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

[Jason Merrill <jason@redhat.com>]

On 4/28/23 08:54, Patrick Palka 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.

OK.

> -- >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>{});
> +}