c++: Fix template arguments comparison with class NTTP [PR91754]

Here we fail to compile the attached test, stating that the use of
T<s> in T<s>::T() {} is "invalid use of incomplete type".  It is a
function definition so grokdeclarator checks that the qualifying type
is complete.

When we parsed the class T, finish_struct gave the class a non-null
TYPE_SIZE, making it COMPLETE_TYPE_P.  But then we're parsing T<s>,
a TEMPLATE_ID, in

  T<s>::T() {}

so try to lookup_template_class T.  This failed because we couldn't
find such a class: comp_template_args told us that the argument lists
don't match, because one of the args was wrapped in a VIEW_CONVERT_EXPR
to make it look const.  It seems to me that we should see through
these artificial wrappers and consider the args same.

2020-01-29  Marek Polacek  <polacek@redhat.com>

	PR c++/91754 - Fix template arguments comparison with class NTTP.
	* pt.c (class_nttp_const_wrapper_p): New.
	(template_args_equal): See through class_nttp_const_wrapper_p
	arguments.

	* g++.dg/cpp2a/nontype-class30.C: New test.

4 files changed, 45 insertions, 0 deletions

diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index 10ef419..3029714 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,5 +1,12 @@
 2020-01-29  Marek Polacek  <polacek@redhat.com>
 
+	PR c++/91754 - Fix template arguments comparison with class NTTP.
+	* pt.c (class_nttp_const_wrapper_p): New.
+	(template_args_equal): See through class_nttp_const_wrapper_p
+	arguments.
+
+2020-01-29  Marek Polacek  <polacek@redhat.com>
+
 	PR c++/92948 - Fix class NTTP with template arguments.
 	* pt.c (convert_nontype_argument): Use IMPLICIT_CONV_EXPR when
 	converting a value-dependent expression to a class type.
diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index fccf4e9..416ff63 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -8989,6 +8989,19 @@ coerce_innermost_template_parms (tree parms,
   return coerced_args;
 }
 
+/* Returns true if T is a wrapper to make a C++20 template parameter
+   object const.  */
+
+static bool
+class_nttp_const_wrapper_p (tree t)
+{
+  if (cxx_dialect < cxx2a)
+    return false;
+  return (TREE_CODE (t) == VIEW_CONVERT_EXPR
+	  && CP_TYPE_CONST_P (TREE_TYPE (t))
+	  && TREE_CODE (TREE_OPERAND (t, 0)) == TEMPLATE_PARM_INDEX);
+}
+
 /* Returns 1 if template args OT and NT are equivalent.  */
 
 int
@@ -9001,6 +9014,11 @@ template_args_equal (tree ot, tree nt, bool partial_order /* = false */)
   if (nt == any_targ_node || ot == any_targ_node)
     return true;
 
+  if (class_nttp_const_wrapper_p (nt))
+    nt = TREE_OPERAND (nt, 0);
+  if (class_nttp_const_wrapper_p (ot))
+    ot = TREE_OPERAND (ot, 0);
+
   if (TREE_CODE (nt) == TREE_VEC)
     /* For member templates */
     return TREE_CODE (ot) == TREE_VEC && comp_template_args (ot, nt);
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index a8db2cb..e38c4da 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,5 +1,10 @@
 2020-01-29  Marek Polacek  <polacek@redhat.com>
 
+	PR c++/91754 - Fix template arguments comparison with class NTTP.
+	* g++.dg/cpp2a/nontype-class30.C: New test.
+
+2020-01-29  Marek Polacek  <polacek@redhat.com>
+
 	PR c++/92948 - Fix class NTTP with template arguments.
 	* g++.dg/cpp2a/nontype-class28.C: New test.
 	* g++.dg/cpp2a/nontype-class29.C: New test.
diff --git a/gcc/testsuite/g++.dg/cpp2a/nontype-class30.C b/gcc/testsuite/g++.dg/cpp2a/nontype-class30.C
new file mode 100644
index 0000000..b2e174c
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/nontype-class30.C
@@ -0,0 +1,15 @@
+// PR c++/91754 - Fix template arguments comparison with class NTTP.
+// { dg-do compile { target c++2a } }
+
+struct S {};
+
+template<S s>
+struct T {
+  T();
+};
+
+template<S s>
+T<s>::T() {} 
+
+S s;
+T<s> t;