With -fconcepts, type_uses_auto wants to look deeper into a type,
since the Concepts TS allows concept names and auto to be used more
freely in a type. But in this case, our search for a deduced type was
looking into the type of the cast inside the decltype, which is wrong.
It turned out that for_each_template_parm_r didn't handle
DECLTYPE_TYPE at all, and then cp_walk_subtrees walked into its
operand. Fixed by clearing *walk_subtrees whether or not we
explicitly walk the operand. We also don't want to look at
non-deduced contexts when considering whether a type needs deducing.
Tested x86_64-pc-linux-gnu, applying to trunk and 8.
commit 716725435749074db5f5e7ef70b8950331e8315d
Author: Jason Merrill <ja...@redhat.com>
Date: Tue May 8 17:39:10 2018 -0400
PR c++/85706 - class deduction under decltype
* pt.c (for_each_template_parm_r): Handle DECLTYPE_TYPE. Clear
*walk_subtrees whether or not we walked into the operand.
(type_uses_auto): Only look at deduced contexts.
diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index c604f46f742..180dfd6861c 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -9829,6 +9829,7 @@ for_each_template_parm_r (tree *tp, int *walk_subtrees, void *d)
break;
case TYPEOF_TYPE:
+ case DECLTYPE_TYPE:
case UNDERLYING_TYPE:
if (pfd->include_nondeduced_p
&& for_each_template_parm (TYPE_VALUES_RAW (t), fn, data,
@@ -9836,6 +9837,7 @@ for_each_template_parm_r (tree *tp, int *walk_subtrees, void *d)
pfd->include_nondeduced_p,
pfd->any_fn))
return error_mark_node;
+ *walk_subtrees = false;
break;
case FUNCTION_DECL:
@@ -26862,7 +26864,7 @@ type_uses_auto (tree type)
them. */
if (uses_template_parms (type))
return for_each_template_parm (type, is_auto_r, /*data*/NULL,
- /*visited*/NULL, /*nondeduced*/true);
+ /*visited*/NULL, /*nondeduced*/false);
else
return NULL_TREE;
}
diff --git a/gcc/testsuite/g++.dg/concepts/class-deduction2.C b/gcc/testsuite/g++.dg/concepts/class-deduction2.C
new file mode 100644
index 00000000000..286e59a5039
--- /dev/null
+++ b/gcc/testsuite/g++.dg/concepts/class-deduction2.C
@@ -0,0 +1,9 @@
+// PR c++/85706
+// { dg-additional-options "-std=c++17 -fconcepts" }
+
+template<class T> struct S {
+ S(T);
+};
+
+template<class = void>
+auto f() -> decltype(S(42)); // error
