Re: [PATCH] c++: outer args for level-lowered ttp [PR109651]

[Patrick Palka <ppalka@redhat.com>]

On Thu, Apr 27, 2023 at 4:46 PM Patrick Palka <ppalka@redhat.com> wrote:
>
> Now that with r14-11-g2245459c85a3f4 made us coerce the template
> arguments of a bound ttp again after level-lowering, this unfortunately
> causes a crash from coerce_template_args_for_ttp in the below testcase.
>
> During the level-lowering substitution T=int into the bound ttp TT<int>
> as part of substitution into the lambda signature, current_template_parms
> is just U=U rather than the ideal TT=TT, U=U.  And because we don't
> consistently set DECL_CONTEXT for level-lowered ttps (it's kind of a
> chicken of the egg problem in this case), we attempt to use
> current_template_parms to obtain the outer arguments during
> coerce_template_args_for_ttp.  But the depth 1 of c_t_p
> current_template_parms is less than the depth 2 of the level-lowered TT,
> and we end up segfaulting from there.
>
> So for level-lowered ttps it seems we need to get the outer arguments a
> different way -- namely, we can look at the trailing parms of its
> DECL_TEMPLATE_PARMS.

Note this is not an ideal solution because TREE_CHAIN of
DECL_TEMPLATE_PARMS in this case is just "2 <empty>, 1 U", so we're
missing tparm information for the level that the ttp belongs to :/ So
the only difference compared to using current_template_parms in this
case is the extra empty level of args corresponding to the ttp's
level.



>
> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> trunk/13?
>
>         PR c++/109651
>
> gcc/cp/ChangeLog:
>
>         * pt.cc (coerce_template_args_for_ttp): For level-lowered
>         ttps with DECL_CONTEXT not set, obtain the outer template
>         arguments via its DECL_TEMPLATE_PARMS.
>
> gcc/testsuite/ChangeLog:
>
>         * g++.dg/template/ttp37.C: New test.
> ---
>  gcc/cp/pt.cc                          |  5 +++++
>  gcc/testsuite/g++.dg/template/ttp37.C | 11 +++++++++++
>  2 files changed, 16 insertions(+)
>  create mode 100644 gcc/testsuite/g++.dg/template/ttp37.C
>
> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> index 678cb7930e3..bbde61061f6 100644
> --- a/gcc/cp/pt.cc
> +++ b/gcc/cp/pt.cc
> @@ -7872,6 +7872,11 @@ coerce_template_args_for_ttp (tree templ, tree arglist,
>    tree outer = DECL_CONTEXT (templ);
>    if (outer)
>      outer = generic_targs_for (outer);
> +  else if (TEMPLATE_TYPE_LEVEL (TREE_TYPE (templ))
> +          != TEMPLATE_TYPE_ORIG_LEVEL (TREE_TYPE (templ)))
> +    /* This is a level-lowered template template parameter, for which
> +       we don't consistently set DECL_CONTEXT (FIXME).  */
> +    outer = template_parms_to_args (TREE_CHAIN (DECL_TEMPLATE_PARMS (templ)));
>    else if (current_template_parms)
>      {
>        /* This is an argument of the current template, so we haven't set
> diff --git a/gcc/testsuite/g++.dg/template/ttp37.C b/gcc/testsuite/g++.dg/template/ttp37.C
> new file mode 100644
> index 00000000000..876e5b6232a
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/template/ttp37.C
> @@ -0,0 +1,11 @@
> +// PR c++/109651
> +// { dg-do compile { target c++20 } }
> +
> +template<class T>
> +void f() {
> +  []<class U>() {
> +    []<template<class> class TT>(TT<int>) { };
> +  };
> +}
> +
> +template void f<int>();
> --
> 2.40.1.423.g2807bd2c10
>