c++: Dependent conversion operator in concept [PR94597]

When building the parameter mapping for an atomic constraint,
find_template_parameters does not spot the template parameter within the
conversion-type-id of a dependent conversion operator, which later leads to an
ICE during substitution when looking up the missing template argument for this
unnoticed template parameter.

gcc/cp/ChangeLog:

	PR c++/94597
	* pt.c (any_template_parm_r) <case IDENTIFIER_NODE>: New case.  If this
	is a conversion operator, visit its TREE_TYPE.

gcc/testsuite/ChangeLog:

	PR c++/94597
	* g++.dg/cpp2a/concepts-conv2.C: New test.

gcc/cp/ChangeLog

+2020-04-21  Patrick Palka  <ppalka@redhat.com>
+
+	PR c++/94597
+	* pt.c (any_template_parm_r) <case IDENTIFIER_NODE>: New case.  If this
+	is a conversion operator, visit its TREE_TYPE.
+
 2020-04-21  Nathan Sidwell  <nathan@acm.org>
 
 	* pt.c (tsubst_copy_and_build) [POINTER_PLUS_EXPR]: Check for

gcc/cp/pt.c

@@ -10529,6 +10529,12 @@ any_template_parm_r (tree t, void *data)
       }
       break;
 
+    case IDENTIFIER_NODE:
+      if (IDENTIFIER_CONV_OP_P (t))
+	/* The conversion-type-id of a conversion operator may be dependent.  */
+	WALK_SUBTREE (TREE_TYPE (t));
+      break;
+
     default:
       break;
     }

gcc/testsuite/ChangeLog

+2020-04-21  Patrick Palka  <ppalka@redhat.com>
+
+	PR c++/94597
+	* g++.dg/cpp2a/concepts-conv2.C: New test.
+
 2020-04-21  Duan bo  <duanbo3@huawei.com>
 
 	PR target/94577

gcc/testsuite/g++.dg/cpp2a/concepts-conv2.C

+// PR c++/94597
+// { dg-do compile { target c++2a } }
+
+template <typename b, typename c> concept d = requires(b e) { e.operator c(); };
+
+template <typename f, typename g> requires(d<f, g>) bool equal(f, g);
+
+template <typename h> struct i {
+  i(h);
+  operator h();
+};
+
+static_assert( d<i<float>, float>);
+static_assert(!d<i<float>, int>);
+
+bool fun() {
+  i a(2.0f);
+  return equal(a, 3.0f);
+}