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ChuanqiXu9ChuanqiXu9
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[C++20] [Modules] [Driver] Don't enable -fdelayed-template-parsing by default on windows with C++20 (#69431)
There are already 3 issues about the broken state of -fdelayed-template-parsing and C++20 modules: - #61068 - #64810 - #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. Also 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.
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‎clang/docs/ReleaseNotes.rst

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@@ -121,6 +121,10 @@ C++ Specific Potentially Breaking Changes
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system headers and macros. It will be turned into a hard (non-downgradable)
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error in the next Clang release.
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- The flag `-fdelayed-template-parsing` won't be enabled by default with C++20
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when targetting MSVC to match the behavior of MSVC.
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(`MSVC Docs <https://learn.microsoft.com/en-us/cpp/build/reference/permissive-standards-conformance?view=msvc-170>`_)
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ABI Changes in This Version
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---------------------------
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- Following the SystemV ABI for x86-64, ``__int128`` arguments will no longer

‎clang/include/clang/Basic/DiagnosticDriverKinds.td

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

‎clang/lib/Driver/ToolChains/Clang.cpp

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

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

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

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