From 90de06a7b3ce6ae8381136e58a2dde91fbbb6eff Mon Sep 17 00:00:00 2001
From: Patrick Palka <ppalka@redhat.com>
Date: Thu, 18 Nov 2021 10:05:13 -0500
Subject: [PATCH] c++: template-id ADL and partial instantiation [PR99911]

Here when partially instantiating the call get<U>(T{}) with T=N::A
(for which earlier unqualified name lookup for 'get' found nothing)
the arguments after substitution are no longer dependent but the callee
still is, so perform_koenig_lookup postpones ADL.  But then we go on to
diagnose the unresolved template name anyway, as if ADL was already
performed and failed.

This patch fixes this by avoiding the error path in question when the
template arguments of an unresolved template-id are still dependent,
mirroring the dependence check in perform_koenig_lookup.

	PR c++/99911

gcc/cp/ChangeLog:

	* pt.c (tsubst_copy_and_build) <case CALL_EXPR>: Don't diagnose
	name lookup failure if the arguments to an unresolved template
	name are still dependent.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/fn-template24.C: New test.
---
 gcc/cp/pt.c                                |  4 +++-
 gcc/testsuite/g++.dg/cpp2a/fn-template24.C | 16 ++++++++++++++++
 2 files changed, 19 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/fn-template24.C

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 2e31663c277..ad51c07347b 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -20434,7 +20434,9 @@ tsubst_copy_and_build (tree t,
 	if (function != NULL_TREE
 	    && (identifier_p (function)
 		|| (TREE_CODE (function) == TEMPLATE_ID_EXPR
-		    && identifier_p (TREE_OPERAND (function, 0))))
+		    && identifier_p (TREE_OPERAND (function, 0))
+		    && !any_dependent_template_arguments_p (TREE_OPERAND
+							    (function, 1))))
 	    && !any_type_dependent_arguments_p (call_args))
 	  {
 	    if (TREE_CODE (function) == TEMPLATE_ID_EXPR)
diff --git a/gcc/testsuite/g++.dg/cpp2a/fn-template24.C b/gcc/testsuite/g++.dg/cpp2a/fn-template24.C
new file mode 100644
index 00000000000..b444ac6a273
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/fn-template24.C
@@ -0,0 +1,16 @@
+// PR c++/99911
+// { dg-do compile { target c++20 } }
+
+namespace N {
+  struct A { };
+  template<class T> void get(A);
+};
+
+template<class T>
+auto f() {
+  return []<class U>(U) { get<U>(T{}); };
+}
+
+int main() {
+  f<N::A>()(0);
+}