c++: Overeager use of deleted function before ADL [PR68942]

Here, at template definition time, ordinary name lookup for 'foo(t)'
finds only the deleted function, and so we form a CALL_EXPR thereof.
Later at instantiation time, when initially substituting into this
CALL_EXPR with T=N::A, we end up calling mark_used on this deleted
function (since it's the only function in the overload set), triggering
a bogus "use of deleted function error", before we get to augment the
overload set via ADL.

This patch fixes this issue by using the tf_conv flag to disable
mark_used during the initial substitution into the callee of a
CALL_EXPR when KOENIG_P, since at this point we're still figuring out
which functions are candidates.

gcc/cp/ChangeLog:

	PR c++/68942
	* pt.c (tsubst_copy_and_build) <case CALL_EXPR>: When KOENIG_P,
	set tf_conv during the initial substitution into the function.

gcc/testsuite/ChangeLog:

	PR c++/68942
	* g++.dg/template/koenig12.C: New test.

2 files changed, 20 insertions, 1 deletions

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index eaf4665..4e9b40f 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -20232,7 +20232,11 @@ tsubst_copy_and_build (tree t,
 	      /* Avoid error about taking the address of a constructor.  */
 	      function = TREE_OPERAND (function, 0);
 
-	    function = tsubst_copy_and_build (function, args, complain,
+	    /* When KOENIG_P, we don't want to mark_used the callee before
+	       augmenting the overload set via ADL, so during this initial
+	       substitution we disable mark_used by setting tf_conv (68942).  */
+	    function = tsubst_copy_and_build (function, args,
+					      complain | (koenig_p * tf_conv),
 					      in_decl,
 					      !qualified_p,
 					      integral_constant_expression_p);
diff --git a/gcc/testsuite/g++.dg/template/koenig12.C b/gcc/testsuite/g++.dg/template/koenig12.C
new file mode 100644
index 0000000..fd05ef5
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/koenig12.C
@@ -0,0 +1,15 @@
+// PR c++/68942
+// { dg-do compile { target c++11 } }
+
+void foo(...) = delete;
+
+template <class T> void lookup(T t) { foo(t); }
+
+namespace N {
+ struct A { };
+ int foo(A);
+}
+
+int main() {
+  lookup(N::A{});
+}