c++: mark_used and ADL with template-id [PR100344]

My r11-295 patch for PR68942 didn't consider that the callee of an
ADL-eligible function call can be a TEMPLATE_ID_EXPR, and we don't want
to disable mark_used when substituting into the template arguments of
this TEMPLATE_ID_EXPR because the arguments are clearly used regardless
of the outcome of ADL.  In the first testcase below, this oversight
causes us to trip over the assert in build_call_a for the call to
find_index<int> because the function no longer had its TREE_USED bit set
from mark_used.

So this patch restricts the original fix to disable mark_used only when
the callee is a FUNCTION_DECL, which seems to be the only case that
matters for PR68942.  For instance, in the second testcase below we
already don't mark_used the deleted function specialization even before
r11-295.

gcc/cp/ChangeLog:

	PR c++/68942
	PR c++/100344
	* pt.c (tsubst_copy_and_build) <case CALL_EXPR>: Set tf_conv
	only when the callee is a FUNCTION_DECL.

gcc/testsuite/ChangeLog:

	PR c++/68942
	PR c++/100344
	* g++.dg/template/call8.C: New test.
	* g++.dg/template/koenig12a.C: New test.

3 files changed, 37 insertions, 5 deletions

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 116bdd2..36a8cb5 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -20227,11 +20227,13 @@ tsubst_copy_and_build (tree t,
 	      /* Avoid error about taking the address of a constructor.  */
 	      function = TREE_OPERAND (function, 0);
 
-	    /* 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),
+	    tsubst_flags_t subcomplain = complain;
+	    if (koenig_p && TREE_CODE (function) == FUNCTION_DECL)
+	      /* 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).  */
+	      subcomplain |= tf_conv;
+	    function = tsubst_copy_and_build (function, args, subcomplain,
 					      in_decl,
 					      !qualified_p,
 					      integral_constant_expression_p);
diff --git a/gcc/testsuite/g++.dg/template/call8.C b/gcc/testsuite/g++.dg/template/call8.C
new file mode 100644
index 0000000..04609dd
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/call8.C
@@ -0,0 +1,14 @@
+// PR c++/100344
+// { dg-do compile { target c++11 } }
+
+template <class> constexpr int find_index() { return 1; }
+
+template <int, class T> void foo(T);
+
+template <class T> void get(T v) {
+  foo<find_index<T>()>(v);
+}
+
+int main() {
+  get(0);
+}
diff --git a/gcc/testsuite/g++.dg/template/koenig12a.C b/gcc/testsuite/g++.dg/template/koenig12a.C
new file mode 100644
index 0000000..96b98b9
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/koenig12a.C
@@ -0,0 +1,16 @@
+// PR c++/68942
+// { dg-do compile { target c++11 } }
+// A template-id analogue of koenig12.C.
+
+template <int> void foo(...) = delete;
+
+template <class T> void lookup(T t) { foo<0>(t); }
+
+namespace N {
+ struct A { };
+ template <int> int foo(A);
+}
+
+int main() {
+  lookup(N::A{});
+}