c++: Failure to lookup using-decl name [PR98231]

In r11-4690 we removed the call to finish_nonmember_using_decl in
tsubst_expr/DECL_EXPR in the USING_DECL block.  This was done not
to perform name lookup twice for a non-dependent using-decl, which
sounds sensible.

However, finish_nonmember_using_decl also pushes the decl's bindings
which we still have to do so that we can find the USING_DECL's name
later.  In this case, we've got a USING_DECL N::operator<<  that we are
tsubstituting.  We already looked it up while parsing the template
"foo", and lookup_using_decl stashed the OVERLOAD it found into
USING_DECL_DECLS.  Now we just have to update the IDENTIFIER_BINDING of
the identifier for operator<< with the overload the name is bound to.

I didn't want to export push_local_binding so I've introduced a new
wrapper.

gcc/cp/ChangeLog:

	PR c++/98231
	* name-lookup.c (push_using_decl_bindings): New.
	* name-lookup.h (push_using_decl_bindings): Declare.
	* pt.c (tsubst_expr): Call push_using_decl_bindings.

gcc/testsuite/ChangeLog:

	PR c++/98231
	* g++.dg/lookup/using63.C: New test.

gcc/cp/name-lookup.c

@@ -9279,4 +9279,14 @@ push_operator_bindings ()
 	}
 }
 
+/* Wrapper around push_local_binding to push the bindings for
+   a non-member USING_DECL DECL that was found during template parsing.  */
+
+void
+push_using_decl_bindings (tree decl)
+{
+  push_local_binding (DECL_NAME (decl), USING_DECL_DECLS (decl),
+		      /*using*/true);
+}
+
 #include "gt-cp-name-lookup.h"

gcc/cp/name-lookup.h

@@ -478,6 +478,7 @@ extern void push_to_top_level (void);
 extern void pop_from_top_level (void);
 extern void maybe_save_operator_binding (tree);
 extern void push_operator_bindings (void);
+extern void push_using_decl_bindings (tree);
 extern void discard_operator_bindings (tree);
 
 /* Lower level interface for modules. */

gcc/cp/pt.c

@@ -18133,6 +18133,9 @@ tsubst_expr (tree t, tree args, tsubst_flags_t complain, tree in_decl,
 	    tree scope = USING_DECL_SCOPE (decl);
 	    gcc_checking_assert (scope
 				 == tsubst (scope, args, complain, in_decl));
+	    /* We still need to push the bindings so that we can look up
+	       this name later.  */
+	    push_using_decl_bindings (decl);
 	  }
 	else if (is_capture_proxy (decl)
 		 && !DECL_TEMPLATE_INSTANTIATION (current_function_decl))

gcc/testsuite/g++.dg/lookup/using63.C

@@ -0,0 +1,17 @@
+// PR c++/98231
+// { dg-do compile }
+
+template <typename, typename = int> struct basic_ostream {};
+namespace N {
+  template <typename Char, typename CharTraits, typename T>
+  void operator<<(basic_ostream<Char, CharTraits>, T);
+}
+basic_ostream<char> os;
+
+template<typename T> void
+foo (T value)
+{
+  using N::operator<<;
+  os << value;
+}
+void bar() { foo (1); }