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

[Patrick Palka via Gcc-patches]

Here, at template definition time, ordinary name lookup for 'foo(t)'
finds 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 before
we augment the overload set via ADL and select the right function.

This patch fixes this issue by using tf_conv in order to disable
mark_used during the initial substitution of the function of a CALL_EXPR
when KOENIG_P.

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk?

gcc/cp/ChangeLog:

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

gcc/testsuite/ChangeLog:

        PR c++/68942
        * g++.dg/template/koenig12.C: New test.
---
 gcc/cp/pt.c                              |  6 +++++-
 gcc/testsuite/g++.dg/template/koenig12.C | 15 +++++++++++++++
 2 files changed, 20 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/template/koenig12.C

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index d79fecd4949..bed9a22193a 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -20238,7 +20238,11 @@ 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 a function until
+              after performing ADL, during this substitution we disable
+              mark_used by adding tf_conv to complain (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 00000000000..fd05ef5719e
--- /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{});
+}
-- 
2.31.1.362.g311531c9de