[C++20] [Modules] [Driver] Don't enable -fdelayed-template-parsing by…

… default on windows after c++20

There are already 3 issues about the broken state of
-fdelayed-template-parsing and C++20 modules:
- llvm#61068
- llvm#64810
- llvm#65027

The problem is more complex than I thought. I am not sure how to fix it
properly now. Given the complexities and -fdelayed-template-parsing is
actually an extension to support old MS codes, I think it may make sense
to not enable the -fdelayed-template-parsing option by default with
C++20 modules to give more user friendly experience. Users who still want
-fdelayed-template-parsing can specify it explicitly.

Given the discussion in llvm#69551,
we decide to not enable -fdelayed-template-parsing by default on windows
after c++20

clang/docs/ReleaseNotes.rst

@@ -100,6 +100,10 @@ C++ Specific Potentially Breaking Changes
   system headers and macros. It will be turned into a hard (non-downgradable)
   error in the next Clang release.
 
+- The flag `-fdelayed-template-parsing` won't be enabled by default with C++20
+  when targetting MSVC to match the behavior of MSVC. 
+  (`MSVC Docs <https://learn.microsoft.com/en-us/cpp/build/reference/permissive-standards-conformance?view=msvc-170>`_)
+
 ABI Changes in This Version
 ---------------------------
 - Following the SystemV ABI for x86-64, ``__int128`` arguments will no longer

clang/include/clang/Basic/DiagnosticDriverKinds.td

@@ -527,6 +527,10 @@ def err_test_module_file_extension_format : Error<
 def err_drv_module_output_with_multiple_arch : Error<
   "option '-fmodule-output' can't be used with multiple arch options">;
 
+def warn_drv_delayed_template_parsing_after_cxx20 : Warning<
+  "-fdelayed-template-parsing is deprecated after C++20">,
+  InGroup<DiagGroup<"delayed-template-parsing-in-cxx20">>;
+
 def err_drv_extract_api_wrong_kind : Error<
   "header file '%0' input '%1' does not match the type of prior input "
   "in api extraction; use '-x %2' to override">;

clang/lib/Driver/ToolChains/Clang.cpp

@@ -3724,20 +3724,10 @@ bool Driver::getDefaultModuleCachePath(SmallVectorImpl<char> &Result) {
 
 static bool RenderModulesOptions(Compilation &C, const Driver &D,
                                  const ArgList &Args, const InputInfo &Input,
-                                 const InputInfo &Output, const Arg *Std,
+                                 const InputInfo &Output, bool HaveStd20,
                                  ArgStringList &CmdArgs) {
   bool IsCXX = types::isCXX(Input.getType());
-  // FIXME: Find a better way to determine whether the input has standard c++
-  // modules support by default.
-  bool HaveStdCXXModules =
-      IsCXX && Std &&
-      (Std->containsValue("c++2a") || Std->containsValue("gnu++2a") ||
-       Std->containsValue("c++20") || Std->containsValue("gnu++20") ||
-       Std->containsValue("c++2b") || Std->containsValue("gnu++2b") ||
-       Std->containsValue("c++23") || Std->containsValue("gnu++23") ||
-       Std->containsValue("c++2c") || Std->containsValue("gnu++2c") ||
-       Std->containsValue("c++26") || Std->containsValue("gnu++26") ||
-       Std->containsValue("c++latest") || Std->containsValue("gnu++latest"));
+  bool HaveStdCXXModules = IsCXX && HaveStd20;
   bool HaveModules = HaveStdCXXModules;
 
   // -fmodules enables the use of precompiled modules (off by default).
@@ -6842,14 +6832,6 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
                         (!IsWindowsMSVC || IsMSVC2015Compatible)))
     CmdArgs.push_back("-fno-threadsafe-statics");
 
-  // -fno-delayed-template-parsing is default, except when targeting MSVC.
-  // Many old Windows SDK versions require this to parse.
-  // FIXME: MSVC introduced /Zc:twoPhase- to disable this behavior in their
-  // compiler. We should be able to disable this by default at some point.
-  if (Args.hasFlag(options::OPT_fdelayed_template_parsing,
-                   options::OPT_fno_delayed_template_parsing, IsWindowsMSVC))
-    CmdArgs.push_back("-fdelayed-template-parsing");
-
   // -fgnu-keywords default varies depending on language; only pass if
   // specified.
   Args.AddLastArg(CmdArgs, options::OPT_fgnu_keywords,
@@ -6870,8 +6852,38 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
 
   Args.AddLastArg(CmdArgs, options::OPT_finline_max_stacksize_EQ);
 
+  // FIXME: Find a better way to determine whether we are in C++20.
+  bool HaveCxx20 =
+      Std &&
+      (Std->containsValue("c++2a") || Std->containsValue("gnu++2a") ||
+       Std->containsValue("c++20") || Std->containsValue("gnu++20") ||
+       Std->containsValue("c++2b") || Std->containsValue("gnu++2b") ||
+       Std->containsValue("c++23") || Std->containsValue("gnu++23") ||
+       Std->containsValue("c++2c") || Std->containsValue("gnu++2c") ||
+       Std->containsValue("c++26") || Std->containsValue("gnu++26") ||
+       Std->containsValue("c++latest") || Std->containsValue("gnu++latest"));
   bool HaveModules =
-      RenderModulesOptions(C, D, Args, Input, Output, Std, CmdArgs);
+      RenderModulesOptions(C, D, Args, Input, Output, HaveCxx20, CmdArgs);
+
+  // -fdelayed-template-parsing is default when targeting MSVC.
+  // Many old Windows SDK versions require this to parse.
+  //
+  // According to
+  // https://learn.microsoft.com/en-us/cpp/build/reference/permissive-standards-conformance?view=msvc-170,
+  // MSVC actually defaults to -fno-delayed-template-parsing (/Zc:twoPhase-
+  // with MSVC CLI) if using C++20. So we match the behavior with MSVC here to
+  // not enable -fdelayed-template-parsing by default after C++20.
+  //
+  // FIXME: Given -fdelayed-template-parsing is a source of bugs, we should be
+  // able to disable this by default at some point.
+  if (Args.hasFlag(options::OPT_fdelayed_template_parsing,
+                   options::OPT_fno_delayed_template_parsing,
+                   IsWindowsMSVC && !HaveCxx20)) {
+    if (HaveCxx20)
+      D.Diag(clang::diag::warn_drv_delayed_template_parsing_after_cxx20);
+
+    CmdArgs.push_back("-fdelayed-template-parsing");
+  }
 
   if (Args.hasFlag(options::OPT_fpch_validate_input_files_content,
                    options::OPT_fno_pch_validate_input_files_content, false))

clang/test/Driver/cl-delayed-template-parsing-cxx20.cpp

@@ -0,0 +1,10 @@
+// RUN: %clang_cl -### -- %s 2>&1 | FileCheck %s --check-prefix=PRE-CXX20
+// RUN: %clang_cl -std:c++20 -### -- %s 2>&1 | FileCheck %s
+// RUN: %clang_cl -std:c++20 -### -fdelayed-template-parsing -- %s 2>&1 | FileCheck %s --check-prefix=CHECK-EXPLICIT
+
+// PRE-CXX20: -fdelayed-template-parsing
+
+// CHECK-NOT: -fdelayed-template-parsing
+
+// CHECK-EXPLICIT: warning: -fdelayed-template-parsing is deprecated after C++20
+// CHECK-EXPLICIT: -fdelayed-template-parsing