cmTargetSourcesCommand: allow `INTERFACE` C++ modules when imported

`PUBLIC` filesets become `INTERFACE` upon installation. Allow
`INTERFACE` scopes for C++ modules when the target is imported.

Help/command/target_sources.rst

@@ -89,7 +89,7 @@ files within those directories. The acceptable types include:
 
   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.
+  ``INTERFACE`` scope except on ``IMPORTED`` targets.
 
 ``CXX_MODULE_HEADER_UNITS``
 
@@ -98,7 +98,7 @@ files within those directories. The acceptable types include:
   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.
+  set type may not have an ``INTERFACE`` scope except on ``IMPORTED`` targets.
 
 The optional default file sets are named after their type. The target may not
 be a custom target or :prop_tgt:`FRAMEWORK` target.

Source/cmTargetSourcesCommand.cxx

@@ -269,7 +269,8 @@ bool TargetSourcesImpl::HandleOneFileSet(
       }
 
       if (cmFileSetVisibilityIsForInterface(visibility) &&
-          !cmFileSetVisibilityIsForSelf(visibility)) {
+          !cmFileSetVisibilityIsForSelf(visibility) &&
+          !this->Target->IsImported()) {
         if (type == "CXX_MODULES"_s || type == "CXX_MODULE_HEADER_UNITS"_s) {
           this->SetError(
             R"(File set TYPEs "CXX_MODULES" and "CXX_MODULE_HEADER_UNITS" may not have "INTERFACE" visibility)");

Tests/RunCMake/CXXModules/FileSetModuleHeaderUnitsInterfaceImported-stderr.txt

@@ -0,0 +1,6 @@
+CMake Warning \(dev\) at FileSetModuleHeaderUnitsInterfaceImported.cmake:2 \(target_sources\):
+  CMake's C\+\+ module support is experimental.  It is meant only for
+  experimentation and feedback to CMake developers.
+Call Stack \(most recent call first\):
+  CMakeLists.txt:6 \(include\)
+This warning is for project developers.  Use -Wno-dev to suppress it.

Tests/RunCMake/CXXModules/FileSetModuleHeaderUnitsInterfaceImported.cmake

@@ -0,0 +1,8 @@
+add_library(module-header SHARED IMPORTED)
+target_sources(module-header
+  INTERFACE
+    FILE_SET fs TYPE CXX_MODULE_HEADER_UNITS FILES
+      sources/module-header.h)
+target_compile_features(module-header
+  INTERFACE
+    cxx_std_20)

Tests/RunCMake/CXXModules/FileSetModulesInterfaceImported-stderr.txt

@@ -0,0 +1,6 @@
+CMake Warning \(dev\) at FileSetModulesInterfaceImported.cmake:2 \(target_sources\):
+  CMake's C\+\+ module support is experimental.  It is meant only for
+  experimentation and feedback to CMake developers.
+Call Stack \(most recent call first\):
+  CMakeLists.txt:6 \(include\)
+This warning is for project developers.  Use -Wno-dev to suppress it.

Tests/RunCMake/CXXModules/FileSetModulesInterfaceImported.cmake

@@ -0,0 +1,8 @@
+add_library(module SHARED IMPORTED)
+target_sources(module
+  INTERFACE
+    FILE_SET fs TYPE CXX_MODULES FILES
+      sources/module.cxx)
+target_compile_features(module
+  INTERFACE
+    cxx_std_20)

Tests/RunCMake/CXXModules/RunCMakeTest.cmake

@@ -60,6 +60,7 @@ foreach (fileset_type IN LISTS fileset_types)
   foreach (scope IN LISTS scopes)
     run_cmake("FileSet${fileset_type}${scope}")
   endforeach ()
+  run_cmake("FileSet${fileset_type}InterfaceImported")
 
   # Test the error message when a non-C++ source file is found in the source
   # list.