c++: Disable -Wdangling-reference when initing T&

Non-const lvalue references can't bind to a temporary, so the
warning should not be emitted if we're initializing something of that
type.  I'm not disabling the warning when the function itself returns
a non-const lvalue reference, that would regress at least

  const int &r = std::any_cast<int&>(std::any());

in Wdangling-reference2.C where the any_cast returns an int&.

Unfortunately, this patch means we'll stop diagnosing

  int& fn(int&& x) { return static_cast<int&>(x); }
  void test ()
  {
    int &r = fn(4);
  }

where there's a genuine dangling reference.  OTOH, the patch
should suppress false positives with iterators, like:

  auto &candidate = *candidates.begin ();

and arguably that's more important than detecting some relatively
obscure cases.  It's probably not worth it making the warning more
complicated by, for instance, not warning when a fn returns 'int&'
but takes 'const int&' (because then it can't return its argument).

gcc/cp/ChangeLog:

	* call.cc (maybe_warn_dangling_reference): Don't warn when initializing
	a non-const lvalue reference.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp23/elision4.C: Remove dg-warning.
	* g++.dg/warn/Wdangling-reference1.C: Turn dg-warning into dg-bogus.
	* g++.dg/warn/Wdangling-reference7.C: New test.

gcc/cp/call.cc

@@ -13679,8 +13679,14 @@ maybe_warn_dangling_reference (const_tree decl, tree init)
 {
   if (!warn_dangling_reference)
     return;
-  if (!(TYPE_REF_OBJ_P (TREE_TYPE (decl))
-	|| std_pair_ref_ref_p (TREE_TYPE (decl))))
+  tree type = TREE_TYPE (decl);
+  /* Only warn if what we're initializing has type T&& or const T&, or
+     std::pair<const T&, const T&>.  (A non-const lvalue reference can't
+     bind to a temporary.)  */
+  if (!((TYPE_REF_OBJ_P (type)
+	 && (TYPE_REF_IS_RVALUE (type)
+	     || CP_TYPE_CONST_P (TREE_TYPE (type))))
+	|| std_pair_ref_ref_p (type)))
     return;
   /* Don't suppress the diagnostic just because the call comes from
      a system header.  If the DECL is not in a system header, or if

gcc/testsuite/g++.dg/cpp23/elision4.C

@@ -34,6 +34,6 @@ T& temporary2(T&& x) { return static_cast<T&>(x); }
 void
 test ()
 {
-  int& r1 = temporary1 (42); // { dg-warning "dangling reference" }
-  int& r2 = temporary2 (42); // { dg-warning "dangling reference" }
+  int& r1 = temporary1 (42);
+  int& r2 = temporary2 (42);
 }

gcc/testsuite/g++.dg/warn/Wdangling-reference1.C

@@ -139,6 +139,6 @@ struct Y {
 // x1 = Y::operator int&& (&TARGET_EXPR <D.2410, {}>)
 int&& x1 = Y(); // { dg-warning "dangling reference" }
 int&& x2 = Y{}; // { dg-warning "dangling reference" }
-int& x3 = Y(); // { dg-warning "dangling reference" }
-int& x4 = Y{}; // { dg-warning "dangling reference" }
+int& x3 = Y(); // { dg-bogus "dangling reference" }
+int& x4 = Y{}; // { dg-bogus "dangling reference" }
 const int& t1 = Y().foo(10); // { dg-warning "dangling reference" }

gcc/testsuite/g++.dg/warn/Wdangling-reference7.C

@@ -0,0 +1,16 @@
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wdangling-reference" }
+
+int& ref(const int&);
+int&& rref(const int&);
+
+void
+g ()
+{
+  const int& r1 = ref (1); // { dg-warning "dangling reference" }
+  int& r2 = ref (2); // { dg-bogus "dangling reference" }
+  auto& r3 = ref (3); // { dg-bogus "dangling reference" }
+  int&& r4 = rref (4); // { dg-warning "dangling reference" }
+  auto&& r5 = rref (5); // { dg-warning "dangling reference" }
+  const int&& r6 = rref (6); // { dg-warning "dangling reference" }
+}