c++: Improve constexpr error for dangling local variables

Currently, when typeck discovers that a return statement will refer to a
local variable it rewrites to return a null pointer. This causes the
error messages for using the return value in a constant expression to be
unhelpful, especially for reference return values.

This patch removes this "optimisation". Relying on this raises a warning
by default and causes UB anyway, so there should be no issue in doing
so. We also suppress additional warnings from later passes that detect
this as a dangling pointer, since we've already indicated this anyway.

gcc/cp/ChangeLog:

	* semantics.cc (finish_return_stmt): Suppress dangling pointer
        reporting on return statement if already reported.
	* typeck.cc (check_return_expr): Don't set return expression to
        zero for dangling addresses.

gcc/testsuite/ChangeLog:

        * g++.dg/cpp1y/constexpr-lifetime5.C: Test reported message is
        correct.
	* g++.dg/warn/Wreturn-local-addr-6.C: Remove check for return
        value optimisation.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>

gcc/cp/semantics.cc

@@ -1246,7 +1246,10 @@ finish_return_stmt (tree expr)
 
   r = build_stmt (input_location, RETURN_EXPR, expr);
   if (no_warning)
-    suppress_warning (r, OPT_Wreturn_type);
+    {
+      suppress_warning (r, OPT_Wreturn_type);
+      suppress_warning (r, OPT_Wdangling_pointer_);
+    }
   r = maybe_cleanup_point_expr_void (r);
   r = add_stmt (r);
 

gcc/cp/typeck.cc

@@ -11232,8 +11232,9 @@ check_return_expr (tree retval, bool *no_warning)
       else if (!processing_template_decl
 	       && maybe_warn_about_returning_address_of_local (retval, loc)
 	       && INDIRECT_TYPE_P (valtype))
-	retval = build2 (COMPOUND_EXPR, TREE_TYPE (retval), retval,
-			 build_zero_cst (TREE_TYPE (retval)));
+	/* Suppress the Wdangling-pointer warning in the return statement
+	   that would otherwise occur.  */
+	*no_warning = true;
     }
 
   if (processing_template_decl)

gcc/testsuite/g++.dg/cpp1y/constexpr-lifetime5.C

@@ -1,11 +1,11 @@
 // { dg-do compile { target c++14 } }
 // { dg-options "-Wno-return-local-addr" }
 
-constexpr const int& id(int x) { return x; }
+constexpr const int& id(int x) { return x; }  // { dg-message "note: declared here" }
 
 constexpr bool test() {
   const int& y = id(3);
   return y == 3;
 }
 
-constexpr bool x = test();  // { dg-error "" }
+constexpr bool x = test();  // { dg-error "accessing object outside its lifetime" }

gcc/testsuite/g++.dg/warn/Wreturn-local-addr-6.C

@@ -24,6 +24,3 @@ return_addr_local_as_intref (void)
 
   return (const intptr_t&)a;   // { dg-warning "\\\[-Wreturn-local-addr]" } */
 }
-
-/* Verify that the return value has been replaced with zero:
-  { dg-final { scan-tree-dump-times "return 0;" 2 "optimized" } } */