Re: [PATCH] c++: ttp matching with constrained auto parm [PR99909]

[Jason Merrill via Gcc-patches]

On 9/28/21 15:15, Patrick Palka wrote:

Here, when unifying TT with S, processing_template_decl is unset, and
this foils the dependence checks in do_auto_deduction for avoiding
checking constraints on an auto when the initializer is dependent.

This patch fixes this issue by making sure processing_template_decl is
set during the call to unify from coerce_template_template_parms; this
seems sensible because we're unifying one set of template parameters
with another, so we're dealing with templated trees throughout.

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

OK.

        PR c++/99909

gcc/cp/ChangeLog:

        * pt.c (coerce_template_template_parms): Keep
        processing_template_decl set during the call to unify as well.

gcc/testsuite/ChangeLog:

        * g++.dg/cpp2a/concepts-ttp3.C: New test.
---
  gcc/cp/pt.c                                |  4 ++--
  gcc/testsuite/g++.dg/cpp2a/concepts-ttp3.C | 11 +++++++++++
  2 files changed, 13 insertions(+), 2 deletions(-)
  create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-ttp3.C

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 41fa7ed5e43..1dcdffe322a 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -7994,12 +7994,12 @@ coerce_template_template_parms (tree parm_parms,
        /* So coerce P's args to apply to A's parms, and then deduce between A's
         args and the converted args.  If that succeeds, A is at least as
         specialized as P, so they match.*/
+      processing_template_decl_sentinel ptds (/*reset*/false);
+      ++processing_template_decl;
        tree pargs = template_parms_level_to_args (parm_parms);
        pargs = add_outermost_template_args (outer_args, pargs);
-      ++processing_template_decl;
        pargs = coerce_template_parms (arg_parms, pargs, NULL_TREE, tf_none,
                                     /*require_all*/true, /*use_default*/true);
-      --processing_template_decl;
        if (pargs != error_mark_node)
        {
          tree targs = make_tree_vec (nargs);
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-ttp3.C 
b/gcc/testsuite/g++.dg/cpp2a/concepts-ttp3.C
new file mode 100644
index 00000000000..898524e0dfa
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-ttp3.C
@@ -0,0 +1,11 @@
+// PR c++/99909
+// { dg-do compile { target c++20 } }
+
+template<class T> constexpr bool always_true = true;
+template<class T> concept C = always_true<T>;
+
+template<C auto> struct S { };
+
+template<template<auto> class TT> void f() { }
+
+template void f<S>();