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

[Jason Merrill <jason@redhat.com>]

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>();
>