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.
2026-09-26 04:09:35 +03:00 · 2023-10-02 10:17:31 -04:00
parent 3cddd11649
commit 437280b127
50 changed files with 129 additions and 41 deletions
@@ -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
 ================
@@ -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>
@@ -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
@@ -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,
@@ -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
@@ -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)                                                                  \
@@ -0,0 +1 @@
+1
@@ -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.
@@ -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)
@@ -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)
@@ -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")
@@ -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")
@@ -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")
@@ -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")
@@ -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")
@@ -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)
@@ -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")
@@ -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)
@@ -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")
@@ -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")
@@ -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")
@@ -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")
@@ -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")
@@ -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)
@@ -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")
@@ -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)
@@ -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")
@@ -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)
@@ -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")
@@ -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)
@@ -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")
@@ -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)
@@ -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")
@@ -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)
@@ -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")
@@ -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")
@@ -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")
@@ -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")
@@ -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")
@@ -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")
@@ -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")
@@ -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")
@@ -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")
@@ -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")
@@ -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")
@@ -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")
@@ -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")
@@ -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")
@@ -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")
@@ -38,6 +38,7 @@
   \* CMP0131
   \* CMP0142
   \* CMP0154
+   \* CMP0155

 Call Stack \(most recent call first\):
  CMakeLists.txt:3 \(include\)