From 4f0410959ec014bdb8d9838a36ae37e31b98318d Mon Sep 17 00:00:00 2001
From: Ben Boeckel <ben.boeckel@kitware.com>
Date: Thu, 21 Sep 2023 09:41:26 -0400
Subject: GCC: add rules for scanning C++ module dependencies

---
 Modules/Compiler/GNU-CXX.cmake | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/Modules/Compiler/GNU-CXX.cmake b/Modules/Compiler/GNU-CXX.cmake
index b35f254..c276f4e 100644
--- a/Modules/Compiler/GNU-CXX.cmake
+++ b/Modules/Compiler/GNU-CXX.cmake
@@ -72,3 +72,24 @@ elseif(CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 8.0)
 endif()
 
 __compiler_check_default_language_standard(CXX 3.4 98 6.0 14 11.1 17)
+
+if(CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 14.0)
+  string(CONCAT CMAKE_EXPERIMENTAL_CXX_SCANDEP_SOURCE
+    "<CMAKE_CXX_COMPILER> <DEFINES> <INCLUDES> <FLAGS> -E -x c++ <SOURCE>"
+    " -MT <DYNDEP_FILE> -MD -MF <DEP_FILE>"
+    " -fmodules-ts -fdeps-file=<DYNDEP_FILE> -fdeps-target=<OBJECT> -fdeps-format=p1689r5"
+    " -o <PREPROCESSED_SOURCE>")
+  set(CMAKE_EXPERIMENTAL_CXX_MODULE_MAP_FORMAT "gcc")
+  string(CONCAT CMAKE_EXPERIMENTAL_CXX_MODULE_MAP_FLAG
+    # Turn on modules.
+    "-fmodules-ts"
+    # Read the module mapper file.
+    " -fmodule-mapper=<MODULE_MAP_FILE>"
+    # Make sure dependency tracking is enabled (missing from `try_*`).
+    " -MD"
+    # Suppress `CXX_MODULES +=` from generated depfile snippets.
+    " -fdeps-format=p1689r5"
+    # Force C++ as a language.
+    " -x c++")
+  set(CMAKE_EXPERIMENTAL_CXX_MODULE_BMI_ONLY_FLAG "-fmodule-only")
+endif()
-- 
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