the acceptable types. All other set names must not start
+  with a capital letter or
   underscore.
 
 ``TYPE <type>``
 
-  Every file set is associated with a particular type of file. ``HEADERS``
-  is currently the only defined type and it is an error to specify anything
-  else. As a special case, if the name of the file set is ``HEADERS``, the
-  type does not need to be specified and the ``TYPE <type>`` arguments can be
+  Every file set is associated with a particular type of file. Only types
+  specified above may be used and it is an error to specify anything else. As
+  a special case, if the name of the file set is one of the types, the type
+  does not need to be specified and the ``TYPE <type>`` arguments can be
   omitted. For all other file set names, ``TYPE`` is required.
 
 ``BASE_DIRS <dirs>...``
@@ -134,6 +159,8 @@ Each ``target_sources(FILE_SET)`` entry starts with ``INTERFACE``, ``PUBLIC``, o
 The following target properties are set by ``target_sources(FILE_SET)``,
 but they should not generally be manipulated directly:
 
+For file sets of type ``HEADERS``:
+
 * :prop_tgt:`HEADER_SETS`
 * :prop_tgt:`INTERFACE_HEADER_SETS`
 * :prop_tgt:`HEADER_SET`
@@ -141,17 +168,37 @@ but they should not generally be manipulated directly:
 * :prop_tgt:`HEADER_DIRS`
 * :prop_tgt:`HEADER_DIRS_<NAME>`
 
+For file sets of type ``CXX_MODULES``:
+
+* :prop_tgt:`CXX_MODULE_SETS`
+* :prop_tgt:`INTERFACE_CXX_MODULE_SETS`
+* :prop_tgt:`CXX_MODULE_SET`
+* :prop_tgt:`CXX_MODULE_SET_<NAME>`
+* :prop_tgt:`CXX_MODULE_DIRS`
+* :prop_tgt:`CXX_MODULE_DIRS_<NAME>`
+
+For file sets of type ``CXX_MODULE_HEADER_UNITS``:
+
+* :prop_tgt:`CXX_MODULE_HEADER_UNIT_SETS`
+* :prop_tgt:`INTERFACE_CXX_MODULE_HEADER_UNIT_SETS`
+* :prop_tgt:`CXX_MODULE_HEADER_UNIT_SET`
+* :prop_tgt:`CXX_MODULE_HEADER_UNIT_SET_<NAME>`
+* :prop_tgt:`CXX_MODULE_HEADER_UNIT_DIRS`
+* :prop_tgt:`CXX_MODULE_HEADER_UNIT_DIRS_<NAME>`
+
 Target properties related to include directories are also modified by
 ``target_sources(FILE_SET)`` as follows:
 
 :prop_tgt:`INCLUDE_DIRECTORIES`
 
-  If the ``TYPE`` is ``HEADERS``, and the scope of the file set is ``PRIVATE``
-  or ``PUBLIC``, all of the ``BASE_DIRS`` of the file set are wrapped in
-  :genex:`$<BUILD_INTERFACE>` and appended to this property.
+  If the ``TYPE`` is ``HEADERS`` or ``CXX_MODULE_HEADER_UNITS``, and the scope
+  of the file set is ``PRIVATE`` or ``PUBLIC``, all of the ``BASE_DIRS`` of
+  the file set are wrapped in :genex:`$<BUILD_INTERFACE>` and appended to this
+  property.
 
 :prop_tgt:`INTERFACE_INCLUDE_DIRECTORIES`
 
-  If the ``TYPE`` is ``HEADERS``, and the scope of the file set is
-  ``INTERFACE`` or ``PUBLIC``, all of the ``BASE_DIRS`` of the file set are
-  wrapped in :genex:`$<BUILD_INTERFACE>` and appended to this property.
+  If the ``TYPE`` is ``HEADERS`` or ``CXX_MODULE_HEADER_UNITS``, and the scope
+  of the file set is ``INTERFACE`` or ``PUBLIC``, all of the ``BASE_DIRS`` of
+  the file set are wrapped in :genex:`$<BUILD_INTERFACE>` and appended to this
+  property.
diff --git a/Help/dev/experimental.rst b/Help/dev/experimental.rst
index 2c2eb21..14e6075 100644
--- a/Help/dev/experimental.rst
+++ b/Help/dev/experimental.rst
@@ -57,11 +57,6 @@ dependencies to the file specified by the ``<DYNDEP_FILE>`` placeholder. The
 ``CMAKE_EXPERIMENTAL_CXX_SCANDEP_DEPFILE_FORMAT`` file may be set to ``msvc``
 for scandep rules which use ``msvc``-style dependency reporting.
 
-For tools which need to know the file set the source belongs to, the
-``CMAKE_EXPERIMENTAL_CXX_MODULE_SOURCE_TYPE_FLAG_<FILE_SET_TYPE>`` flag may
-be provided so that different source types can be distinguished prior to
-scanning.
-
 The module dependencies should be written in the format described
 by the `P1689r4`_ paper.
 
diff --git a/Help/manual/cmake-properties.7.rst b/Help/manual/cmake-properties.7.rst
index d88322c..dc2c591 100644
--- a/Help/manual/cmake-properties.7.rst
+++ b/Help/manual/cmake-properties.7.rst
@@ -184,6 +184,16 @@ Properties on Targets
    /prop_tgt/CUDA_STANDARD
    /prop_tgt/CUDA_STANDARD_REQUIRED
    /prop_tgt/CXX_EXTENSIONS
+   /prop_tgt/CXX_MODULE_DIRS
+   /prop_tgt/CXX_MODULE_DIRS_NAME
+   /prop_tgt/CXX_MODULE_SET
+   /prop_tgt/CXX_MODULE_SET_NAME
+   /prop_tgt/CXX_MODULE_SETS
+   /prop_tgt/CXX_MODULE_HEADER_UNIT_DIRS
+   /prop_tgt/CXX_MODULE_HEADER_UNIT_DIRS_NAME
+   /prop_tgt/CXX_MODULE_HEADER_UNIT_SET
+   /prop_tgt/CXX_MODULE_HEADER_UNIT_SET_NAME
+   /prop_tgt/CXX_MODULE_HEADER_UNIT_SETS
    /prop_tgt/CXX_STANDARD
    /prop_tgt/CXX_STANDARD_REQUIRED
    /prop_tgt/DEBUG_POSTFIX
@@ -262,6 +272,8 @@ Properties on Targets
    /prop_tgt/INTERFACE_COMPILE_DEFINITIONS
    /prop_tgt/INTERFACE_COMPILE_FEATURES
    /prop_tgt/INTERFACE_COMPILE_OPTIONS
+   /prop_tgt/INTERFACE_CXX_MODULE_SETS
+   /prop_tgt/INTERFACE_CXX_MODULE_HEADER_UNIT_SETS
    /prop_tgt/INTERFACE_HEADER_SETS
    /prop_tgt/INTERFACE_HEADER_SETS_TO_VERIFY
    /prop_tgt/INTERFACE_INCLUDE_DIRECTORIES
diff --git a/Help/prop_tgt/CXX_MODULE_DIRS.rst b/Help/prop_tgt/CXX_MODULE_DIRS.rst
new file mode 100644
index 0000000..fdf3831
--- /dev/null
+++ b/Help/prop_tgt/CXX_MODULE_DIRS.rst
@@ -0,0 +1,17 @@
+CXX_MODULE_DIRS
+---------------
+
+.. note ::
+
+  Experimental. Gated by ``CMAKE_EXPERIMENTAL_CXX_MODULE_CMAKE_API``
+
+Semicolon-separated list of base directories of the target's default
+C++ module set (i.e. the file set with name and type ``CXX_MODULES``). The
+property supports
+:manual:`generator expressions <cmake-generator-expressions(7)>`.
+
+This property is normally only set by :command:`target_sources(FILE_SET)`
+rather than being manipulated directly.
+
+See :prop_tgt:`CXX_MODULE_DIRS_<NAME>` for the list of base directories in
+other C++ module sets.
diff --git a/Help/prop_tgt/CXX_MODULE_DIRS_NAME.rst b/Help/prop_tgt/CXX_MODULE_DIRS_NAME.rst
new file mode 100644
index 0000000..8c27d45
--- /dev/null
+++ b/Help/prop_tgt/CXX_MODULE_DIRS_NAME.rst
@@ -0,0 +1,17 @@
+CXX_MODULE_DIRS_<NAME>
+----------------------
+
+.. note ::
+
+  Experimental. Gated by ``CMAKE_EXPERIMENTAL_CXX_MODULE_CMAKE_API``
+
+Semicolon-separated list of base directories of the target's ``<NAME>`` C++
+module set, which has the set type ``CXX_MODULES``. The property supports
+:manual:`generator expressions <cmake-generator-expressions(7)>`.
+
+This property is normally only set by :command:`target_sources(FILE_SET)`
+rather than being manipulated directly.
+
+See :prop_tgt:`CXX_MODULE_DIRS` for the list of base directories in the
+default C++ module set. See :prop_tgt:`CXX_MODULE_SETS` for the file set names
+of all C++ module sets.
diff --git a/Help/prop_tgt/CXX_MODULE_HEADER_UNIT_DIRS.rst b/Help/prop_tgt/CXX_MODULE_HEADER_UNIT_DIRS.rst
new file mode 100644
index 0000000..17e5cf0
--- /dev/null
+++ b/Help/prop_tgt/CXX_MODULE_HEADER_UNIT_DIRS.rst
@@ -0,0 +1,17 @@
+CXX_MODULE_HEADER_UNIT_DIRS
+---------------------------
+
+.. note ::
+
+  Experimental. Gated by ``CMAKE_EXPERIMENTAL_CXX_MODULE_CMAKE_API``
+
+Semicolon-separated list of base directories of the target's default C++
+module header set (i.e. the file set with name and type
+``CXX_MODULE_HEADER_UNITS``). The property supports
+:manual:`generator expressions <cmake-generator-expressions(7)>`.
+
+This property is normally only set by :command:`target_sources(FILE_SET)`
+rather than being manipulated directly.
+
+See :prop_tgt:`CXX_MODULE_HEADER_UNIT_DIRS_<NAME>` for the list of base directories
+in other C++ module header sets.
diff --git a/Help/prop_tgt/CXX_MODULE_HEADER_UNIT_DIRS_NAME.rst b/Help/prop_tgt/CXX_MODULE_HEADER_UNIT_DIRS_NAME.rst
new file mode 100644
index 0000000..ca30f23
--- /dev/null
+++ b/Help/prop_tgt/CXX_MODULE_HEADER_UNIT_DIRS_NAME.rst
@@ -0,0 +1,19 @@
+CXX_MODULE_HEADER_UNIT_DIRS_<NAME>
+----------------------------------
+
+.. note ::
+
+  Experimental. Gated by ``CMAKE_EXPERIMENTAL_CXX_MODULE_CMAKE_API``
+
+Semicolon-separated list of base directories of the target's ``<NAME>`` C++
+module header set, which has the set type ``CXX_MODULE_HEADER_UNITS``. The
+property supports
+:manual:`generator expressions <cmake-generator-expressions(7)>`.
+
+This property is normally only set by :command:`target_sources(FILE_SET)`
+rather than being manipulated directly.
+
+See :prop_tgt:`CXX_MODULE_HEADER_UNIT_DIRS` for the list of base directories
+in the default C++ module header set. See
+:prop_tgt:`CXX_MODULE_HEADER_UNIT_SETS` for the file set names of all C++
+module header sets.
diff --git a/Help/prop_tgt/CXX_MODULE_HEADER_UNIT_SET.rst b/Help/prop_tgt/CXX_MODULE_HEADER_UNIT_SET.rst
new file mode 100644
index 0000000..f67a848
--- /dev/null
+++ b/Help/prop_tgt/CXX_MODULE_HEADER_UNIT_SET.rst
@@ -0,0 +1,18 @@
+CXX_MODULE_HEADER_UNIT_SET
+--------------------------
+
+.. note ::
+
+  Experimental. Gated by ``CMAKE_EXPERIMENTAL_CXX_MODULE_CMAKE_API``
+
+Semicolon-separated list of files in the target's default C++ module header
+set, (i.e. the file set with name and type ``CXX_MODULE_HEADER_UNITS``). If
+any of the paths are relative, they are computed relative to the target's
+source directory. The property supports
+:manual:`generator expressions <cmake-generator-expressions(7)>`.
+
+This property is normally only set by :command:`target_sources(FILE_SET)`
+rather than being manipulated directly.
+
+See :prop_tgt:`CXX_MODULE_HEADER_UNIT_SET_<NAME>` for the list of files in
+other C++ module header sets.
diff --git a/Help/prop_tgt/CXX_MODULE_HEADER_UNIT_SETS.rst b/Help/prop_tgt/CXX_MODULE_HEADER_UNIT_SETS.rst
new file mode 100644
index 0000000..7b4bd3f
--- /dev/null
+++ b/Help/prop_tgt/CXX_MODULE_HEADER_UNIT_SETS.rst
@@ -0,0 +1,18 @@
+CXX_MODULE_HEADER_UNIT_SETS
+---------------------------
+
+.. note ::
+
+  Experimental. Gated by ``CMAKE_EXPERIMENTAL_CXX_MODULE_CMAKE_API``
+
+Read-only list of the target's ``PRIVATE`` and ``PUBLIC`` C++ module header
+sets (i.e. all file sets with the type ``CXX_MODULE_HEADER_UNITS``). Files
+listed in these file sets are treated as source files for the purpose of IDE
+integration.
+
+C++ module header sets may be defined using the :command:`target_sources`
+command ``FILE_SET`` option with type ``CXX_MODULE_HEADER_UNITS``.
+
+See also :prop_tgt:`CXX_MODULE_HEADER_UNIT_SET_<NAME>`,
+:prop_tgt:`CXX_MODULE_HEADER_UNIT_SET` and
+:prop_tgt:`INTERFACE_CXX_MODULE_HEADER_UNIT_SETS`.
diff --git a/Help/prop_tgt/CXX_MODULE_HEADER_UNIT_SET_NAME.rst b/Help/prop_tgt/CXX_MODULE_HEADER_UNIT_SET_NAME.rst
new file mode 100644
index 0000000..d328950
--- /dev/null
+++ b/Help/prop_tgt/CXX_MODULE_HEADER_UNIT_SET_NAME.rst
@@ -0,0 +1,19 @@
+CXX_MODULE_HEADER_UNIT_SET_<NAME>
+---------------------------------
+
+.. note ::
+
+  Experimental. Gated by ``CMAKE_EXPERIMENTAL_CXX_MODULE_CMAKE_API``
+
+Semicolon-separated list of files in the target's ``<NAME>`` C++ module header
+set, which has the set type ``CXX_MODULE_HEADER_UNITS``. If any of the paths
+are relative, they are computed relative to the target's source directory. The
+property supports
+:manual:`generator expressions <cmake-generator-expressions(7)>`.
+
+This property is normally only set by :command:`target_sources(FILE_SET)`
+rather than being manipulated directly.
+
+See :prop_tgt:`CXX_MODULE_HEADER_UNIT_SET` for the list of files in the
+default C++ module header set. See :prop_tgt:`CXX_MODULE_HEADER_UNIT_SETS` for
+the file set names of all C++ module header sets.
diff --git a/Help/prop_tgt/CXX_MODULE_SET.rst b/Help/prop_tgt/CXX_MODULE_SET.rst
new file mode 100644
index 0000000..ae9000e
--- /dev/null
+++ b/Help/prop_tgt/CXX_MODULE_SET.rst
@@ -0,0 +1,18 @@
+CXX_MODULE_SET
+--------------
+
+.. note ::
+
+  Experimental. Gated by ``CMAKE_EXPERIMENTAL_CXX_MODULE_CMAKE_API``
+
+Semicolon-separated list of files in the target's default C++ module set,
+(i.e. the file set with name and type ``CXX_MODULES``). If any of the paths
+are relative, they are computed relative to the target's source directory. The
+property supports
+:manual:`generator expressions <cmake-generator-expressions(7)>`.
+
+This property is normally only set by :command:`target_sources(FILE_SET)`
+rather than being manipulated directly.
+
+See :prop_tgt:`CXX_MODULE_SET_<NAME>` for the list of files in other C++
+module sets.
diff --git a/Help/prop_tgt/CXX_MODULE_SETS.rst b/Help/prop_tgt/CXX_MODULE_SETS.rst
new file mode 100644
index 0000000..c03df39
--- /dev/null
+++ b/Help/prop_tgt/CXX_MODULE_SETS.rst
@@ -0,0 +1,16 @@
+CXX_MODULE_SETS
+---------------
+
+.. note ::
+
+  Experimental. Gated by ``CMAKE_EXPERIMENTAL_CXX_MODULE_CMAKE_API``
+
+Read-only list of the target's ``PRIVATE`` and ``PUBLIC`` C++ module sets (i.e.
+all file sets with the type ``CXX_MODULES``). Files listed in these file sets are
+treated as source files for the purpose of IDE integration.
+
+C++ module sets may be defined using the :command:`target_sources` command
+``FILE_SET`` option with type ``CXX_MODULES``.
+
+See also :prop_tgt:`CXX_MODULE_SET_<NAME>`, :prop_tgt:`CXX_MODULE_SET` and
+:prop_tgt:`INTERFACE_CXX_MODULE_SETS`.
diff --git a/Help/prop_tgt/CXX_MODULE_SET_NAME.rst b/Help/prop_tgt/CXX_MODULE_SET_NAME.rst
new file mode 100644
index 0000000..27c88f3
--- /dev/null
+++ b/Help/prop_tgt/CXX_MODULE_SET_NAME.rst
@@ -0,0 +1,18 @@
+CXX_MODULE_SET_<NAME>
+---------------------
+
+.. note ::
+
+  Experimental. Gated by ``CMAKE_EXPERIMENTAL_CXX_MODULE_CMAKE_API``
+
+Semicolon-separated list of files in the target's ``<NAME>`` C++ module set,
+which has the set type ``CXX_MODULES``. If any of the paths are relative, they
+are computed relative to the target's source directory. The property supports
+:manual:`generator expressions <cmake-generator-expressions(7)>`.
+
+This property is normally only set by :command:`target_sources(FILE_SET)`
+rather than being manipulated directly.
+
+See :prop_tgt:`CXX_MODULE_SET` for the list of files in the default C++ module
+set. See :prop_tgt:`CXX_MODULE_SETS` for the file set names of all C++ module
+sets.
diff --git a/Help/prop_tgt/INTERFACE_CXX_MODULE_HEADER_UNIT_SETS.rst b/Help/prop_tgt/INTERFACE_CXX_MODULE_HEADER_UNIT_SETS.rst
new file mode 100644
index 0000000..eb3a9ff
--- /dev/null
+++ b/Help/prop_tgt/INTERFACE_CXX_MODULE_HEADER_UNIT_SETS.rst
@@ -0,0 +1,16 @@
+INTERFACE_CXX_MODULE_HEADER_UNIT_SETS
+-------------------------------------
+
+.. note ::
+
+  Experimental. Gated by ``CMAKE_EXPERIMENTAL_CXX_MODULE_CMAKE_API``
+
+Read-only list of the target's ``PUBLIC`` C++ module header sets (i.e. all
+file sets with the type ``CXX_MODULE_HEADER_UNITS``). Files listed in these
+C++ module header sets can be installed with :command:`install(TARGETS)` and
+exported with :command:`install(EXPORT)` and :command:`export`.
+
+C++ module header sets may be defined using the :command:`target_sources`
+command ``FILE_SET`` option with type ``CXX_MODULE_HEADER_UNITS``.
+
+See also :prop_tgt:`CXX_MODULE_HEADER_UNIT_SETS`.
diff --git a/Help/prop_tgt/INTERFACE_CXX_MODULE_SETS.rst b/Help/prop_tgt/INTERFACE_CXX_MODULE_SETS.rst
new file mode 100644
index 0000000..cc30386
--- /dev/null
+++ b/Help/prop_tgt/INTERFACE_CXX_MODULE_SETS.rst
@@ -0,0 +1,16 @@
+INTERFACE_CXX_MODULE_SETS
+-------------------------
+
+.. note ::
+
+  Experimental. Gated by ``CMAKE_EXPERIMENTAL_CXX_MODULE_CMAKE_API``
+
+Read-only list of the target's ``PUBLIC`` C++ module sets (i.e. all file sets
+with the type ``CXX_MODULES``). Files listed in these C++ module sets can be
+installed with :command:`install(TARGETS)` and exported with
+:command:`install(EXPORT)` and :command:`export`.
+
+C++ module sets may be defined using the :command:`target_sources` command
+``FILE_SET`` option with type ``CXX_MODULES``.
+
+See also :prop_tgt:`CXX_MODULE_SETS`.