c++: Use error_at rather than warning_at for missing return in constexpr functions [PR96182]

For C++11 we already emit an error if a constexpr function doesn't contain
a return statement, because in C++11 that is the only thing it needs to
contain, but for C++14 we would normally issue a -Wreturn-type warning.

As mentioned by Jonathan, such constexpr functions are invalid, no
diagnostics required, because there doesn't exist any arguments for
which it would result in valid constant expression.

This raises it to an error in such cases.  The !LAMBDA_TYPE_P case
is to avoid error on g++.dg/pr81194.C where the user didn't write
constexpr anywhere and the operator() is compiler generated.

2020-07-31  Jakub Jelinek  <jakub@redhat.com>

	PR c++/96182
	* decl.c (finish_function): In constexpr functions use for C++14 and
	later error instead of warning if no return statement is present and
	diagnose it regardless of warn_return_type.  Move the warn_return_type
	diagnostics earlier in the function.

	* g++.dg/cpp1y/constexpr-96182.C: New test.
	* g++.dg/other/error35.C (S<T>::g()): Add return statement.
	* g++.dg/cpp1y/pr63996.C (foo): Likewise.
	* g++.dg/cpp1y/constexpr-return2.C (f): Likewise.
	* g++.dg/cpp1y/var-templ44.C (make_array): Add throw 1.

gcc/cp/decl.c

@@ -17112,6 +17112,51 @@ finish_function (bool inline_p)
 				      DECL_ATTRIBUTES (fndecl)))
       omp_declare_variant_finalize (fndecl, attr);
 
+  /* Complain if there's just no return statement.  */
+  if ((warn_return_type
+       || (cxx_dialect >= cxx14
+	   && DECL_DECLARED_CONSTEXPR_P (fndecl)))
+      && !VOID_TYPE_P (TREE_TYPE (fntype))
+      && !dependent_type_p (TREE_TYPE (fntype))
+      && !current_function_returns_value && !current_function_returns_null
+      /* Don't complain if we abort or throw.  */
+      && !current_function_returns_abnormally
+      /* Don't complain if there's an infinite loop.  */
+      && !current_function_infinite_loop
+      /* Don't complain if we are declared noreturn.  */
+      && !TREE_THIS_VOLATILE (fndecl)
+      && !DECL_NAME (DECL_RESULT (fndecl))
+      && !TREE_NO_WARNING (fndecl)
+      /* Structor return values (if any) are set by the compiler.  */
+      && !DECL_CONSTRUCTOR_P (fndecl)
+      && !DECL_DESTRUCTOR_P (fndecl)
+      && targetm.warn_func_return (fndecl))
+    {
+      gcc_rich_location richloc (input_location);
+      /* Potentially add a "return *this;" fix-it hint for
+	 assignment operators.  */
+      if (IDENTIFIER_ASSIGN_OP_P (DECL_NAME (fndecl)))
+	{
+	  tree valtype = TREE_TYPE (DECL_RESULT (fndecl));
+	  if (TREE_CODE (valtype) == REFERENCE_TYPE
+	      && current_class_ref
+	      && same_type_ignoring_top_level_qualifiers_p
+		  (TREE_TYPE (valtype), TREE_TYPE (current_class_ref))
+	      && global_dc->option_enabled (OPT_Wreturn_type,
+					    global_dc->lang_mask,
+					    global_dc->option_state))
+	    add_return_star_this_fixit (&richloc, fndecl);
+	}
+      if (cxx_dialect >= cxx14
+	  && DECL_DECLARED_CONSTEXPR_P (fndecl))
+	error_at (&richloc, "no return statement in %<constexpr%> function "
+			    "returning non-void");
+      else if (warning_at (&richloc, OPT_Wreturn_type,
+			   "no return statement in function returning "
+			   "non-void"))
+	TREE_NO_WARNING (fndecl) = 1;
+    }
+
   /* Lambda closure members are implicitly constexpr if possible.  */
   if (cxx_dialect >= cxx17
       && LAMBDA_TYPE_P (CP_DECL_CONTEXT (fndecl)))
@@ -17163,44 +17208,6 @@ finish_function (bool inline_p)
      to the FUNCTION_DECL node itself.  */
   BLOCK_SUPERCONTEXT (DECL_INITIAL (fndecl)) = fndecl;
 
-  /* Complain if there's just no return statement.  */
-  if (warn_return_type
-      && !VOID_TYPE_P (TREE_TYPE (fntype))
-      && !dependent_type_p (TREE_TYPE (fntype))
-      && !current_function_returns_value && !current_function_returns_null
-      /* Don't complain if we abort or throw.  */
-      && !current_function_returns_abnormally
-      /* Don't complain if there's an infinite loop.  */
-      && !current_function_infinite_loop
-      /* Don't complain if we are declared noreturn.  */
-      && !TREE_THIS_VOLATILE (fndecl)
-      && !DECL_NAME (DECL_RESULT (fndecl))
-      && !TREE_NO_WARNING (fndecl)
-      /* Structor return values (if any) are set by the compiler.  */
-      && !DECL_CONSTRUCTOR_P (fndecl)
-      && !DECL_DESTRUCTOR_P (fndecl)
-      && targetm.warn_func_return (fndecl))
-    {
-      gcc_rich_location richloc (input_location);
-      /* Potentially add a "return *this;" fix-it hint for
-	 assignment operators.  */
-      if (IDENTIFIER_ASSIGN_OP_P (DECL_NAME (fndecl)))
-	{
-	  tree valtype = TREE_TYPE (DECL_RESULT (fndecl));
-	  if (TREE_CODE (valtype) == REFERENCE_TYPE
-	      && current_class_ref
-	      && same_type_ignoring_top_level_qualifiers_p
-		  (TREE_TYPE (valtype), TREE_TYPE (current_class_ref))
-	      && global_dc->option_enabled (OPT_Wreturn_type,
-					    global_dc->lang_mask,
-					    global_dc->option_state))
-	    add_return_star_this_fixit (&richloc, fndecl);
-	}
-      if (warning_at (&richloc, OPT_Wreturn_type,
-	  "no return statement in function returning non-void"))
-	TREE_NO_WARNING (fndecl) = 1;
-    }
-
   /* Store the end of the function, so that we get good line number
      info for the epilogue.  */
   cfun->function_end_locus = input_location;

gcc/testsuite/g++.dg/cpp1y/constexpr-96182.C

@@ -0,0 +1,6 @@
+// PR c++/96182
+// { dg-do compile { target c++11 } }
+
+constexpr int foo () {} // { dg-error "no return statement in 'constexpr' function returning non-void" "" { target c++14 } }
+// { dg-error "body of 'constexpr' function 'constexpr int foo\\\(\\\)' not a return-statement" "" { target c++11_only } .-1 }
+// { dg-warning "no return statement in function returning non-void" "" { target c++11_only } .-2 }

gcc/testsuite/g++.dg/cpp1y/constexpr-return2.C

@@ -3,6 +3,7 @@
 
 constexpr int f (int i)
 {
+  if (i == -1) return 0;
 }
 
 constexpr int i = f(42);	// { dg-error "flows off the end|in .constexpr. expansion of " }

gcc/testsuite/g++.dg/cpp1y/pr63996.C

@@ -5,6 +5,7 @@ constexpr int
 foo (int i)
 {
   int a[i] = { }; // { dg-error "7:ISO C\\+\\+ forbids variable length array .a" }
+  if (i == 23) return 0;
 }
 
 constexpr int j = foo (1); // { dg-error "flows off the end|in .constexpr. expansion of" }

gcc/testsuite/g++.dg/cpp1y/var-templ44.C

@@ -26,5 +26,6 @@ constexpr auto make_array()
     -> array<conditional_t<is_void_v<_Dest>, common_type_t<>, _Dest>,
              sizeof...(_Types)> {
   static_assert(__or_<__not_<is_void<_Dest>>, __and_<>>::value, ""); // { dg-error "static assert" }
+  throw 1;
 }
 auto d = make_array();

gcc/testsuite/g++.dg/other/error35.C

@@ -9,6 +9,6 @@ template <typename> struct S {
 enum S<char>::E;
 template <typename T> enum S<T>::E : int { b };
 template <typename T>
-constexpr int S<T>::g() const { b; } // { dg-error "not declared" }
+constexpr int S<T>::g() const { b; if (false) return 0; } // { dg-error "not declared" }
 static_assert(S<char>().g() == 1, ""); // { dg-error "" }
 // { dg-message "in .constexpr. expansion of" "" { target *-*-* } .-1 }