cxxmodules: scan C++ sources for imports by default

Existing projects are not using C++ modules in their sources,
so introduce policy CMP0155 to enable scanning by default.

Help/manual/cmake-cxxmodules.7.rst

@@ -27,7 +27,7 @@ following queries. The first query that provides a yes/no answer is used.
 - If the :prop_tgt:`CXX_SCAN_FOR_MODULES` target property is set, its value
   will be used.  Set the :variable:`CMAKE_CXX_SCAN_FOR_MODULES` variable
   to initialize this property on all targets as they are created.
-- Otherwise the source will be scanned.
+- Otherwise, the source file will be scanned.  See policy :policy:`CMP0155`.
 
 Compiler Support
 ================

Help/manual/cmake-policies.7.rst

@@ -57,6 +57,7 @@ Policies Introduced by CMake 3.28
 .. toctree::
    :maxdepth: 1
 
+   CMP0155: C++ sources in targets with at least C++20 are scanned for imports. </policy/CMP0155>
    CMP0154: Generated files are private by default in targets using file sets. </policy/CMP0154>
    CMP0153: The exec_program command should not be called. </policy/CMP0153>
    CMP0152: file(REAL_PATH) resolves symlinks before collapsing ../ components.  </policy/CMP0152>

Help/policy/CMP0155.rst

@@ -0,0 +1,26 @@
+CMP0155
+-------
+
+.. versionadded:: 3.28
+
+C++ sources in targets with at least C++20 are scanned for imports.
+
+CMake 3.27 and below assume that C++ sources do not ``import`` modules.
+CMake 3.28 and above prefer to assume that C++ sources in targets using C++20
+or higher might ``import`` modules, and must be scanned before compiling,
+unless explicitly disabled.  This policy provides compatibility for projects
+that use C++20 or higher, without modules, that have not been updated to turn
+off scanning, e.g., via the :variable:`CMAKE_CXX_SCAN_FOR_MODULES` variable.
+See the :manual:`cmake-cxxmodules(7)` manual for more details on C++ module
+support.
+
+The ``OLD`` behavior for this policy is to assume that C++ 20 and newer
+sources do not import modules.  The ``NEW`` behavior for this policy is to
+assume that C++ 20 and newer files may import modules, and need to be scanned.
+
+This policy was introduced in CMake version 3.28.  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

Source/cmCoreTryCompile.cxx

@@ -871,6 +871,13 @@ cm::optional<cmTryCompileResult> cmCoreTryCompile::TryCompileCode(
               ? "NEW"
               : "OLD");
 
+    /* Set the appropriate policy information for C++ module support */
+    fprintf(fout, "cmake_policy(SET CMP0155 %s)\n",
+            this->Makefile->GetPolicyStatus(cmPolicies::CMP0155) ==
+                cmPolicies::NEW
+              ? "NEW"
+              : "OLD");
+
     // Workaround for -Wl,-headerpad_max_install_names issue until we can avoid
     // adding that flag in the platform and compiler language files
     fprintf(fout,

Source/cmGeneratorTarget.cxx

@@ -9261,7 +9261,22 @@ bool cmGeneratorTarget::NeedDyndepForSource(std::string const& lang,
   if (tgtProp.IsSet()) {
     return tgtProp.IsOn();
   }
-  return true;
+
+  bool policyAnswer = false;
+  switch (this->GetPolicyStatusCMP0155()) {
+    case cmPolicies::WARN:
+    case cmPolicies::OLD:
+      // The OLD behavior is to not scan the source.
+      policyAnswer = false;
+      break;
+    case cmPolicies::REQUIRED_ALWAYS:
+    case cmPolicies::REQUIRED_IF_USED:
+    case cmPolicies::NEW:
+      // The NEW behavior is to scan the source.
+      policyAnswer = true;
+      break;
+  }
+  return policyAnswer;
 }
 
 void cmGeneratorTarget::BuildFileSetInfoCache(std::string const& config) const

Source/cmPolicies.h

@@ -469,6 +469,10 @@ class cmMakefile;
   SELECT(                                                                     \
     POLICY, CMP0154,                                                          \
     "Generated files are private by default in targets using file sets.", 3,  \
+    28, 0, cmPolicies::WARN)                                                  \
+  SELECT(                                                                     \
+    POLICY, CMP0155,                                                          \
+    "C++ sources in targets with at least C++20 are scanned for imports", 3,  \
     28, 0, cmPolicies::WARN)
 
 #define CM_SELECT_ID(F, A1, A2, A3, A4, A5, A6) F(A1)
@@ -508,7 +512,8 @@ class cmMakefile;
   F(CMP0119)                                                                  \
   F(CMP0131)                                                                  \
   F(CMP0142)                                                                  \
-  F(CMP0154)
+  F(CMP0154)                                                                  \
+  F(CMP0155)
 
 #define CM_FOR_EACH_CUSTOM_COMMAND_POLICY(F)                                  \
   F(CMP0116)                                                                  \

Tests/RunCMake/CXXModules/CMP0155-NEW-result.txt

@@ -0,0 +1 @@
+1

Tests/RunCMake/CXXModules/CMP0155-NEW-stderr.txt

@@ -0,0 +1,8 @@
+(CMake Error in CMakeLists.txt:
+(  The target named "cmp0155-new" has C\+\+ sources that use modules but the
+  compiler does not provide a way to discover the import graph dependencies
+|  The target named "cmp0155-new" contains C\+\+ sources that use modules which
+  is not supported by the generator
+)
+)*
+CMake Generate step failed.  Build files cannot be regenerated correctly.

Tests/RunCMake/CXXModules/CMP0155-NEW.cmake

@@ -0,0 +1,11 @@
+enable_language(CXX)
+unset(CMAKE_CXX_SCANDEP_SOURCE)
+
+cmake_policy(SET CMP0155 NEW)
+
+add_executable(cmp0155-new
+  sources/module-use.cxx)
+set_target_properties(cmp0155-new
+  PROPERTIES
+    CXX_STANDARD 20
+    CXX_STANDARD_REQUIRED ON)

Tests/RunCMake/CXXModules/CMP0155-OLD.cmake

@@ -0,0 +1,11 @@
+enable_language(CXX)
+unset(CMAKE_CXX_SCANDEP_SOURCE)
+
+cmake_policy(SET CMP0155 OLD)
+
+add_executable(cmp0155-old
+  sources/module-use.cxx)
+set_target_properties(cmp0155-old
+  PROPERTIES
+    CXX_STANDARD 20
+    CXX_STANDARD_REQUIRED ON)

Tests/RunCMake/CXXModules/RunCMakeTest.cmake

@@ -17,6 +17,8 @@ if ("cxx_std_20" IN_LIST CMAKE_CXX_COMPILE_FEATURES)
 
   run_cmake(NoScanningSourceFileProperty)
   run_cmake(NoScanningTargetProperty)
+  run_cmake(CMP0155-OLD)
+  run_cmake(CMP0155-NEW)
 endif ()
 
 if (RunCMake_GENERATOR MATCHES "Ninja")

Tests/RunCMake/CXXModules/examples/circular/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.24)
+cmake_minimum_required(VERSION 3.24...3.28)
 project(cxx_modules_circular CXX)
 
 include("${CMAKE_SOURCE_DIR}/../cxx-modules-rules.cmake")

Tests/RunCMake/CXXModules/examples/deep-chain/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.24)
+cmake_minimum_required(VERSION 3.24...3.28)
 project(cxx_modules_deep_chain CXX)
 
 include("${CMAKE_SOURCE_DIR}/../cxx-modules-rules.cmake")

Tests/RunCMake/CXXModules/examples/duplicate/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.24)
+cmake_minimum_required(VERSION 3.24...3.28)
 project(cxx_modules_duplicate CXX)
 
 include("${CMAKE_SOURCE_DIR}/../cxx-modules-rules.cmake")

Tests/RunCMake/CXXModules/examples/export-bmi-and-interface-build/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.24)
+cmake_minimum_required(VERSION 3.24...3.28)
 project(cxx_modules_export_bmi_and_interfaces CXX)
 
 include("${CMAKE_SOURCE_DIR}/../cxx-modules-rules.cmake")

Tests/RunCMake/CXXModules/examples/export-bmi-and-interface-build/test/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.24)
+cmake_minimum_required(VERSION 3.24...3.28)
 project(cxx_modules_library NONE)
 
 find_package(export_bmi_and_interfaces REQUIRED)

Tests/RunCMake/CXXModules/examples/export-bmi-and-interface-install/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.24)
+cmake_minimum_required(VERSION 3.24...3.28)
 project(cxx_modules_export_bmi_and_interfaces CXX)
 
 include("${CMAKE_SOURCE_DIR}/../cxx-modules-rules.cmake")

Tests/RunCMake/CXXModules/examples/export-bmi-and-interface-install/test/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.24)
+cmake_minimum_required(VERSION 3.24...3.28)
 project(cxx_modules_library NONE)
 
 find_package(export_bmi_and_interfaces REQUIRED)

Tests/RunCMake/CXXModules/examples/export-compile-commands/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.24)
+cmake_minimum_required(VERSION 3.24...3.28)
 project(cxx_modules_export_compile_commands CXX)
 
 include("${CMAKE_SOURCE_DIR}/../cxx-modules-rules.cmake")

Tests/RunCMake/CXXModules/examples/export-include-directories-build/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.24)
+cmake_minimum_required(VERSION 3.24...3.28)
 project(cxx_modules_export_include_directories CXX)
 
 include("${CMAKE_SOURCE_DIR}/../cxx-modules-rules.cmake")

Tests/RunCMake/CXXModules/examples/export-include-directories-install/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.24)
+cmake_minimum_required(VERSION 3.24...3.28)
 project(cxx_modules_export_include_directories CXX)
 
 include("${CMAKE_SOURCE_DIR}/../cxx-modules-rules.cmake")

Tests/RunCMake/CXXModules/examples/export-include-directories-install/test/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.24)
+cmake_minimum_required(VERSION 3.24...3.28)
 project(cxx_modules_library NONE)
 
 set(CMAKE_EXPERIMENTAL_CXX_MODULE_CMAKE_API "ac01f462-0f5f-432a-86aa-acef252918a6")

Tests/RunCMake/CXXModules/examples/export-interface-build/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.24)
+cmake_minimum_required(VERSION 3.24...3.28)
 project(cxx_modules_export_interfaces CXX)
 
 include("${CMAKE_SOURCE_DIR}/../cxx-modules-rules.cmake")

Tests/RunCMake/CXXModules/examples/export-interface-build/test/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.24)
+cmake_minimum_required(VERSION 3.24...3.28)
 project(cxx_modules_library NONE)
 
 find_package(export_interfaces REQUIRED)

Tests/RunCMake/CXXModules/examples/export-interface-install/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.24)
+cmake_minimum_required(VERSION 3.24...3.28)
 project(cxx_modules_export_interfaces CXX)
 
 include("${CMAKE_SOURCE_DIR}/../cxx-modules-rules.cmake")

Tests/RunCMake/CXXModules/examples/export-interface-install/test/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.24)
+cmake_minimum_required(VERSION 3.24...3.28)
 project(cxx_modules_library NONE)
 
 find_package(export_interfaces REQUIRED)

Tests/RunCMake/CXXModules/examples/export-interface-no-properties-build/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.24)
+cmake_minimum_required(VERSION 3.24...3.28)
 project(cxx_modules_export_interfaces_no_properties CXX)
 
 include("${CMAKE_SOURCE_DIR}/../cxx-modules-rules.cmake")

Tests/RunCMake/CXXModules/examples/export-interface-no-properties-build/test/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.24)
+cmake_minimum_required(VERSION 3.24...3.28)
 project(cxx_modules_library NONE)
 
 find_package(export_interfaces_no_properties REQUIRED)

Tests/RunCMake/CXXModules/examples/export-interface-no-properties-install/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.24)
+cmake_minimum_required(VERSION 3.24...3.28)
 project(cxx_modules_export_interfaces CXX)
 
 include("${CMAKE_SOURCE_DIR}/../cxx-modules-rules.cmake")

Tests/RunCMake/CXXModules/examples/export-interface-no-properties-install/test/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.24)
+cmake_minimum_required(VERSION 3.24...3.28)
 project(cxx_modules_library NONE)
 
 find_package(export_interfaces_no_properties REQUIRED)

Tests/RunCMake/CXXModules/examples/export-usage-build/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.24)
+cmake_minimum_required(VERSION 3.24...3.28)
 project(cxx_modules_export_usage CXX)
 
 include("${CMAKE_SOURCE_DIR}/../cxx-modules-rules.cmake")

Tests/RunCMake/CXXModules/examples/export-usage-build/test/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.24)
+cmake_minimum_required(VERSION 3.24...3.28)
 project(cxx_modules_library NONE)
 
 find_package(export_usage REQUIRED)

Tests/RunCMake/CXXModules/examples/export-usage-install/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.24)
+cmake_minimum_required(VERSION 3.24...3.28)
 project(cxx_modules_export_usage CXX)
 
 include("${CMAKE_SOURCE_DIR}/../cxx-modules-rules.cmake")

Tests/RunCMake/CXXModules/examples/export-usage-install/test/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.24)
+cmake_minimum_required(VERSION 3.24...3.28)
 project(cxx_modules_library NONE)
 
 find_package(export_usage REQUIRED)

Tests/RunCMake/CXXModules/examples/generated/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.24)
+cmake_minimum_required(VERSION 3.24...3.28)
 project(cxx_modules_generated CXX)
 
 include("${CMAKE_SOURCE_DIR}/../cxx-modules-rules.cmake")

Tests/RunCMake/CXXModules/examples/import-modules/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.24)
+cmake_minimum_required(VERSION 3.24...3.28)
 project(cxx_modules_import_interfaces CXX)
 
 include("${CMAKE_SOURCE_DIR}/../cxx-modules-rules.cmake")

Tests/RunCMake/CXXModules/examples/install-bmi-and-interfaces/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.24)
+cmake_minimum_required(VERSION 3.24...3.28)
 project(cxx_modules_install_bmi_and_interfaces CXX)
 
 include("${CMAKE_SOURCE_DIR}/../cxx-modules-rules.cmake")

Tests/RunCMake/CXXModules/examples/install-bmi/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.24)
+cmake_minimum_required(VERSION 3.24...3.28)
 project(cxx_modules_install_bmi CXX)
 
 include("${CMAKE_SOURCE_DIR}/../cxx-modules-rules.cmake")

Tests/RunCMake/CXXModules/examples/internal-partitions/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.24)
+cmake_minimum_required(VERSION 3.24...3.28)
 project(cxx_modules_internal_partitions CXX)
 
 include("${CMAKE_SOURCE_DIR}/../cxx-modules-rules.cmake")

Tests/RunCMake/CXXModules/examples/library/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.24)
+cmake_minimum_required(VERSION 3.24...3.28)
 project(cxx_modules_library CXX)
 
 include("${CMAKE_SOURCE_DIR}/../cxx-modules-rules.cmake")

Tests/RunCMake/CXXModules/examples/object-library/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.24)
+cmake_minimum_required(VERSION 3.24...3.28)
 project(cxx_modules_objlib CXX)
 
 include("${CMAKE_SOURCE_DIR}/../cxx-modules-rules.cmake")

Tests/RunCMake/CXXModules/examples/partitions/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.24)
+cmake_minimum_required(VERSION 3.24...3.28)
 project(cxx_modules_partitions CXX)
 
 include("${CMAKE_SOURCE_DIR}/../cxx-modules-rules.cmake")

Tests/RunCMake/CXXModules/examples/public-req-private/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.24)
+cmake_minimum_required(VERSION 3.24...3.28)
 project(cxx_modules_public_req_private CXX)
 
 include("${CMAKE_SOURCE_DIR}/../cxx-modules-rules.cmake")

Tests/RunCMake/CXXModules/examples/req-private-other-target/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.26)
+cmake_minimum_required(VERSION 3.26...3.28)
 project(req_private_other_target CXX)
 
 include("${CMAKE_SOURCE_DIR}/../cxx-modules-rules.cmake")

Tests/RunCMake/CXXModules/examples/same-src-name/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.27)
+cmake_minimum_required(VERSION 3.27...3.28)
 project(cxx_modules_same_src_name CXX)
 
 include("${CMAKE_SOURCE_DIR}/../cxx-modules-rules.cmake")

Tests/RunCMake/CXXModules/examples/scan_properties/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.24)
+cmake_minimum_required(VERSION 3.24...3.28)
 project(scan_properties CXX)
 
 include("${CMAKE_SOURCE_DIR}/../cxx-modules-rules.cmake")

Tests/RunCMake/CXXModules/examples/simple/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.24)
+cmake_minimum_required(VERSION 3.24...3.28)
 project(cxx_modules_simple CXX)
 
 include("${CMAKE_SOURCE_DIR}/../cxx-modules-rules.cmake")

Tests/RunCMake/CXXModules/examples/try-compile/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.24)
+cmake_minimum_required(VERSION 3.24...3.28)
 project(cxx_modules_try_compile CXX)
 
 include("${CMAKE_SOURCE_DIR}/../cxx-modules-rules.cmake")

Tests/RunCMake/CXXModules/examples/try-run/CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.24)
+cmake_minimum_required(VERSION 3.24...3.28)
 project(cxx_modules_try_run CXX)
 
 include("${CMAKE_SOURCE_DIR}/../cxx-modules-rules.cmake")

Tests/RunCMake/TargetPolicies/PolicyList-stderr.txt

@@ -38,6 +38,7 @@
    \* CMP0131
    \* CMP0142
    \* CMP0154
+   \* CMP0155
 
 Call Stack \(most recent call first\):
   CMakeLists.txt:3 \(include\)