c++: Avoid calling const copy ctor on implicit move. [PR91212]

Our implementation of C++11 implicit move was wrong for return; we didn't
actually hit the check for the type of the first parameter of the selected
constructor, because we didn't see LOOKUP_PREFER_RVALUE set properly.

Fixing that to look at the right flags fixed the issue for this testcase,
but broke implicit move for a by-value converting constructor (PR58051).  I
think this was not allowed in C++17, but it is allowed under the implicit
move changes from C++20, and those changes were voted to apply as a DR to
earlier standards as well, so I don't want to break it now.

So after fixing the flags check I changed the test to allow value
parameters.

gcc/cp/ChangeLog:

	PR c++/91212
	* call.c (build_over_call): Don't call a const ref
	overload for implicit move.

gcc/testsuite/ChangeLog:

	PR c++/91212
	* g++.dg/cpp0x/move-return3.C: New test.

gcc/cp/call.c

@@ -8678,15 +8678,18 @@ build_over_call (struct z_candidate *cand, int flags, tsubst_flags_t complain)
 	  parm = TREE_CHAIN (parm);
 	}
 
-      if (flags & LOOKUP_PREFER_RVALUE)
+      if (cand->flags & LOOKUP_PREFER_RVALUE)
 	{
 	  /* The implicit move specified in 15.8.3/3 fails "...if the type of
 	     the first parameter of the selected constructor is not an rvalue
 	     reference to the object's type (possibly cv-qualified)...." */
 	  gcc_assert (!(complain & tf_error));
 	  tree ptype = convs[0]->type;
-	  if (!TYPE_REF_P (ptype)
-	      || !TYPE_REF_IS_RVALUE (ptype)
+	  /* Allow calling a by-value converting constructor even though it
+	     isn't permitted by the above, because we've allowed it since GCC 5
+	     (PR58051) and it's allowed in C++20.  But don't call a copy
+	     constructor.  */
+	  if ((TYPE_REF_P (ptype) && !TYPE_REF_IS_RVALUE (ptype))
 	      || CONVERSION_RANK (convs[0]) > cr_exact)
 	    return error_mark_node;
 	}

gcc/testsuite/g++.dg/cpp0x/move-return3.C

@@ -0,0 +1,23 @@
+// PR c++/91212
+// Test that C++11 implicit move semantics don't call the const copy.
+// { dg-do link }
+
+struct T { int i; };
+
+struct X {
+  X(T&) { }    // #1
+  X(const T&); // #2
+};
+
+X
+fn ()
+{
+  T buf;
+  return buf;
+}
+
+int
+main()
+{
+  X c = fn ();
+}