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

[Patrick Palka <ppalka@redhat.com>]

On Fri, Apr 28, 2023 at 8:54 AM Patrick Palka <ppalka@redhat.com> wrote:
>
> On Thu, 27 Apr 2023, Patrick Palka wrote:
>
> > 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.
>
> And on the other hand, this issue seems specific to lambdas because
> it's in tsubst_lambda_expr that we substitute the function type _before_
> substituting and installing the template parameters, which is opposite
> to the typical order that tsubst_template_decl does things in.  And
> that's ultimately the reason the current_template_parms fallback in
> coerce_template_args_for_ttp misbehaves in this testcase.
>
> So the following seems to be a better fix.  With it, current_template_parms
> is correctly 2 TT, 1 U during substitution the lambda's function type,
> which makes coerce_template_args_for_ttp happy when level lowering
> the bound ttp within the function type.

Ping.

>
> -- >8 --
>
> Subject: [PATCH] c++: bound ttp in lambda function type [PR109651]
>
>         PR c++/109651
>
> gcc/cp/ChangeLog:
>
>         * pt.cc (tsubst_template_decl): Add default argument to
>         lambda_fntype parameter.  Add defaulted lambda_tparms parameter.
>         Prefer to use lambda_tparms instead of substituting
>         DECL_TEMPLATE_PARMS.
>         (tsubst_decl) <case TEMPLATE_DECL>: Adjust tsubst_template_decl
>         call.
>         (tsubst_lambda_expr): For a generic lambda, substitute
>         DECL_TEMPLATE_PARMS and update current_template_parms
>         before substituting the function type.  Pass the substituted
>         DECL_TEMPLATE_PARMS to tsubst_template_decl.
>
> gcc/testsuite/ChangeLog:
>
>         * g++.dg/cpp2a/lambda-generic-ttp1.C: New test.
>         * g++.dg/cpp2a/lambda-generic-ttp2.C: New test.
> ---
>  gcc/cp/pt.cc                                  | 30 ++++++++++++++-----
>  .../g++.dg/cpp2a/lambda-generic-ttp1.C        | 11 +++++++
>  .../g++.dg/cpp2a/lambda-generic-ttp2.C        | 13 ++++++++
>  3 files changed, 47 insertions(+), 7 deletions(-)
>  create mode 100644 gcc/testsuite/g++.dg/cpp2a/lambda-generic-ttp1.C
>  create mode 100644 gcc/testsuite/g++.dg/cpp2a/lambda-generic-ttp2.C
>
> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> index 678cb7930e3..43713d9ab72 100644
> --- a/gcc/cp/pt.cc
> +++ b/gcc/cp/pt.cc
> @@ -14629,7 +14629,8 @@ tsubst_function_decl (tree t, tree args, tsubst_flags_t complain,
>
>  static tree
>  tsubst_template_decl (tree t, tree args, tsubst_flags_t complain,
> -                     tree lambda_fntype)
> +                     tree lambda_fntype = NULL_TREE,
> +                     tree lambda_tparms = NULL_TREE)
>  {
>    /* We can get here when processing a member function template,
>       member class template, or template template parameter.  */
> @@ -14719,8 +14720,10 @@ tsubst_template_decl (tree t, tree args, tsubst_flags_t complain,
>    auto tparm_guard = make_temp_override (current_template_parms);
>    DECL_TEMPLATE_PARMS (r)
>      = current_template_parms
> -    = tsubst_template_parms (DECL_TEMPLATE_PARMS (t), args,
> -                            complain);
> +    = (lambda_tparms
> +       ? lambda_tparms
> +       : tsubst_template_parms (DECL_TEMPLATE_PARMS (t), args,
> +                               complain));
>
>    bool class_p = false;
>    tree inner = decl;
> @@ -14888,7 +14891,7 @@ tsubst_decl (tree t, tree args, tsubst_flags_t complain)
>    switch (TREE_CODE (t))
>      {
>      case TEMPLATE_DECL:
> -      r = tsubst_template_decl (t, args, complain, /*lambda*/NULL_TREE);
> +      r = tsubst_template_decl (t, args, complain);
>        break;
>
>      case FUNCTION_DECL:
> @@ -20130,12 +20133,24 @@ tsubst_lambda_expr (tree t, tree args, tsubst_flags_t complain, tree in_decl)
>                   ? DECL_TI_TEMPLATE (oldfn)
>                   : NULL_TREE);
>
> +  tree tparms = NULL_TREE;
> +  if (oldtmpl)
> +    tparms = tsubst_template_parms (DECL_TEMPLATE_PARMS (oldtmpl), args, complain);
> +
>    tree fntype = static_fn_type (oldfn);
> +
> +  tree saved_ctp = current_template_parms;
>    if (oldtmpl)
> -    ++processing_template_decl;
> +    {
> +      ++processing_template_decl;
> +      current_template_parms = tparms;
> +    }
>    fntype = tsubst (fntype, args, complain, in_decl);
>    if (oldtmpl)
> -    --processing_template_decl;
> +    {
> +      current_template_parms = saved_ctp;
> +      --processing_template_decl;
> +    }
>
>    if (fntype == error_mark_node)
>      r = error_mark_node;
> @@ -20151,7 +20166,8 @@ tsubst_lambda_expr (tree t, tree args, tsubst_flags_t complain, tree in_decl)
>                                  type_memfn_quals (fntype),
>                                  type_memfn_rqual (fntype));
>        tree inst = (oldtmpl
> -                  ? tsubst_template_decl (oldtmpl, args, complain, fntype)
> +                  ? tsubst_template_decl (oldtmpl, args, complain,
> +                                          fntype, tparms)
>                    : tsubst_function_decl (oldfn, args, complain, fntype));
>        if (inst == error_mark_node)
>         {
> diff --git a/gcc/testsuite/g++.dg/cpp2a/lambda-generic-ttp1.C b/gcc/testsuite/g++.dg/cpp2a/lambda-generic-ttp1.C
> new file mode 100644
> index 00000000000..876e5b6232a
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/lambda-generic-ttp1.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>();
> diff --git a/gcc/testsuite/g++.dg/cpp2a/lambda-generic-ttp2.C b/gcc/testsuite/g++.dg/cpp2a/lambda-generic-ttp2.C
> new file mode 100644
> index 00000000000..dc2029d7227
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/lambda-generic-ttp2.C
> @@ -0,0 +1,13 @@
> +// PR c++/109651
> +// { dg-do compile { target c++20 } }
> +
> +template<class T>
> +auto f() {
> +  return []<class U, template<class V, U, V> class TT>(U, TT<int, 1, 2>) { };
> +}
> +
> +template<class T, int N, int M> struct A { };
> +
> +int main() {
> +  f<int>()(0, A<int, 1, 2>{});
> +}
> --
> 2.40.1.423.g2807bd2c10
>
>
> >
> >
> >
> > >
> > > 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
> > >
> >