PR c++/80452 - Core 1579, implicit move semantics on return/throw

	* cp-tree.h (LOOKUP_PREFER_RVALUE): Now means that we've already
	tentatively changed the lvalue to an rvalue.
	* call.c (reference_binding): Remove LOOKUP_PREFER_RVALUE handling.
	(build_over_call): If LOOKUP_PREFER_RVALUE, check that the first
	parameter is an rvalue reference.
	* except.c (build_throw): Do maybe-rvalue overload resolution twice.
	* typeck.c (check_return_expr): Likewise.

From-SVN: r251035

gcc/cp/ChangeLog

+2017-08-10  Jason Merrill  <jason@redhat.com>
+
+	PR c++/80452 - Core 1579, implicit move semantics on return/throw
+	* cp-tree.h (LOOKUP_PREFER_RVALUE): Now means that we've already
+	tentatively changed the lvalue to an rvalue.
+	* call.c (reference_binding): Remove LOOKUP_PREFER_RVALUE handling.
+	(build_over_call): If LOOKUP_PREFER_RVALUE, check that the first
+	parameter is an rvalue reference.
+	* except.c (build_throw): Do maybe-rvalue overload resolution twice.
+	* typeck.c (check_return_expr): Likewise.
+
 2017-08-10  David Malcolm  <dmalcolm@redhat.com>
 
 	* parser.c (cp_parser_error): Update for new param to

gcc/cp/call.c

@@ -101,7 +101,7 @@ struct conversion {
   /* If KIND is ck_ref_bind, true when either an lvalue reference is
      being bound to an lvalue expression or an rvalue reference is
      being bound to an rvalue expression.  If KIND is ck_rvalue,
-     true when we should treat an lvalue as an rvalue (12.8p33).  If
+     true when we are treating an lvalue as an rvalue (12.8p33).  If
      KIND is ck_base, always false.  */
   BOOL_BITFIELD rvaluedness_matches_p: 1;
   BOOL_BITFIELD check_narrowing: 1;
@@ -1161,6 +1161,7 @@ standard_conversion (tree to, tree from, tree expr, bool c_cast_p,
 	}
       conv = build_conv (ck_rvalue, from, conv);
       if (flags & LOOKUP_PREFER_RVALUE)
+	/* Tell convert_like_real to set LOOKUP_PREFER_RVALUE.  */
 	conv->rvaluedness_matches_p = true;
     }
 
@@ -1629,11 +1630,7 @@ reference_binding (tree rto, tree rfrom, tree expr, bool c_cast_p, int flags,
       conv = build_identity_conv (tfrom, expr);
       conv = direct_reference_binding (rto, conv);
 
-      if (flags & LOOKUP_PREFER_RVALUE)
-	/* The top-level caller requested that we pretend that the lvalue
-	   be treated as an rvalue.  */
-	conv->rvaluedness_matches_p = TYPE_REF_IS_RVALUE (rto);
-      else if (TREE_CODE (rfrom) == REFERENCE_TYPE)
+      if (TREE_CODE (rfrom) == REFERENCE_TYPE)
 	/* Handle rvalue reference to function properly.  */
 	conv->rvaluedness_matches_p
 	  = (TYPE_REF_IS_RVALUE (rto) == TYPE_REF_IS_RVALUE (rfrom));
@@ -1659,8 +1656,7 @@ reference_binding (tree rto, tree rfrom, tree expr, bool c_cast_p, int flags,
       /* Don't allow binding of lvalues (other than function lvalues) to
 	 rvalue references.  */
       if (is_lvalue && TYPE_REF_IS_RVALUE (rto)
-	  && TREE_CODE (to) != FUNCTION_TYPE
-          && !(flags & LOOKUP_PREFER_RVALUE))
+	  && TREE_CODE (to) != FUNCTION_TYPE)
 	conv->bad_p = true;
 
       /* Nor the reverse.  */
@@ -6917,6 +6913,7 @@ convert_like_real (conversion *convs, tree expr, tree fn, int argnum,
       else
 	flags |= LOOKUP_ONLYCONVERTING;
       if (convs->rvaluedness_matches_p)
+	/* standard_conversion got LOOKUP_PREFER_RVALUE.  */
 	flags |= LOOKUP_PREFER_RVALUE;
       if (TREE_CODE (expr) == TARGET_EXPR
 	  && TARGET_EXPR_LIST_INIT_P (expr))
@@ -7716,6 +7713,19 @@ build_over_call (struct z_candidate *cand, int flags, tsubst_flags_t complain)
 	  ++arg_index;
 	  parm = TREE_CHAIN (parm);
 	}
+
+      if (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 (TREE_CODE (ptype) != REFERENCE_TYPE
+	      || !TYPE_REF_IS_RVALUE (ptype)
+	      || CONVERSION_RANK (convs[0]) > cr_exact)
+	    return error_mark_node;
+	}
     }
   /* Bypass access control for 'this' parameter.  */
   else if (TREE_CODE (TREE_TYPE (fn)) == METHOD_TYPE)

gcc/cp/cp-tree.h

@@ -5296,7 +5296,7 @@ enum overload_flags { NO_SPECIAL = 0, DTOR_FLAG, TYPENAME_FLAG };
    (Normally, these entities are registered in the symbol table, but
    not found by lookup.)  */
 #define LOOKUP_HIDDEN (LOOKUP_PREFER_NAMESPACES << 1)
-/* Prefer that the lvalue be treated as an rvalue.  */
+/* We're trying to treat an lvalue as an rvalue.  */
 #define LOOKUP_PREFER_RVALUE (LOOKUP_HIDDEN << 1)
 /* We're inside an init-list, so narrowing conversions are ill-formed.  */
 #define LOOKUP_NO_NARROWING (LOOKUP_PREFER_RVALUE << 1)

gcc/cp/except.c

@@ -665,6 +665,7 @@ build_throw (tree exp)
 	{
 	  int flags = LOOKUP_NORMAL | LOOKUP_ONLYCONVERTING;
 	  vec<tree, va_gc> *exp_vec;
+	  bool converted = false;
 
 	  /* Under C++0x [12.8/16 class.copy], a thrown lvalue is sometimes
 	     treated as an rvalue for the purposes of overload resolution
@@ -675,14 +676,31 @@ build_throw (tree exp)
 	      && ! TREE_STATIC (exp)
 	      /* The variable must not have the `volatile' qualifier.  */
 	      && !(cp_type_quals (TREE_TYPE (exp)) & TYPE_QUAL_VOLATILE))
-	    flags = flags | LOOKUP_PREFER_RVALUE;
+	    {
+	      tree moved = move (exp);
+	      exp_vec = make_tree_vector_single (moved);
+	      moved = (build_special_member_call
+		       (object, complete_ctor_identifier, &exp_vec,
+			TREE_TYPE (object), flags|LOOKUP_PREFER_RVALUE,
+			tf_none));
+	      release_tree_vector (exp_vec);
+	      if (moved != error_mark_node)
+		{
+		  exp = moved;
+		  converted = true;
+		}
+	    }
 
 	  /* Call the copy constructor.  */
-	  exp_vec = make_tree_vector_single (exp);
-	  exp = (build_special_member_call
-		 (object, complete_ctor_identifier, &exp_vec,
-		  TREE_TYPE (object), flags, tf_warning_or_error));
-	  release_tree_vector (exp_vec);
+	  if (!converted)
+	    {
+	      exp_vec = make_tree_vector_single (exp);
+	      exp = (build_special_member_call
+		     (object, complete_ctor_identifier, &exp_vec,
+		      TREE_TYPE (object), flags, tf_warning_or_error));
+	      release_tree_vector (exp_vec);
+	    }
+
 	  if (exp == error_mark_node)
 	    {
 	      error ("  in thrown expression");

gcc/cp/typeck.c

@@ -9156,6 +9156,7 @@ check_return_expr (tree retval, bool *no_warning)
 
          Note that these conditions are similar to, but not as strict as,
 	 the conditions for the named return value optimization.  */
+      bool converted = false;
       if ((cxx_dialect != cxx98)
           && ((VAR_P (retval) && !DECL_HAS_VALUE_EXPR_P (retval))
 	      || TREE_CODE (retval) == PARM_DECL)
@@ -9163,14 +9164,25 @@ check_return_expr (tree retval, bool *no_warning)
 	  && !TREE_STATIC (retval)
 	  /* This is only interesting for class type.  */
 	  && CLASS_TYPE_P (functype))
-	flags = flags | LOOKUP_PREFER_RVALUE;
+	{
+	  tree moved = move (retval);
+	  moved = convert_for_initialization
+	    (NULL_TREE, functype, moved, flags|LOOKUP_PREFER_RVALUE,
+	     ICR_RETURN, NULL_TREE, 0, tf_none);
+	  if (moved != error_mark_node)
+	    {
+	      retval = moved;
+	      converted = true;
+	    }
+	}
 
       /* First convert the value to the function's return type, then
 	 to the type of return value's location to handle the
 	 case that functype is smaller than the valtype.  */
-      retval = convert_for_initialization
-	(NULL_TREE, functype, retval, flags, ICR_RETURN, NULL_TREE, 0,
-         tf_warning_or_error);
+      if (!converted)
+	retval = convert_for_initialization
+	  (NULL_TREE, functype, retval, flags, ICR_RETURN, NULL_TREE, 0,
+	   tf_warning_or_error);
       retval = convert (valtype, retval);
 
       /* If the conversion failed, treat this just like `return;'.  */

gcc/testsuite/g++.dg/cpp0x/move-return1.C

+// PR c++/80452
+// { dg-do compile { target c++11 } }
+
+template<typename> struct check { };
+template<typename T> struct check<T&>;
+
+struct A {
+  A() = default;
+  A(A&&) = default;
+  A(const A&) = delete;
+};
+
+template <class T>
+struct B {
+  template <class U> B(U&&) { check<U> u; }
+};
+
+B<A> f()
+{
+  A a;
+  return a;
+}