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).

54 files changed, 562 insertions, 2 deletions

diff --git a/Tests/RunCMake/CMakeLists.txt b/Tests/RunCMake/CMakeLists.txt
index 5325a3a..3d2654c 100644
--- a/Tests/RunCMake/CMakeLists.txt
+++ b/Tests/RunCMake/CMakeLists.txt
@@ -564,6 +564,9 @@ if(CMake_TEST_CUDA)
 endif()
 add_RunCMake_test(DependencyGraph -DCMAKE_Fortran_COMPILER=${CMAKE_Fortran_COMPILER})
 
+# Add C++ Module tests.
+add_RunCMake_test(CXXModules)
+
 # ctresalloc links against CMakeLib and CTestLib, which means it can't be built
 # if CMake_TEST_EXTERNAL_CMAKE is activated (the compiler might be different.)
 # So, it has to be provided in the original build tree.
diff --git a/Tests/RunCMake/CXXModules/CMakeLists.txt b/Tests/RunCMake/CXXModules/CMakeLists.txt
new file mode 100644
index 0000000..338a412
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/CMakeLists.txt
@@ -0,0 +1,6 @@
+cmake_minimum_required(VERSION 3.23)
+project(${RunCMake_TEST} NONE)
+
+set(CMAKE_EXPERIMENTAL_CXX_MODULE_CMAKE_API "17be90bd-a850-44e0-be50-448de847d652")
+
+include(${RunCMake_TEST}.cmake)
diff --git a/Tests/RunCMake/CXXModules/FileSetModuleHeaderUnitsInterface-result.txt b/Tests/RunCMake/CXXModules/FileSetModuleHeaderUnitsInterface-result.txt
new file mode 100644
index 0000000..d00491f
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/FileSetModuleHeaderUnitsInterface-result.txt
@@ -0,0 +1 @@
+1
diff --git a/Tests/RunCMake/CXXModules/FileSetModuleHeaderUnitsInterface-stderr.txt b/Tests/RunCMake/CXXModules/FileSetModuleHeaderUnitsInterface-stderr.txt
new file mode 100644
index 0000000..d573a02
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/FileSetModuleHeaderUnitsInterface-stderr.txt
@@ -0,0 +1,12 @@
+CMake Warning \(dev\) at FileSetModuleHeaderUnitsInterface.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.
+
+CMake Error at FileSetModuleHeaderUnitsInterface.cmake:2 \(target_sources\):
+  target_sources File set TYPEs "CXX_MODULES" and "CXX_MODULE_HEADER_UNITS"
+  may not have "INTERFACE" visibility
+Call Stack \(most recent call first\):
+  CMakeLists.txt:6 \(include\)
diff --git a/Tests/RunCMake/CXXModules/FileSetModuleHeaderUnitsInterface.cmake b/Tests/RunCMake/CXXModules/FileSetModuleHeaderUnitsInterface.cmake
new file mode 100644
index 0000000..03ca17e
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/FileSetModuleHeaderUnitsInterface.cmake
@@ -0,0 +1,8 @@
+add_library(module-header)
+target_sources(module-header
+  INTERFACE
+    FILE_SET fs TYPE CXX_MODULE_HEADER_UNITS FILES
+      sources/module-header.h)
+target_compile_features(module-header
+  PRIVATE
+    cxx_std_20)
diff --git a/Tests/RunCMake/CXXModules/FileSetModuleHeaderUnitsPrivate-stderr.txt b/Tests/RunCMake/CXXModules/FileSetModuleHeaderUnitsPrivate-stderr.txt
new file mode 100644
index 0000000..a7ac88e
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/FileSetModuleHeaderUnitsPrivate-stderr.txt
@@ -0,0 +1,11 @@
+CMake Warning \(dev\) at FileSetModuleHeaderUnitsPrivate.cmake:7 \(target_sources\):
+  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.
+
+CMake Warning \(dev\):
+  C\+\+20 modules support via CMAKE_EXPERIMENTAL_CXX_MODULE_DYNDEP is
+  experimental.  It is meant only for compiler developers to try.
+This warning is for project developers.  Use -Wno-dev to suppress it.
diff --git a/Tests/RunCMake/CXXModules/FileSetModuleHeaderUnitsPrivate.cmake b/Tests/RunCMake/CXXModules/FileSetModuleHeaderUnitsPrivate.cmake
new file mode 100644
index 0000000..ebf9853
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/FileSetModuleHeaderUnitsPrivate.cmake
@@ -0,0 +1,13 @@
+enable_language(CXX)
+set(CMAKE_EXPERIMENTAL_CXX_MODULE_DYNDEP 1)
+set(CMAKE_EXPERIMENTAL_CXX_SCANDEP_SOURCE "")
+
+add_library(module-header
+  sources/cxx-anchor.cxx)
+target_sources(module-header
+  PRIVATE
+    FILE_SET fs TYPE CXX_MODULE_HEADER_UNITS FILES
+      sources/module-header.h)
+target_compile_features(module-header
+  PRIVATE
+    cxx_std_20)
diff --git a/Tests/RunCMake/CXXModules/FileSetModuleHeaderUnitsPublic-stderr.txt b/Tests/RunCMake/CXXModules/FileSetModuleHeaderUnitsPublic-stderr.txt
new file mode 100644
index 0000000..a5b4ede
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/FileSetModuleHeaderUnitsPublic-stderr.txt
@@ -0,0 +1,11 @@
+CMake Warning \(dev\) at FileSetModuleHeaderUnitsPublic.cmake:7 \(target_sources\):
+  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.
+
+CMake Warning \(dev\):
+  C\+\+20 modules support via CMAKE_EXPERIMENTAL_CXX_MODULE_DYNDEP is
+  experimental.  It is meant only for compiler developers to try.
+This warning is for project developers.  Use -Wno-dev to suppress it.
diff --git a/Tests/RunCMake/CXXModules/FileSetModuleHeaderUnitsPublic.cmake b/Tests/RunCMake/CXXModules/FileSetModuleHeaderUnitsPublic.cmake
new file mode 100644
index 0000000..3dfccbb5
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/FileSetModuleHeaderUnitsPublic.cmake
@@ -0,0 +1,13 @@
+enable_language(CXX)
+set(CMAKE_EXPERIMENTAL_CXX_MODULE_DYNDEP 1)
+set(CMAKE_EXPERIMENTAL_CXX_SCANDEP_SOURCE "")
+
+add_library(module-header
+  sources/cxx-anchor.cxx)
+target_sources(module-header
+  PUBLIC
+    FILE_SET fs TYPE CXX_MODULE_HEADER_UNITS FILES
+      sources/module-header.h)
+target_compile_features(module-header
+  PRIVATE
+    cxx_std_20)
diff --git a/Tests/RunCMake/CXXModules/FileSetModulesInterface-result.txt b/Tests/RunCMake/CXXModules/FileSetModulesInterface-result.txt
new file mode 100644
index 0000000..d00491f
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/FileSetModulesInterface-result.txt
@@ -0,0 +1 @@
+1
diff --git a/Tests/RunCMake/CXXModules/FileSetModulesInterface-stderr.txt b/Tests/RunCMake/CXXModules/FileSetModulesInterface-stderr.txt
new file mode 100644
index 0000000..81a35e8
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/FileSetModulesInterface-stderr.txt
@@ -0,0 +1,12 @@
+CMake Warning \(dev\) at FileSetModulesInterface.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.
+
+CMake Error at FileSetModulesInterface.cmake:2 \(target_sources\):
+  target_sources File set TYPEs "CXX_MODULES" and "CXX_MODULE_HEADER_UNITS"
+  may not have "INTERFACE" visibility
+Call Stack \(most recent call first\):
+  CMakeLists.txt:6 \(include\)
diff --git a/Tests/RunCMake/CXXModules/FileSetModulesInterface.cmake b/Tests/RunCMake/CXXModules/FileSetModulesInterface.cmake
new file mode 100644
index 0000000..24cec3e
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/FileSetModulesInterface.cmake
@@ -0,0 +1,8 @@
+add_library(module)
+target_sources(module
+  INTERFACE
+    FILE_SET fs TYPE CXX_MODULES FILES
+      sources/module.cxx)
+target_compile_features(module
+  PRIVATE
+    cxx_std_20)
diff --git a/Tests/RunCMake/CXXModules/FileSetModulesPrivate-stderr.txt b/Tests/RunCMake/CXXModules/FileSetModulesPrivate-stderr.txt
new file mode 100644
index 0000000..03e06cc
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/FileSetModulesPrivate-stderr.txt
@@ -0,0 +1,11 @@
+CMake Warning \(dev\) at FileSetModulesPrivate.cmake:6 \(target_sources\):
+  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.
+
+CMake Warning \(dev\):
+  C\+\+20 modules support via CMAKE_EXPERIMENTAL_CXX_MODULE_DYNDEP is
+  experimental.  It is meant only for compiler developers to try.
+This warning is for project developers.  Use -Wno-dev to suppress it.
diff --git a/Tests/RunCMake/CXXModules/FileSetModulesPrivate.cmake b/Tests/RunCMake/CXXModules/FileSetModulesPrivate.cmake
new file mode 100644
index 0000000..ca18982
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/FileSetModulesPrivate.cmake
@@ -0,0 +1,12 @@
+enable_language(CXX)
+set(CMAKE_EXPERIMENTAL_CXX_MODULE_DYNDEP 1)
+set(CMAKE_EXPERIMENTAL_CXX_SCANDEP_SOURCE "")
+
+add_library(module)
+target_sources(module
+  PRIVATE
+    FILE_SET fs TYPE CXX_MODULES FILES
+      sources/module.cxx)
+target_compile_features(module
+  PRIVATE
+    cxx_std_20)
diff --git a/Tests/RunCMake/CXXModules/FileSetModulesPublic-stderr.txt b/Tests/RunCMake/CXXModules/FileSetModulesPublic-stderr.txt
new file mode 100644
index 0000000..0c110c3
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/FileSetModulesPublic-stderr.txt
@@ -0,0 +1,11 @@
+CMake Warning \(dev\) at FileSetModulesPublic.cmake:6 \(target_sources\):
+  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.
+
+CMake Warning \(dev\):
+  C\+\+20 modules support via CMAKE_EXPERIMENTAL_CXX_MODULE_DYNDEP is
+  experimental.  It is meant only for compiler developers to try.
+This warning is for project developers.  Use -Wno-dev to suppress it.
diff --git a/Tests/RunCMake/CXXModules/FileSetModulesPublic.cmake b/Tests/RunCMake/CXXModules/FileSetModulesPublic.cmake
new file mode 100644
index 0000000..58de174
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/FileSetModulesPublic.cmake
@@ -0,0 +1,12 @@
+enable_language(CXX)
+set(CMAKE_EXPERIMENTAL_CXX_MODULE_DYNDEP 1)
+set(CMAKE_EXPERIMENTAL_CXX_SCANDEP_SOURCE "")
+
+add_library(module)
+target_sources(module
+  PUBLIC
+    FILE_SET fs TYPE CXX_MODULES FILES
+      sources/module.cxx)
+target_compile_features(module
+  PRIVATE
+    cxx_std_20)
diff --git a/Tests/RunCMake/CXXModules/NoCXX-result.txt b/Tests/RunCMake/CXXModules/NoCXX-result.txt
new file mode 100644
index 0000000..d00491f
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/NoCXX-result.txt
@@ -0,0 +1 @@
+1
diff --git a/Tests/RunCMake/CXXModules/NoCXX-stderr.txt b/Tests/RunCMake/CXXModules/NoCXX-stderr.txt
new file mode 100644
index 0000000..aa7f406
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/NoCXX-stderr.txt
@@ -0,0 +1,20 @@
+CMake Warning \(dev\) at NoCXX.cmake:4 \(target_sources\):
+  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.
+
+CMake Error in CMakeLists.txt:
+  The "nocxx" target has C\+\+ module sources but the "CXX" language has not
+  been enabled
+
+(
+CMake Error in CMakeLists.txt:
+(  The "nocxx" target has C\+\+ module sources but the "CXX" language has not
+  been enabled
+|  The "nocxx" target contains C\+\+ module sources which are not supported by
+  the generator
+)
+)*
+CMake Generate step failed.  Build files cannot be regenerated correctly.
diff --git a/Tests/RunCMake/CXXModules/NoCXX.cmake b/Tests/RunCMake/CXXModules/NoCXX.cmake
new file mode 100644
index 0000000..3c46f9d
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/NoCXX.cmake
@@ -0,0 +1,9 @@
+enable_language(C)
+
+add_library(nocxx)
+target_sources(nocxx
+  PRIVATE
+    sources/c-anchor.c
+  PUBLIC
+    FILE_SET fs TYPE CXX_MODULES FILES
+      sources/module.cxx)
diff --git a/Tests/RunCMake/CXXModules/NoCXX20-result.txt b/Tests/RunCMake/CXXModules/NoCXX20-result.txt
new file mode 100644
index 0000000..d00491f
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/NoCXX20-result.txt
@@ -0,0 +1 @@
+1
diff --git a/Tests/RunCMake/CXXModules/NoCXX20-stderr.txt b/Tests/RunCMake/CXXModules/NoCXX20-stderr.txt
new file mode 100644
index 0000000..95d73b1
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/NoCXX20-stderr.txt
@@ -0,0 +1,20 @@
+CMake Warning \(dev\) at NoCXX20.cmake:4 \(target_sources\):
+  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.
+
+CMake Error in CMakeLists.txt:
+  The "nocxx20" target has C\+\+ module sources but is not using at least
+  "cxx_std_20"
+
+(
+CMake Error in CMakeLists.txt:
+(  The "nocxx20" target has C\+\+ module sources but is not using at least
+  "cxx_std_20"
+|  The "nocxx20" target contains C\+\+ module sources which are not supported by
+  the generator
+)
+)*
+CMake Generate step failed.  Build files cannot be regenerated correctly.
diff --git a/Tests/RunCMake/CXXModules/NoCXX20.cmake b/Tests/RunCMake/CXXModules/NoCXX20.cmake
new file mode 100644
index 0000000..d502f7c
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/NoCXX20.cmake
@@ -0,0 +1,10 @@
+enable_language(CXX)
+
+add_library(nocxx20)
+target_sources(nocxx20
+  PUBLIC
+    FILE_SET fs TYPE CXX_MODULES FILES
+      sources/module.cxx)
+target_compile_features(nocxx20
+  PRIVATE
+    cxx_std_17)
diff --git a/Tests/RunCMake/CXXModules/NoCXX20ModuleFlag-result.txt b/Tests/RunCMake/CXXModules/NoCXX20ModuleFlag-result.txt
new file mode 100644
index 0000000..d00491f
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/NoCXX20ModuleFlag-result.txt
@@ -0,0 +1 @@
+1
diff --git a/Tests/RunCMake/CXXModules/NoCXX20ModuleFlag-stderr.txt b/Tests/RunCMake/CXXModules/NoCXX20ModuleFlag-stderr.txt
new file mode 100644
index 0000000..5f90ec8
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/NoCXX20ModuleFlag-stderr.txt
@@ -0,0 +1,20 @@
+CMake Warning \(dev\) at NoCXX20ModuleFlag.cmake:4 \(target_sources\):
+  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.
+
+CMake Error in CMakeLists.txt:
+  The "noexperimentalflag" target has C\+\+ module sources but its experimental
+  support has not been requested
+
+(
+CMake Error in CMakeLists.txt:
+(  The "noexperimentalflag" target has C\+\+ module sources but its experimental
+  support has not been requested
+|  The "noexperimentalflag" target contains C\+\+ module sources which are not
+  supported by the generator
+)
+)*
+CMake Generate step failed.  Build files cannot be regenerated correctly.
diff --git a/Tests/RunCMake/CXXModules/NoCXX20ModuleFlag.cmake b/Tests/RunCMake/CXXModules/NoCXX20ModuleFlag.cmake
new file mode 100644
index 0000000..5f896f9
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/NoCXX20ModuleFlag.cmake
@@ -0,0 +1,10 @@
+enable_language(CXX)
+
+add_library(noexperimentalflag)
+target_sources(noexperimentalflag
+  PUBLIC
+    FILE_SET fs TYPE CXX_MODULES FILES
+      sources/module.cxx)
+target_compile_features(noexperimentalflag
+  PRIVATE
+    cxx_std_20)
diff --git a/Tests/RunCMake/CXXModules/NoDyndepSupport-result.txt b/Tests/RunCMake/CXXModules/NoDyndepSupport-result.txt
new file mode 100644
index 0000000..d00491f
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/NoDyndepSupport-result.txt
@@ -0,0 +1 @@
+1
diff --git a/Tests/RunCMake/CXXModules/NoDyndepSupport-stderr.txt b/Tests/RunCMake/CXXModules/NoDyndepSupport-stderr.txt
new file mode 100644
index 0000000..52f781f
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/NoDyndepSupport-stderr.txt
@@ -0,0 +1,30 @@
+CMake Warning \(dev\) at NoDyndepSupport.cmake:10 \(target_sources\):
+  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.
+
+(CMake Warning \(dev\):
+  C\+\+20 modules support via CMAKE_EXPERIMENTAL_CXX_MODULE_DYNDEP is
+  experimental.  It is meant only for compiler developers to try.
+This warning is for project developers.  Use -Wno-dev to suppress it.
+
+CMake Error:
+  The Ninja generator does not support C\+\+20 modules using Ninja version
+
+    .*
+
+  due to lack of required features.  Ninja 1.10 or higher is required.
+
+|CMake Error in CMakeLists.txt:
+  The "nodyndep" target contains C\+\+ module sources which are not supported
+  by the generator
+
+(
+CMake Error in CMakeLists.txt:
+  The "nodyndep" target contains C\+\+ module sources which are not supported
+  by the generator
+
+)*)
+CMake Generate step failed.  Build files cannot be regenerated correctly.
diff --git a/Tests/RunCMake/CXXModules/NoDyndepSupport.cmake b/Tests/RunCMake/CXXModules/NoDyndepSupport.cmake
new file mode 100644
index 0000000..0954400
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/NoDyndepSupport.cmake
@@ -0,0 +1,16 @@
+enable_language(CXX)
+set(CMAKE_EXPERIMENTAL_CXX_MODULE_DYNDEP 1)
+set(CMAKE_EXPERIMENTAL_CXX_SCANDEP_SOURCE "")
+
+if (NOT CMAKE_CXX_STANDARD_DEFAULT)
+  set(CMAKE_CXX_STANDARD_DEFAULT "11")
+endif ()
+
+add_library(nodyndep)
+target_sources(nodyndep
+  PUBLIC
+    FILE_SET fs TYPE CXX_MODULES FILES
+      sources/module.cxx)
+target_compile_features(nodyndep
+  PRIVATE
+    cxx_std_20)
diff --git a/Tests/RunCMake/CXXModules/NotCXXSourceModuleHeaderUnits-result.txt b/Tests/RunCMake/CXXModules/NotCXXSourceModuleHeaderUnits-result.txt
new file mode 100644
index 0000000..d00491f
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/NotCXXSourceModuleHeaderUnits-result.txt
@@ -0,0 +1 @@
+1
diff --git a/Tests/RunCMake/CXXModules/NotCXXSourceModuleHeaderUnits-stderr.txt b/Tests/RunCMake/CXXModules/NotCXXSourceModuleHeaderUnits-stderr.txt
new file mode 100644
index 0000000..a93eb40
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/NotCXXSourceModuleHeaderUnits-stderr.txt
@@ -0,0 +1,22 @@
+CMake Warning \(dev\) at NotCXXSourceModuleHeaderUnits.cmake:7 \(target_sources\):
+  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.
+
+CMake Warning \(dev\):
+  C\+\+20 modules support via CMAKE_EXPERIMENTAL_CXX_MODULE_DYNDEP is
+  experimental.  It is meant only for compiler developers to try.
+This warning is for project developers.  Use -Wno-dev to suppress it.
+
+CMake Error in CMakeLists.txt:
+  Target "not-cxx-source" contains the source
+
+    .*/Tests/RunCMake/CXXModules/sources/c-anchor.c
+
+  in a file set of type "CXX_MODULE_HEADER_UNITS" but the source is not
+  classified as a "CXX" source.
+
+
+CMake Generate step failed.  Build files cannot be regenerated correctly.
diff --git a/Tests/RunCMake/CXXModules/NotCXXSourceModuleHeaderUnits.cmake b/Tests/RunCMake/CXXModules/NotCXXSourceModuleHeaderUnits.cmake
new file mode 100644
index 0000000..af4ddac
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/NotCXXSourceModuleHeaderUnits.cmake
@@ -0,0 +1,15 @@
+enable_language(C)
+enable_language(CXX)
+set(CMAKE_EXPERIMENTAL_CXX_MODULE_DYNDEP 1)
+set(CMAKE_EXPERIMENTAL_CXX_SCANDEP_SOURCE "")
+
+add_library(not-cxx-source)
+target_sources(not-cxx-source
+  PRIVATE
+    sources/cxx-anchor.cxx
+  PUBLIC
+    FILE_SET fs TYPE CXX_MODULE_HEADER_UNITS FILES
+      sources/c-anchor.c)
+target_compile_features(not-cxx-source
+  PRIVATE
+    cxx_std_20)
diff --git a/Tests/RunCMake/CXXModules/NotCXXSourceModules-result.txt b/Tests/RunCMake/CXXModules/NotCXXSourceModules-result.txt
new file mode 100644
index 0000000..d00491f
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/NotCXXSourceModules-result.txt
@@ -0,0 +1 @@
+1
diff --git a/Tests/RunCMake/CXXModules/NotCXXSourceModules-stderr.txt b/Tests/RunCMake/CXXModules/NotCXXSourceModules-stderr.txt
new file mode 100644
index 0000000..d341c1f
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/NotCXXSourceModules-stderr.txt
@@ -0,0 +1,17 @@
+CMake Warning \(dev\) at NotCXXSourceModules.cmake:7 \(target_sources\):
+  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.
+
+CMake Error in CMakeLists.txt:
+  Target "not-cxx-source" contains the source
+
+    .*/Tests/RunCMake/CXXModules/sources/c-anchor.c
+
+  in a file set of type "CXX_MODULES" but the source is not classified as a
+  "CXX" source.
+
+
+CMake Generate step failed.  Build files cannot be regenerated correctly.
diff --git a/Tests/RunCMake/CXXModules/NotCXXSourceModules.cmake b/Tests/RunCMake/CXXModules/NotCXXSourceModules.cmake
new file mode 100644
index 0000000..f7a6060
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/NotCXXSourceModules.cmake
@@ -0,0 +1,13 @@
+enable_language(C)
+enable_language(CXX)
+set(CMAKE_EXPERIMENTAL_CXX_MODULE_DYNDEP 1)
+set(CMAKE_EXPERIMENTAL_CXX_SCANDEP_SOURCE "")
+
+add_library(not-cxx-source)
+target_sources(not-cxx-source
+  PUBLIC
+    FILE_SET fs TYPE CXX_MODULES FILES
+      sources/c-anchor.c)
+target_compile_features(not-cxx-source
+  PRIVATE
+    cxx_std_20)
diff --git a/Tests/RunCMake/CXXModules/RunCMakeTest.cmake b/Tests/RunCMake/CXXModules/RunCMakeTest.cmake
new file mode 100644
index 0000000..00c1a8b
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/RunCMakeTest.cmake
@@ -0,0 +1,67 @@
+include(RunCMake)
+
+# For `if (IN_LIST)`
+cmake_policy(SET CMP0057 NEW)
+
+run_cmake(compiler_introspection)
+include("${RunCMake_BINARY_DIR}/compiler_introspection-build/info.cmake")
+
+# Test negative cases where C++20 modules do not work.
+run_cmake(NoCXX)
+if ("cxx_std_20" IN_LIST CMAKE_CXX_COMPILE_FEATURES)
+  # This test requires that the compiler be told to compile in an older-than-20
+  # standard. If the compiler forces a standard to be used, skip it.
+  if (NOT forced_cxx_standard)
+    run_cmake(NoCXX20)
+  endif ()
+
+  # This test uses C++20, but another prerequisite is missing, so forced
+  # standards don't matter.
+  run_cmake(NoCXX20ModuleFlag)
+endif ()
+
+if (RunCMake_GENERATOR MATCHES "Ninja")
+  execute_process(
+    COMMAND "${CMAKE_MAKE_PROGRAM}" --version
+    RESULT_VARIABLE res
+    OUTPUT_VARIABLE ninja_version
+    ERROR_VARIABLE err
+    OUTPUT_STRIP_TRAILING_WHITESPACE
+    ERROR_STRIP_TRAILING_WHITESPACE)
+
+  if (res)
+    message(WARNING
+      "Failed to determine `ninja` version: ${err}")
+    set(ninja_version "0")
+  endif ()
+endif ()
+
+# Test behavior when the generator does not support C++20 modules.
+if (NOT RunCMake_GENERATOR MATCHES "Ninja" OR
+    ninja_version VERSION_LESS "1.10" OR
+    NOT "cxx_std_20" IN_LIST CMAKE_CXX_COMPILE_FEATURES)
+  if ("cxx_std_20" IN_LIST CMAKE_CXX_COMPILE_FEATURES)
+    run_cmake(NoDyndepSupport)
+  endif ()
+
+  # Bail; the remaining tests require the generator to successfully generate
+  # with C++20 modules in the source list.
+  return ()
+endif ()
+
+set(fileset_types
+  Modules
+  ModuleHeaderUnits)
+set(scopes
+  Interface
+  Private
+  Public)
+foreach (fileset_type IN LISTS fileset_types)
+  foreach (scope IN LISTS scopes)
+    run_cmake("FileSet${fileset_type}${scope}")
+  endforeach ()
+
+  # Test the error message when a non-C++ source file is found in the source
+  # list.
+  run_cmake("NotCXXSource${fileset_type}")
+endforeach ()
diff --git a/Tests/RunCMake/CXXModules/compiler_introspection.cmake b/Tests/RunCMake/CXXModules/compiler_introspection.cmake
new file mode 100644
index 0000000..7a2df3d
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/compiler_introspection.cmake
@@ -0,0 +1,25 @@
+enable_language(CXX)
+
+set(info "")
+
+# See `Modules/Compiler/MSVC-CXX.cmake` for this. If there is explicitly no
+# default, the feature list is populated to be everything.
+if (DEFINED CMAKE_CXX_STANDARD_DEFAULT AND
+    CMAKE_CXX_STANDARD_DEFAULT STREQUAL "")
+  set(CMAKE_CXX_COMPILE_FEATURES "")
+endif ()
+
+# Detect if the environment forces a C++ standard, let the test selection know.
+set(forced_cxx_standard 0)
+if (CMAKE_CXX_FLAGS MATCHES "-std=")
+  set(forced_cxx_standard 1)
+endif ()
+
+# Forward information about the C++ compile features.
+string(APPEND info "\
+set(CMAKE_CXX_COMPILE_FEATURES \"${CMAKE_CXX_COMPILE_FEATURES}\")
+set(CMAKE_MAKE_PROGRAM \"${CMAKE_MAKE_PROGRAM}\")
+set(forced_cxx_standard \"${forced_cxx_standard}\")
+")
+
+file(WRITE "${CMAKE_CURRENT_BINARY_DIR}/info.cmake" "${info}")
diff --git a/Tests/RunCMake/CXXModules/sources/c-anchor.c b/Tests/RunCMake/CXXModules/sources/c-anchor.c
new file mode 100644
index 0000000..c782188
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/sources/c-anchor.c
@@ -0,0 +1,4 @@
+int c_anchor()
+{
+  return 0;
+}
diff --git a/Tests/RunCMake/CXXModules/sources/cxx-anchor.cxx b/Tests/RunCMake/CXXModules/sources/cxx-anchor.cxx
new file mode 100644
index 0000000..9c94ec1
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/sources/cxx-anchor.cxx
@@ -0,0 +1,4 @@
+int cxx_anchor()
+{
+  return 0;
+}
diff --git a/Tests/RunCMake/CXXModules/sources/module-header.h b/Tests/RunCMake/CXXModules/sources/module-header.h
new file mode 100644
index 0000000..982617e
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/sources/module-header.h
@@ -0,0 +1,9 @@
+#ifndef module_header_h
+#define module_header_h
+
+inline int h()
+{
+  return 0;
+}
+
+#endif
diff --git a/Tests/RunCMake/CXXModules/sources/module-impl.cxx b/Tests/RunCMake/CXXModules/sources/module-impl.cxx
new file mode 100644
index 0000000..4718999
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/sources/module-impl.cxx
@@ -0,0 +1,6 @@
+module M;
+
+int f()
+{
+  return 0;
+}
diff --git a/Tests/RunCMake/CXXModules/sources/module-internal-part-impl.cxx b/Tests/RunCMake/CXXModules/sources/module-internal-part-impl.cxx
new file mode 100644
index 0000000..be77b0d
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/sources/module-internal-part-impl.cxx
@@ -0,0 +1,11 @@
+#ifdef _MSC_VER
+// Only MSVC supports this pattern.
+module M : internal_part;
+#else
+module M;
+#endif
+
+int i()
+{
+  return 0;
+}
diff --git a/Tests/RunCMake/CXXModules/sources/module-internal-part.cxx b/Tests/RunCMake/CXXModules/sources/module-internal-part.cxx
new file mode 100644
index 0000000..fa82afb
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/sources/module-internal-part.cxx
@@ -0,0 +1,3 @@
+module M : internal_part;
+
+int i();
diff --git a/Tests/RunCMake/CXXModules/sources/module-part-impl.cxx b/Tests/RunCMake/CXXModules/sources/module-part-impl.cxx
new file mode 100644
index 0000000..46d5d9f
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/sources/module-part-impl.cxx
@@ -0,0 +1,13 @@
+#ifdef _MSC_VER
+// Only MSVC supports this pattern.
+module M : part;
+#else
+module M;
+#endif
+
+import M : internal_part;
+
+int p()
+{
+  return i();
+}
diff --git a/Tests/RunCMake/CXXModules/sources/module-part.cxx b/Tests/RunCMake/CXXModules/sources/module-part.cxx
new file mode 100644
index 0000000..137c16f
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/sources/module-part.cxx
@@ -0,0 +1,3 @@
+export module M : part;
+
+int p();
diff --git a/Tests/RunCMake/CXXModules/sources/module-use.cxx b/Tests/RunCMake/CXXModules/sources/module-use.cxx
new file mode 100644
index 0000000..2d060cd
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/sources/module-use.cxx
@@ -0,0 +1,6 @@
+import M;
+
+int main(int argc, char* argv[])
+{
+  return f() + p();
+}
diff --git a/Tests/RunCMake/CXXModules/sources/module.cxx b/Tests/RunCMake/CXXModules/sources/module.cxx
new file mode 100644
index 0000000..a631354
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/sources/module.cxx
@@ -0,0 +1,5 @@
+export module M;
+export import M : part;
+import M : internal_part;
+
+int f();
diff --git a/Tests/RunCMake/target_sources/FileSetDefaultWrongTypeExperimental-result.txt b/Tests/RunCMake/target_sources/FileSetDefaultWrongTypeExperimental-result.txt
new file mode 100644
index 0000000..d00491f
--- /dev/null
+++ b/Tests/RunCMake/target_sources/FileSetDefaultWrongTypeExperimental-result.txt
@@ -0,0 +1 @@
+1
diff --git a/Tests/RunCMake/target_sources/FileSetDefaultWrongTypeExperimental-stderr.txt b/Tests/RunCMake/target_sources/FileSetDefaultWrongTypeExperimental-stderr.txt
new file mode 100644
index 0000000..042d67d
--- /dev/null
+++ b/Tests/RunCMake/target_sources/FileSetDefaultWrongTypeExperimental-stderr.txt
@@ -0,0 +1,12 @@
+^CMake Warning \(dev\) at FileSetDefaultWrongTypeExperimental.cmake:6 \(target_sources\):
+  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:3 \(include\)
+This warning is for project developers.  Use -Wno-dev to suppress it.
+
+CMake Error at FileSetDefaultWrongTypeExperimental\.cmake:[0-9]+ \(target_sources\):
+  target_sources File set TYPE may only be "HEADERS", "CXX_MODULES", or
+  "CXX_MODULE_HEADER_UNITS"
+Call Stack \(most recent call first\):
+  CMakeLists\.txt:[0-9]+ \(include\)$
diff --git a/Tests/RunCMake/target_sources/FileSetDefaultWrongTypeExperimental.cmake b/Tests/RunCMake/target_sources/FileSetDefaultWrongTypeExperimental.cmake
new file mode 100644
index 0000000..2dbcee7
--- /dev/null
+++ b/Tests/RunCMake/target_sources/FileSetDefaultWrongTypeExperimental.cmake
@@ -0,0 +1,6 @@
+enable_language(C)
+
+set(CMAKE_EXPERIMENTAL_CXX_MODULE_CMAKE_API "17be90bd-a850-44e0-be50-448de847d652")
+
+add_library(lib1 STATIC empty.c)
+target_sources(lib1 PRIVATE FILE_SET UNKNOWN)
diff --git a/Tests/RunCMake/target_sources/FileSetWrongTypeExperimental-result.txt b/Tests/RunCMake/target_sources/FileSetWrongTypeExperimental-result.txt
new file mode 100644
index 0000000..d00491f
--- /dev/null
+++ b/Tests/RunCMake/target_sources/FileSetWrongTypeExperimental-result.txt
@@ -0,0 +1 @@
+1
diff --git a/Tests/RunCMake/target_sources/FileSetWrongTypeExperimental-stderr.txt b/Tests/RunCMake/target_sources/FileSetWrongTypeExperimental-stderr.txt
new file mode 100644
index 0000000..a1b784f
--- /dev/null
+++ b/Tests/RunCMake/target_sources/FileSetWrongTypeExperimental-stderr.txt
@@ -0,0 +1,12 @@
+^CMake Warning \(dev\) at FileSetWrongTypeExperimental.cmake:6 \(target_sources\):
+  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:3 \(include\)
+This warning is for project developers.  Use -Wno-dev to suppress it.
+
+CMake Error at FileSetWrongTypeExperimental\.cmake:[0-9]+ \(target_sources\):
+  target_sources File set TYPE may only be "HEADERS", "CXX_MODULES", or
+  "CXX_MODULE_HEADER_UNITS"
+Call Stack \(most recent call first\):
+  CMakeLists\.txt:[0-9]+ \(include\)$
diff --git a/Tests/RunCMake/target_sources/FileSetWrongTypeExperimental.cmake b/Tests/RunCMake/target_sources/FileSetWrongTypeExperimental.cmake
new file mode 100644
index 0000000..024c2cb
--- /dev/null
+++ b/Tests/RunCMake/target_sources/FileSetWrongTypeExperimental.cmake
@@ -0,0 +1,6 @@
+enable_language(C)
+
+set(CMAKE_EXPERIMENTAL_CXX_MODULE_CMAKE_API "17be90bd-a850-44e0-be50-448de847d652")
+
+add_library(lib1 STATIC empty.c)
+target_sources(lib1 PRIVATE FILE_SET a TYPE UNKNOWN)
diff --git a/Tests/RunCMake/target_sources/RunCMakeTest.cmake b/Tests/RunCMake/target_sources/RunCMakeTest.cmake
index e78ee9d..135777b 100644
--- a/Tests/RunCMake/target_sources/RunCMakeTest.cmake
+++ b/Tests/RunCMake/target_sources/RunCMakeTest.cmake
@@ -26,6 +26,8 @@ run_cmake(FileSetProperties)
 run_cmake(FileSetNoType)
 run_cmake(FileSetWrongType)
 run_cmake(FileSetDefaultWrongType)
+run_cmake(FileSetWrongTypeExperimental)
+run_cmake(FileSetDefaultWrongTypeExperimental)
 run_cmake(FileSetChangeScope)
 run_cmake(FileSetChangeType)
 run_cmake(FileSetWrongBaseDirs)
diff --git a/Tests/RunCMake/try_compile/CxxStandard-stderr.txt b/Tests/RunCMake/try_compile/CxxStandard-stderr.txt
index ec7245f..cee1b44 100644
--- a/Tests/RunCMake/try_compile/CxxStandard-stderr.txt
+++ b/Tests/RunCMake/try_compile/CxxStandard-stderr.txt
@@ -1,6 +1,17 @@
-^CMake Error at .*/Tests/RunCMake/try_compile/CxxStandard-build/CMakeFiles/CMakeTmp/CMakeLists.txt:[0-9]+ \(add_executable\):
+^(CMake Error in .*/Tests/RunCMake/try_compile/CxxStandard-build/CMakeFiles/CMakeTmp/CMakeLists.txt:
+  The CXX_STANDARD property on target "cmTC_[0-9a-f]*" contained an invalid
+  value: "3".
+
+
+)?CMake Error at .*/Tests/RunCMake/try_compile/CxxStandard-build/CMakeFiles/CMakeTmp/CMakeLists.txt:[0-9]+ \(add_executable\):
   CXX_STANDARD is set to invalid value '3'
-+
+
+(
+CMake Error in .*/Tests/RunCMake/try_compile/CxxStandard-build/CMakeFiles/CMakeTmp/CMakeLists.txt:
+  The CXX_STANDARD property on target "cmTC_[0-9a-f]*" contained an invalid
+  value: "3".
+
+)?
 CMake Error at CxxStandard.cmake:[0-9]+ \(try_compile\):
   Failed to generate test project build system.
 Call Stack \(most recent call first\):