[clang] Don't emit redundant warnings for 'return;'

when the function declaration's return type is already invalid for
some reason. This is relevant to #49188
because another way that the declaration's return type could become
invalid is that it might be `C auto` where `C<void>` is false.

Differential Revision: https://reviews.llvm.org/D119094

clang/lib/Sema/SemaStmt.cpp

@@ -4098,7 +4098,9 @@ StmtResult Sema::BuildReturnStmt(SourceLocation ReturnLoc, Expr *RetValExp,
   } else if (!RetValExp && !HasDependentReturnType) {
     FunctionDecl *FD = getCurFunctionDecl();
 
-    if (getLangOpts().CPlusPlus11 && FD && FD->isConstexpr()) {
+    if ((FD && FD->isInvalidDecl()) || FnRetType->containsErrors()) {
+      // The intended return type might have been "void", so don't warn.
+    } else if (getLangOpts().CPlusPlus11 && FD && FD->isConstexpr()) {
       // C++11 [stmt.return]p2
       Diag(ReturnLoc, diag::err_constexpr_return_missing_expr)
           << FD << FD->isConsteval();

clang/test/SemaCXX/deduced-return-void.cpp

@@ -0,0 +1,23 @@
+// RUN: %clang_cc1 -std=c++14 -fsyntax-only -verify %s
+// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify %s
+
+// Check that we don't get any extra warning for "return" without an
+// expression, in a function that might have been intended to return
+// void all along.
+auto f1() {
+  return 1;
+  return; // expected-error {{deduced as 'void' here but deduced as 'int' in earlier return statement}}
+}
+
+decltype(auto) f2() {
+  return 1;
+  return; // expected-error {{deduced as 'void' here but deduced as 'int' in earlier return statement}}
+}
+
+auto *g() {
+  return; // expected-error {{cannot deduce return type 'auto *' from omitted return expression}}
+}
+
+decltype(h1) h1() { // expected-error {{use of undeclared identifier 'h1'}}
+  return;
+}