Disable constexpr function body checking in more situations

clang/docs/ReleaseNotes.rst

@@ -207,6 +207,9 @@ C++23 Feature Support
 - Implemented `P1774R8: Portable assumptions <https://wg21.link/P1774R8>`_.
 
 - Implemented `P2448R2: Relaxing some constexpr restrictions <https://wg21.link/P2448R2>`_.
+  Note, the ``-Winvalid-constexpr`` diagnostic is now disabled in C++23 mode,
+  but can be explicitly specified to retain the old diagnostic checking
+  behavior.
 
 - Added a ``__reference_converts_from_temporary`` builtin, completing the necessary compiler support for
   `P2255R2: Type trait to determine if a reference binds to a temporary <https://wg21.link/P2255R2>`_.
@@ -323,6 +326,17 @@ Non-comprehensive list of changes in this release
 - Builtins ``__builtin_shufflevector()`` and ``__builtin_convertvector()`` may
   now be used within constant expressions.
 
+- When compiling a constexpr function, Clang will check to see whether the
+  function can *never* be used in a constant expression context and issues a
+  diagnostic under the ``-Winvalid-constexpr`` diagostic flag (which defaults
+  to an error). This check can be expensive because the mere presence of a
+  function marked ``constexpr`` will cause us to undergo constant expression
+  evaluation, even if the function is not called within the translation unit
+  being compiled. Due to the expense, Clang no longer checks constexpr function
+  bodies when the function is defined in a system header file or when
+  ``-Winvalid-constexpr`` is not enabled for the function definition, which
+  should result in mild compile-time performance improvements.
+
 New Compiler Flags
 ------------------
 - ``-fsanitize=implicit-bitfield-conversion`` checks implicit truncation and

clang/include/clang/Basic/LangOptions.def

@@ -505,6 +505,14 @@ COMPATIBLE_LANGOPT(IncrementalExtensions, 1, 0, " True if we want to process sta
 
 BENIGN_LANGOPT(CheckNew, 1, 0, "Do not assume C++ operator new may not return NULL")
 
+// FIXME: It would be better for us to find a way to encode the state of this
+// diagnostic in tablegen so that we can specify a particular diagnostic option
+// is disabled or enabled based on other language options or made it easier to
+// do this from the compiler invocation without hitting option round-tripping
+// issues.
+BENIGN_LANGOPT(CheckConstexprFunctionBodies, 1, 1,
+               "Emit diagnostics for a constexpr function body that can never "
+               "be used in a constant expression.")
 #undef LANGOPT
 #undef COMPATIBLE_LANGOPT
 #undef BENIGN_LANGOPT

clang/include/clang/Driver/Options.td

@@ -557,6 +557,17 @@ multiclass BoolMOption<string flag_base, KeyPathAndMacro kpm,
               Group<m_Group>;
 }
 
+/// Creates a BoolOption where both of the flags are prefixed with "W", are in
+/// the Group<W_Group>.
+/// Used for -cc1 frontend options. Driver-only options do not map to
+/// CompilerInvocation.
+multiclass BoolWOption<string flag_base, KeyPathAndMacro kpm,
+                       Default default, FlagDef flag1, FlagDef flag2,
+                       BothFlags both = BothFlags<[]>> {
+  defm NAME : BoolOption<"W", flag_base, kpm, default, flag1, flag2, both>,
+              Group<W_Group>;
+}
+
 // Works like BoolOption except without marshalling
 multiclass BoolOptionWithoutMarshalling<string prefix = "", string spelling_base,
                                         FlagDef flag1_base, FlagDef flag2_base,
@@ -606,6 +617,7 @@ defvar cpp11 = LangOpts<"CPlusPlus11">;
 defvar cpp14 = LangOpts<"CPlusPlus14">;
 defvar cpp17 = LangOpts<"CPlusPlus17">;
 defvar cpp20 = LangOpts<"CPlusPlus20">;
+defvar cpp23 = LangOpts<"CPlusPlus23">;
 defvar c99 = LangOpts<"C99">;
 defvar c23 = LangOpts<"C23">;
 defvar lang_std = LangOpts<"LangStd">;
@@ -961,6 +973,12 @@ def Wdeprecated : Flag<["-"], "Wdeprecated">, Group<W_Group>,
   HelpText<"Enable warnings for deprecated constructs and define __DEPRECATED">;
 def Wno_deprecated : Flag<["-"], "Wno-deprecated">, Group<W_Group>,
   Visibility<[ClangOption, CC1Option]>;
+defm invalid_constexpr : BoolWOption<"invalid-constexpr",
+  LangOpts<"CheckConstexprFunctionBodies">,
+  Default<!strconcat("!", cpp23.KeyPath)>,
+  NegFlag<SetFalse, [], [ClangOption, CC1Option], "Disable">,
+  PosFlag<SetTrue, [], [ClangOption, CC1Option], "Enable">,
+  BothFlags<[], [ClangOption, CC1Option], " checking of constexpr function bodies for validity within a constant expression context">>;
 def Wl_COMMA : CommaJoined<["-"], "Wl,">, Visibility<[ClangOption, FlangOption]>,
   Flags<[LinkerInput, RenderAsInput]>,
   HelpText<"Pass the comma separated arguments in <arg> to the linker">,

clang/lib/Frontend/CompilerInvocation.cpp

@@ -2407,6 +2407,9 @@ void CompilerInvocationBase::GenerateDiagnosticArgs(
     // This option is automatically generated from UndefPrefixes.
     if (Warning == "undef-prefix")
       continue;
+    // This option is automatically generated from CheckConstexprFunctionBodies.
+    if (Warning == "invalid-constexpr" || Warning == "no-invalid-constexpr")
+      continue;
     Consumer(StringRef("-W") + Warning);
   }
 

clang/lib/Sema/SemaDeclCXX.cpp

@@ -2469,11 +2469,18 @@ static bool CheckConstexprFunctionBody(Sema &SemaRef, const FunctionDecl *Dcl,
   //     base class sub-objects shall be a constexpr constructor.
   //
   // Note that this rule is distinct from the "requirements for a constexpr
-  // function", so is not checked in CheckValid mode.
+  // function", so is not checked in CheckValid mode. Because the check for
+  // constexpr potential is expensive, skip the check if the diagnostic is
+  // disabled, the function is declared in a system header, or we're in C++23
+  // or later mode (see https://wg21.link/P2448).
+  bool SkipCheck =
+      !SemaRef.getLangOpts().CheckConstexprFunctionBodies ||
+      SemaRef.getSourceManager().isInSystemHeader(Dcl->getLocation()) ||
+      SemaRef.getDiagnostics().isIgnored(
+          diag::ext_constexpr_function_never_constant_expr, Dcl->getLocation());
   SmallVector<PartialDiagnosticAt, 8> Diags;
-  if (Kind == Sema::CheckConstexprKind::Diagnose &&
-      !Expr::isPotentialConstantExpr(Dcl, Diags) &&
-      !SemaRef.getLangOpts().CPlusPlus23) {
+  if (Kind == Sema::CheckConstexprKind::Diagnose && !SkipCheck &&
+      !Expr::isPotentialConstantExpr(Dcl, Diags)) {
     SemaRef.Diag(Dcl->getLocation(),
                  diag::ext_constexpr_function_never_constant_expr)
         << isa<CXXConstructorDecl>(Dcl) << Dcl->isConsteval()

clang/test/SemaCXX/constexpr-never-constant.cpp

@@ -0,0 +1,26 @@
+// RUN: %clang_cc1 -fsyntax-only -std=c++20 -verify -fcxx-exceptions %s
+// RUN: %clang_cc1 -fsyntax-only -std=c++23 -Winvalid-constexpr -verify -fcxx-exceptions %s
+// Note: for a diagnostic that defaults to an error, -Wno-foo -Wfoo will
+// disable the diagnostic and then re-enable it *as a warning* rather than as
+// an error. So we manually enable it as an error again with -Werror to keep
+// the diagnostic checks consistent.
+// RUN: %clang_cc1 -fsyntax-only -std=c++23 -Wno-invalid-constexpr -Winvalid-constexpr -Werror=invalid-constexpr -verify -fcxx-exceptions %s
+
+// RUN: %clang_cc1 -fsyntax-only -Wno-invalid-constexpr -verify=good -fcxx-exceptions %s
+// RUN: %clang_cc1 -fsyntax-only -std=c++23 -verify=good -fcxx-exceptions %s
+// RUN: %clang_cc1 -fsyntax-only -std=c++23 -Wno-invalid-constexpr -verify=good -fcxx-exceptions %s
+// RUN: %clang_cc1 -fsyntax-only -std=c++23 -Winvalid-constexpr -Wno-invalid-constexpr -verify=good -fcxx-exceptions %s
+// RUN: %clang_cc1 -fsyntax-only -Wno-invalid-constexpr -verify=good -fcxx-exceptions %s
+// good-no-diagnostics
+
+constexpr void func() { // expected-error {{constexpr function never produces a constant expression}}
+  throw 12;             // expected-note {{subexpression not valid in a constant expression}}
+}
+
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Winvalid-constexpr"
+constexpr void other_func() {
+#pragma clang diagnostic pop
+
+  throw 12;
+}