4 files changed, 64 insertions, 10 deletions
diff --git a/Help/command/add_library.rst b/Help/command/add_library.rst
index ed8447e..f7e6792 100644
--- a/Help/command/add_library.rst
+++ b/Help/command/add_library.rst
@@ -118,8 +118,35 @@ target using the commands:
 and then it is used as an argument to :command:`target_link_libraries`
 like any other target.
 
-An interface library has no source files itself and is not included
-as a target in the generated buildsystem.
+An interface library created with the above signature has no source files
+itself and is not included as a target in the generated buildsystem.
+
+Since CMake 3.19, an interface library target may be created with
+source files:
+
+.. code-block:: cmake
+
+  add_library(<name> INTERFACE [<source>...] [EXCLUDE_FROM_ALL])
+
+Source files may be listed directly in the ``add_library`` call or added
+later by calls to :command:`target_sources` with the ``PRIVATE`` or
+``PUBLIC`` keywords.
+
+If an interface library has source files (i.e. the :prop_tgt:`SOURCES`
+target property is set), it will appear in the generated buildsystem
+as a build target much like a target defined by the
+:command:`add_custom_target` command.  It does not compile any sources,
+but does contain build rules for custom commands created by the
+:command:`add_custom_command` command.
+
+.. note::
+  In most command signatures where the ``INTERFACE`` keyword appears,
+  the items listed after it only become part of that target's usage
+  requirements and are not part of the target's own settings.  However,
+  in this signature of ``add_library``, the ``INTERFACE`` keyword refers
+  to the library type only.  Sources listed after it in the ``add_library``
+  call are ``PRIVATE`` to the interface library and do not appear in its
+  :prop_tgt:`INTERFACE_SOURCES` target property.
 
 Imported Libraries
 ^^^^^^^^^^^^^^^^^^
diff --git a/Help/manual/cmake-buildsystem.7.rst b/Help/manual/cmake-buildsystem.7.rst
index 6eea191..cd27316 100644
--- a/Help/manual/cmake-buildsystem.7.rst
+++ b/Help/manual/cmake-buildsystem.7.rst
@@ -922,8 +922,8 @@ property from it:
 Interface Libraries
 -------------------
 
-An ``INTERFACE`` target has no :prop_tgt:`LOCATION` and is mutable, but is
-otherwise similar to an :prop_tgt:`IMPORTED` target.
+An ``INTERFACE`` library target does not compile sources and does not
+produce a library artifact on disk, so it has no :prop_tgt:`LOCATION`.
 
 It may specify usage requirements such as
 :prop_tgt:`INTERFACE_INCLUDE_DIRECTORIES`,
@@ -937,11 +937,22 @@ Only the ``INTERFACE`` modes of the :command:`target_include_directories`,
 :command:`target_sources`, and :command:`target_link_libraries` commands
 may be used with ``INTERFACE`` libraries.
 
+Since CMake 3.19, an ``INTERFACE`` library target may optionally contain
+source files.  An interface library that contains source files will be
+included as a build target in the generated buildsystem.  It does not
+compile sources, but may contain custom commands to generate other sources.
+Additionally, IDEs will show the source files as part of the target for
+interactive reading and editing.
+
 A primary use-case for ``INTERFACE`` libraries is header-only libraries.
 
 .. code-block:: cmake
 
-  add_library(Eigen INTERFACE)
+  add_library(Eigen INTERFACE
+    src/eigen.h
+    src/vector.h
+    src/matrix.h
+    )
   target_include_directories(Eigen INTERFACE
     $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/src>
     $<INSTALL_INTERFACE:include/Eigen>
@@ -980,7 +991,12 @@ to must be installed separately:
 
 .. code-block:: cmake
 
-  add_library(Eigen INTERFACE)
+  set(Eigen_headers
+    src/eigen.h
+    src/vector.h
+    src/matrix.h
+    )
+  add_library(Eigen INTERFACE ${Eigen_headers})
   target_include_directories(Eigen INTERFACE
     $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/src>
     $<INSTALL_INTERFACE:include/Eigen>
@@ -990,9 +1006,6 @@ to must be installed separately:
   install(EXPORT eigenExport NAMESPACE Upstream::
     DESTINATION lib/cmake/Eigen
   )
-  install(FILES
-      ${CMAKE_CURRENT_SOURCE_DIR}/src/eigen.h
-      ${CMAKE_CURRENT_SOURCE_DIR}/src/vector.h
-      ${CMAKE_CURRENT_SOURCE_DIR}/src/matrix.h
+  install(FILES ${Eigen_headers}
     DESTINATION include/Eigen
   )
diff --git a/Help/prop_tgt/EXPORT_PROPERTIES.rst b/Help/prop_tgt/EXPORT_PROPERTIES.rst
index 34c054f..2d54f8b 100644
--- a/Help/prop_tgt/EXPORT_PROPERTIES.rst
+++ b/Help/prop_tgt/EXPORT_PROPERTIES.rst
@@ -14,3 +14,11 @@ Properties starting with ``INTERFACE_`` or ``IMPORTED_`` are not allowed as
 they are reserved for internal CMake use.
 
 Properties containing generator expressions are also not allowed.
+
+.. note::
+
+  Since CMake 3.19, :ref:`Interface Libraries` may have arbitrary
+  target properties.  If a project exports an interface library
+  with custom properties, the resulting package may not work with
+  dependents configured by older versions of CMake that reject the
+  custom properties.
diff --git a/Help/release/dev/build-interface-targets.rst b/Help/release/dev/build-interface-targets.rst
new file mode 100644
index 0000000..37bded4
--- /dev/null
+++ b/Help/release/dev/build-interface-targets.rst
@@ -0,0 +1,6 @@
+build-interface-targets
+-----------------------
+
+* :ref:`Interface Libraries` may now have source files added via
+  :command:`add_library` or :command:`target_sources`.  Those
+  with sources will be generated as part of the build system.