[clang] [NFC] More exhaustive tests for deducing void return types

[• Quuxplusone]

Summary

This is a preliminary ahead of D119184 (I'll rebase that one on top of this). This shows what Clang's current behavior is for various deduced-return-type situations. Then, in D119184, I'll be able to demonstrate exactly how the diagnostics on each of these situations change.

[ChuanqiXu]

LGTM.

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

 // 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(h1) h1() { // expected-error {{use of undeclared identifier 'h1'}}
+  return;
+}
+
+namespace JustAuto {
+int i;
+auto f1() { }
+auto f2() { return; }
+auto f3() { return void(); }
+auto f4() {
+  return i;
+  return; // expected-error {{'auto' in return type deduced as 'void' here but deduced as 'int' in earlier return statement}}
+}
+auto f5() {
+  return i;
+  return void(); // expected-error {{'auto' in return type 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 l1 = []() { };
+auto l2 = []() { return; };
+auto l3 = []() { return void(); };
+auto l4 = []() {
+  return i;
+  return; // expected-error {{return type 'void' must match previous return type 'int' when lambda expression has unspecified explicit return type}}
+};
+auto l5 = []() {
+  return i;
+  return void(); // expected-error {{return type 'void' must match previous return type 'int' when lambda expression has unspecified explicit return type}}
+};
+
+} // namespace JustAuto
+
+namespace DecltypeAuto {
+int i;
+decltype(auto) f1() { }
+decltype(auto) f2() { return; }
+decltype(auto) f3() { return void(); }
+decltype(auto) f4() {
+  return i;
+  return; // expected-error {{'decltype(auto)' in return type deduced as 'void' here but deduced as 'int' in earlier return statement}}
+}
+decltype(auto) f5() {
+  return i;
+  return void(); // expected-error {{'decltype(auto)' in return type deduced as 'void' here but deduced as 'int' in earlier return statement}}
 }
 
-auto *g() {
+auto l1 = []() -> decltype(auto) { };
+auto l2 = []() -> decltype(auto) { return; };
+auto l3 = []() -> decltype(auto) { return void(); };
+auto l4 = []() -> decltype(auto) {
+  return i;
+  return; // expected-error {{'decltype(auto)' in return type deduced as 'void' here but deduced as 'int' in earlier return statement}}
+};
+auto l5 = []() -> decltype(auto) {
+  return i;
+  return void(); // expected-error {{'decltype(auto)' in return type deduced as 'void' here but deduced as 'int' in earlier return statement}}
+};
+
+} // namespace DecltypeAuto
+
+namespace AutoPtr {
+int i;
+auto *f1() { } // expected-error {{cannot deduce return type 'auto *' for function with no return statements}}
+auto *f2() {
+  return; // expected-error {{cannot deduce return type 'auto *' from omitted return expression}}
+}
+auto *f3() {
+  return void(); // expected-error {{cannot deduce return type 'auto *' from returned value of type 'void'}}
+}
+auto *f4() {
+  return &i;
   return; // expected-error {{cannot deduce return type 'auto *' from omitted return expression}}
 }
+auto *f5() {
+  return &i;
+  return void(); // expected-error {{cannot deduce return type 'auto *' from returned value of type 'void'}}
+}
 
-decltype(h1) h1() { // expected-error {{use of undeclared identifier 'h1'}}
-  return;
+auto l1 = []() -> auto* { }; // expected-error {{cannot deduce return type 'auto *' for function with no return statements}}
+auto l2 = []() -> auto* {
+  return; // expected-error {{cannot deduce return type 'auto *' from omitted return expression}}
+};
+auto l3 = []() -> auto* {
+  return void(); // expected-error {{cannot deduce return type 'auto *' from returned value of type 'void'}}
+};
+auto l4 = []() -> auto* {
+  return &i;
+  return; // expected-error {{cannot deduce return type 'auto *' from omitted return expression}}
+};
+auto l5 = []() -> auto* {
+  return &i;
+  return void(); // expected-error {{cannot deduce return type 'auto *' from returned value of type 'void'}}
+};
+} // namespace AutoPtr
+
+namespace AutoRef {
+int i;
+auto& f1() { // expected-error {{cannot deduce return type 'auto &' for function with no return statements}}
+}
+auto& f2() {
+  return; // expected-error {{cannot deduce return type 'auto &' from omitted return expression}}
+}
+auto& f3() {
+  return void(); // expected-error@-1 {{cannot form a reference to 'void'}}
 }
+auto& f4() {
+  return i;
+  return; // expected-error {{cannot deduce return type 'auto &' from omitted return expression}}
+}
+auto& f5() {
+  return i;
+  return void(); // expected-error@-2 {{cannot form a reference to 'void'}}
+}
+auto& f6() { return 42; } // expected-error {{non-const lvalue reference to type 'int' cannot bind to a temporary of type 'int'}}
+
+auto l1 = []() -> auto& { }; // expected-error {{cannot deduce return type 'auto &' for function with no return statements}}
+auto l2 = []() -> auto& {
+  return; // expected-error {{cannot deduce return type 'auto &' from omitted return expression}}
+};
+auto l3 = []() -> auto& { // expected-error {{cannot form a reference to 'void'}}
+  return void();
+};
+auto l4 = []() -> auto& {
+  return i;
+  return; // expected-error {{cannot deduce return type 'auto &' from omitted return expression}}
+};
+auto l5 = []() -> auto& { // expected-error {{cannot form a reference to 'void'}}
+  return i;
+  return void();
+};
+auto l6 = []() -> auto& {
+  return 42; // expected-error {{non-const lvalue reference to type 'int' cannot bind to a temporary of type 'int'}}
+};
+} // namespace AutoRef