c++: lvalueness of non-dependent assignment expr [PR114994]

r14-4111-g6e92a6a2a72d3b made us check non-dependent simple assignment
expressions ahead of time and give them a type, as was already done for
compound assignments.  Unlike for compound assignments however, if a
simple assignment resolves to an operator overload we represent it as a
(typed) MODOP_EXPR instead of a CALL_EXPR to the selected overload.
(I reckoned this was at worst a pessimization -- we'll just have to repeat
overload resolution at instantiatiation time.)

But this turns out to break the below testcase ultimately because
MODOP_EXPR (of non-reference type) is always treated as an lvalue
according to lvalue_kind, which is incorrect for the MODOP_EXPR
representing x=42.

We can fix this by representing such class assignment expressions as
CALL_EXPRs as well, but this turns out to require some tweaking of our
-Wparentheses warning logic and may introduce other fallout making it
unsuitable for backporting.

So this patch instead fixes lvalue_kind to consider the type of a
MODOP_EXPR representing a class assignment.

	PR c++/114994

gcc/cp/ChangeLog:

	* tree.cc (lvalue_kind) <case MODOP_EXPR>: For a class
	assignment, consider the result type.

gcc/testsuite/ChangeLog:

	* g++.dg/template/non-dependent32.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>

2 files changed, 22 insertions, 1 deletions

diff --git a/gcc/cp/tree.cc b/gcc/cp/tree.cc
index f1a23ff..9d37d25 100644
--- a/gcc/cp/tree.cc
+++ b/gcc/cp/tree.cc
@@ -275,7 +275,10 @@ lvalue_kind (const_tree ref)
       /* We expect to see unlowered MODOP_EXPRs only during
 	 template processing.  */
       gcc_assert (processing_template_decl);
-      return clk_ordinary;
+      if (CLASS_TYPE_P (TREE_TYPE (TREE_OPERAND (ref, 0))))
+	goto default_;
+      else
+	return clk_ordinary;
 
     case MODIFY_EXPR:
     case TYPEID_EXPR:
diff --git a/gcc/testsuite/g++.dg/template/non-dependent32.C b/gcc/testsuite/g++.dg/template/non-dependent32.C
new file mode 100644
index 0000000..54252c7
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/non-dependent32.C
@@ -0,0 +1,18 @@
+// PR c++/114994
+// { dg-do compile { target c++11 } }
+
+struct udl_arg {
+  udl_arg operator=(int);
+};
+
+void f(udl_arg&&);
+
+template<class>
+void g() {
+  udl_arg x;
+  f(x=42); // { dg-bogus "cannot bind" }
+}
+
+int main() {
+  g<int>();
+}