c++: UNBOUND_CLASS_TEMPLATE context substitution [PR119981]

In r15-123 and r14-11434 we unconditionally set processing_template_decl
when substituting the context of an UNBOUND_CLASS_TEMPLATE, in order to
handle instantiation of the dependently scoped friend declaration

  template<int N>
  template<class T>
  friend class A<N>::B;

where the scope A<N> remains dependent after instantiation.  But this
turns out to misbehave for the UNBOUND_CLASS_TEMPLATE in the below
testcase representing

  g<[]{}>::template fn

since with the flag set substituting the args of test3 into the lambda
causes us to defer the substitution and yield a lambda that still looks
dependent, which in turn makes g<[]{}> still dependent and not suitable
for qualified name lookup.

This patch restricts setting processing_template_decl during
UNBOUND_CLASS_TEMPLATE substitution to the case where there are multiple
levels of introduced template parameters, as in the friend declaration.
(This means we need to substitute the template parameter list(s) first,
which makes sense since they lexically appear first.)

	PR c++/119981
	PR c++/119378

gcc/cp/ChangeLog:

	* pt.cc (tsubst) <case UNBOUND_CLASS_TEMPLATE>: Substitute
	into template parameter list first.  When substituting the
	context, only set processing_template_decl if there's more
	than one level of introduced template parameters.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/lambda-targ15.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index e8d342f99f6dbc43291a9c6217e964a906b254d9..26ed9de430c0e7dd230206652462828577157f0a 100644 (file)
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -17181,18 +17181,24 @@ tsubst (tree t, tree args, tsubst_flags_t complain, tree in_decl)
 
     case UNBOUND_CLASS_TEMPLATE:
       {
-       ++processing_template_decl;
-       tree ctx = tsubst_entering_scope (TYPE_CONTEXT (t), args,
-                                         complain, in_decl);
-       --processing_template_decl;
        tree name = TYPE_IDENTIFIER (t);
+       if (name == error_mark_node)
+         return error_mark_node;
+
        tree parm_list = DECL_TEMPLATE_PARMS (TYPE_NAME (t));
+       parm_list = tsubst_template_parms (parm_list, args, complain);
+       if (parm_list == error_mark_node)
+         return error_mark_node;
 
-       if (ctx == error_mark_node || name == error_mark_node)
+       if (parm_list && TMPL_PARMS_DEPTH (parm_list) > 1)
+         ++processing_template_decl;
+       tree ctx = tsubst_entering_scope (TYPE_CONTEXT (t), args,
+                                         complain, in_decl);
+       if (parm_list && TMPL_PARMS_DEPTH (parm_list) > 1)
+         --processing_template_decl;
+       if (ctx == error_mark_node)
          return error_mark_node;
 
-       if (parm_list)
-         parm_list = tsubst_template_parms (parm_list, args, complain);
        return make_unbound_class_template (ctx, name, parm_list, complain);
       }
 

diff --git a/gcc/testsuite/g++.dg/cpp2a/lambda-targ15.C b/gcc/testsuite/g++.dg/cpp2a/lambda-targ15.C
new file mode 100644 (file)
index 0000000..90160a5
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/lambda-targ15.C
@@ -0,0 +1,17 @@
+// PR c++/119981
+// { dg-do compile { target c++20 } }
+
+template<template<class> class P>
+struct mp_copy_if{};
+
+template<auto Fn>
+struct g {
+  template<class> struct fn{};
+};
+
+template<typename>
+void test3() {
+  mp_copy_if<g<[]{}>::template fn> b;
+}
+
+template void test3<int>();