PCH: Fix Makefile dependencies to rebuild PCH on header changes

Teach the Makefile generator to scan the implicit dependencies of PCH
creation.  When a header named by `target_precompile_headers` changes
the corresponding PCH must be rebuilt and all consumers recompiled.

Fixes: #19830

4 files changed, 29 insertions, 4 deletions

diff --git a/Tests/BuildDepends/CMakeLists.txt b/Tests/BuildDepends/CMakeLists.txt
index 39a5131fed..c1ad17cd35 100644
--- a/Tests/BuildDepends/CMakeLists.txt
+++ b/Tests/BuildDepends/CMakeLists.txt
@@ -62,6 +62,12 @@ file(WRITE ${BuildDepends_BINARY_DIR}/Project/zot_macro_dir.hxx
   "static const char* zot_macro_dir = \"zot_macro_dir\";\n")
 file(WRITE ${BuildDepends_BINARY_DIR}/Project/zot_macro_tgt.hxx
   "static const char* zot_macro_tgt = \"zot_macro_tgt\";\n")
+file(WRITE ${BuildDepends_BINARY_DIR}/Project/zot_pch.hxx
+  "#ifndef ZOT_PCH_HXX\n"
+  "#define ZOT_PCH_HXX\n"
+  "static const char* zot_pch = \"zot_pch\";\n"
+  "#endif\n"
+  )
 
 file(WRITE ${BuildDepends_BINARY_DIR}/Project/link_depends_no_shared_lib.h
   "#define link_depends_no_shared_lib_value 1\n")
@@ -155,7 +161,7 @@ execute_process(COMMAND ${zot} OUTPUT_VARIABLE out RESULT_VARIABLE runResult)
 string(REGEX REPLACE "[\r\n]" " " out "${out}")
 message("Run result: ${runResult} Output: \"${out}\"")
 
-set(VALUE_UNCHANGED "[zot] [zot_custom] [zot_macro_dir] [zot_macro_tgt] ")
+set(VALUE_UNCHANGED "[zot] [zot_custom] [zot_macro_dir] [zot_macro_tgt] [zot_pch] ")
 if("${out}" STREQUAL "${VALUE_UNCHANGED}")
   message("Worked!")
 else()
@@ -245,6 +251,12 @@ file(WRITE ${BuildDepends_BINARY_DIR}/Project/zot_macro_dir.hxx
   "static const char* zot_macro_dir = \"zot_macro_dir changed\";\n")
 file(WRITE ${BuildDepends_BINARY_DIR}/Project/zot_macro_tgt.hxx
   "static const char* zot_macro_tgt = \"zot_macro_tgt changed\";\n")
+file(WRITE ${BuildDepends_BINARY_DIR}/Project/zot_pch.hxx
+  "#ifndef ZOT_PCH_HXX\n"
+  "#define ZOT_PCH_HXX\n"
+  "static const char* zot_pch = \"zot_pch changed\";\n"
+  "#endif\n"
+  )
 
 file(WRITE ${BuildDepends_BINARY_DIR}/Project/link_depends_no_shared_lib.h
   "#define link_depends_no_shared_lib_value 0\n")
@@ -326,7 +338,7 @@ string(REGEX REPLACE "[\r\n]" " " out "${out}")
 message("Run result: ${runResult} Output: \"${out}\"")
 
 set(VALUE_CHANGED
-  "[zot changed] [zot_custom changed] [zot_macro_dir changed] [zot_macro_tgt changed] "
+  "[zot changed] [zot_custom changed] [zot_macro_dir changed] [zot_macro_tgt changed] [zot_pch changed] "
   )
 if("${out}" STREQUAL "${VALUE_CHANGED}")
   message("Worked!")
diff --git a/Tests/BuildDepends/Project/CMakeLists.txt b/Tests/BuildDepends/Project/CMakeLists.txt
index 3f41b26d09..833880098c 100644
--- a/Tests/BuildDepends/Project/CMakeLists.txt
+++ b/Tests/BuildDepends/Project/CMakeLists.txt
@@ -93,6 +93,12 @@ add_executable(zot zot.cxx ${CMAKE_CURRENT_BINARY_DIR}/zot.hxx
   zot_macro_dir.cxx zot_macro_tgt.cxx)
 add_dependencies(zot zot_custom)
 
+add_library(zot_pch zot_pch.cxx)
+target_link_libraries(zot zot_pch)
+if(NOT CMAKE_OSX_ARCHITECTURES MATCHES "[;$]")
+  target_precompile_headers(zot_pch PRIVATE ${CMAKE_CURRENT_BINARY_DIR}/zot_pch.hxx)
+endif()
+
 # Test the #include line macro transformation rule support.
 set_property(
   TARGET zot
diff --git a/Tests/BuildDepends/Project/zot.cxx b/Tests/BuildDepends/Project/zot.cxx
index faee7d3cec..2d08c4b78d 100644
--- a/Tests/BuildDepends/Project/zot.cxx
+++ b/Tests/BuildDepends/Project/zot.cxx
@@ -4,11 +4,12 @@
 
 const char* zot_macro_dir_f();
 const char* zot_macro_tgt_f();
+const char* zot_pch_f();
 
 int main()
 {
-  printf("[%s] [%s] [%s] [%s]\n", zot, zot_custom, zot_macro_dir_f(),
-         zot_macro_tgt_f());
+  printf("[%s] [%s] [%s] [%s] [%s]\n", zot, zot_custom, zot_macro_dir_f(),
+         zot_macro_tgt_f(), zot_pch_f());
   fflush(stdout);
   return 0;
 }
diff --git a/Tests/BuildDepends/Project/zot_pch.cxx b/Tests/BuildDepends/Project/zot_pch.cxx
new file mode 100644
index 0000000000..d9d04c7fad
--- /dev/null
+++ b/Tests/BuildDepends/Project/zot_pch.cxx
@@ -0,0 +1,6 @@
+#include <zot_pch.hxx>
+
+const char* zot_pch_f()
+{
+  return zot_pch;
+}