[Clang][C++23] update constexpr diagnostics for missing return statements per P2448 (#94123)

Fixes #92583

Author: a-tarasyuk

clang/lib/Sema/SemaDeclCXX.cpp

@@ -1806,6 +1806,7 @@ static unsigned getRecordDiagFromTagKind(TagTypeKind Tag) {
 static bool CheckConstexprFunctionBody(Sema &SemaRef, const FunctionDecl *Dcl,
                                        Stmt *Body,
                                        Sema::CheckConstexprKind Kind);
+static bool CheckConstexprMissingReturn(Sema &SemaRef, const FunctionDecl *Dcl);
 
 // Check whether a function declaration satisfies the requirements of a
 // constexpr function definition or a constexpr constructor definition. If so,
@@ -2411,20 +2412,9 @@ static bool CheckConstexprFunctionBody(Sema &SemaRef, const FunctionDecl *Dcl,
     }
   } else {
     if (ReturnStmts.empty()) {
-      // C++1y doesn't require constexpr functions to contain a 'return'
-      // statement. We still do, unless the return type might be void, because
-      // otherwise if there's no return statement, the function cannot
-      // be used in a core constant expression.
-      bool OK = SemaRef.getLangOpts().CPlusPlus14 &&
-                (Dcl->getReturnType()->isVoidType() ||
-                 Dcl->getReturnType()->isDependentType());
       switch (Kind) {
       case Sema::CheckConstexprKind::Diagnose:
-        SemaRef.Diag(Dcl->getLocation(),
-                     OK ? diag::warn_cxx11_compat_constexpr_body_no_return
-                        : diag::err_constexpr_body_no_return)
-            << Dcl->isConsteval();
-        if (!OK)
+        if (!CheckConstexprMissingReturn(SemaRef, Dcl))
           return false;
         break;
 
@@ -2494,6 +2484,28 @@ static bool CheckConstexprFunctionBody(Sema &SemaRef, const FunctionDecl *Dcl,
   return true;
 }
 
+static bool CheckConstexprMissingReturn(Sema &SemaRef,
+                                        const FunctionDecl *Dcl) {
+  bool IsVoidOrDependentType = Dcl->getReturnType()->isVoidType() ||
+                               Dcl->getReturnType()->isDependentType();
+  // Skip emitting a missing return error diagnostic for non-void functions
+  // since C++23 no longer mandates constexpr functions to yield constant
+  // expressions.
+  if (SemaRef.getLangOpts().CPlusPlus23 && !IsVoidOrDependentType)
+    return true;
+
+  // C++14 doesn't require constexpr functions to contain a 'return'
+  // statement. We still do, unless the return type might be void, because
+  // otherwise if there's no return statement, the function cannot
+  // be used in a core constant expression.
+  bool OK = SemaRef.getLangOpts().CPlusPlus14 && IsVoidOrDependentType;
+  SemaRef.Diag(Dcl->getLocation(),
+               OK ? diag::warn_cxx11_compat_constexpr_body_no_return
+                  : diag::err_constexpr_body_no_return)
+      << Dcl->isConsteval();
+  return OK;
+}
+
 bool Sema::CheckImmediateEscalatingFunctionDefinition(
     FunctionDecl *FD, const sema::FunctionScopeInfo *FSI) {
   if (!getLangOpts().CPlusPlus20 || !FD->isImmediateEscalating())

clang/test/CXX/dcl.dcl/dcl.spec/dcl.constexpr/p3.cpp

@@ -212,7 +212,7 @@ constexpr int ClassDecl3() {
   return 0;
 }
 
-constexpr int NoReturn() {} // expected-error {{no return statement in constexpr function}}
+constexpr int NoReturn() {} // beforecxx23-error {{no return statement in constexpr function}}
 constexpr int MultiReturn() {
   return 0; // beforecxx14-note {{return statement}}
   return 0; // beforecxx14-warning {{multiple return statements in constexpr function}}

clang/test/SemaCXX/constant-expression-cxx14.cpp

@@ -82,7 +82,7 @@ constexpr void k() {
 
 // If the return type is not 'void', no return statements => never a constant
 // expression, so still diagnose that case.
-[[noreturn]] constexpr int fn() { // expected-error {{no return statement in constexpr function}}
+[[noreturn]] constexpr int fn() { // cxx14_20-error {{no return statement in constexpr function}}
   fn();
 }
 

clang/test/SemaCXX/constexpr-return-non-void-cxx2b.cpp

@@ -0,0 +1,7 @@
+// RUN: %clang_cc1 -std=c++23 -fsyntax-only -verify %s
+
+constexpr int f() { } // expected-warning {{non-void function does not return a value}}
+static_assert(__is_same(decltype([] constexpr -> int { }( )), int)); // expected-warning {{non-void lambda does not return a value}}
+
+consteval int g() { } // expected-warning {{non-void function does not return a value}}
+static_assert(__is_same(decltype([] consteval -> int { }( )), int)); // expected-warning {{non-void lambda does not return a value}}