Ninja: Avoid preprocessing twice with explicit Fortran_PREPROCESS

Fix spurious warnings from gfortran+Ninja for preprocessing.

Fixes: #23248

Source/cmCommonTargetGenerator.cxx

@@ -101,7 +101,8 @@ void cmCommonTargetGenerator::AppendFortranFormatFlags(
 }
 
 void cmCommonTargetGenerator::AppendFortranPreprocessFlags(
-  std::string& flags, cmSourceFile const& source)
+  std::string& flags, cmSourceFile const& source,
+  PreprocessFlagsRequired requires_pp)
 {
   const std::string srcpp = source.GetSafeProperty("Fortran_PREPROCESS");
   cmOutputConverter::FortranPreprocess preprocess =
@@ -114,7 +115,9 @@ void cmCommonTargetGenerator::AppendFortranPreprocessFlags(
   const char* var = nullptr;
   switch (preprocess) {
     case cmOutputConverter::FortranPreprocess::Needed:
-      var = "CMAKE_Fortran_COMPILE_OPTIONS_PREPROCESS_ON";
+      if (requires_pp == PreprocessFlagsRequired::YES) {
+        var = "CMAKE_Fortran_COMPILE_OPTIONS_PREPROCESS_ON";
+      }
       break;
     case cmOutputConverter::FortranPreprocess::NotNeeded:
       var = "CMAKE_Fortran_COMPILE_OPTIONS_PREPROCESS_OFF";

Source/cmCommonTargetGenerator.h

@@ -46,8 +46,14 @@ protected:
   void AppendFortranFormatFlags(std::string& flags,
                                 cmSourceFile const& source);
 
-  void AppendFortranPreprocessFlags(std::string& flags,
-                                    cmSourceFile const& source);
+  enum class PreprocessFlagsRequired
+  {
+    YES,
+    NO
+  };
+  void AppendFortranPreprocessFlags(
+    std::string& flags, cmSourceFile const& source,
+    PreprocessFlagsRequired requires_pp = PreprocessFlagsRequired::YES);
 
   virtual void AddIncludeFlags(std::string& flags, std::string const& lang,
                                const std::string& config) = 0;

Source/cmNinjaTargetGenerator.cxx

@@ -216,7 +216,8 @@ std::string cmNinjaTargetGenerator::ComputeFlagsForObject(
   // Add Fortran format flags.
   if (language == "Fortran") {
     this->AppendFortranFormatFlags(flags, *source);
-    this->AppendFortranPreprocessFlags(flags, *source);
+    this->AppendFortranPreprocessFlags(flags, *source,
+                                       PreprocessFlagsRequired::NO);
   }
 
   // Add source file specific flags.

Tests/FortranOnly/CMakeLists.txt

@@ -129,6 +129,12 @@ if(CMAKE_Fortran_COMPILE_OPTIONS_PREPROCESS_ON AND
   add_executable(preprocess_target preprocess2.f)
   set_property(TARGET preprocess_target PROPERTY Fortran_PREPROCESS ON)
 
+  if(CMAKE_Fortran_COMPILER_ID STREQUAL "GNU")
+    # gfortran might report spurious warnings if we include the
+    # preprocessing flags at the compilation stage
+    target_compile_options(preprocess_target PRIVATE -Wall -Werror)
+  endif()
+
   # Test that we can preprocess a single source file
   add_executable(preprocess_source preprocess3.f)
   set_property(SOURCE preprocess3.f PROPERTY Fortran_PREPROCESS ON)

Tests/FortranOnly/preprocess2.f

@@ -1,4 +1,6 @@
 #define int INTEGER
+       ! This single unmatched quote ' should not cause an error during compilation
        PROGRAM PREPRO
-       int f
+       int f = 1
+       PRINT*, f
        END