c++: deduction guides and ttp rewriting [PR102479]

  The problem here is ultimately that rewrite_tparm_list when rewriting a
TEMPLATE_TEMPLATE_PARM introduces a tree cycle in the rewritten
ttp that structural_comptypes can't cope with.  In particular the
DECL_TEMPLATE_PARMS of a ttp's TEMPLATE_DECL normally captures an empty
parameter list at its own level (and so the TEMPLATE_DECL doesn't appear
in its own DECL_TEMPLATE_PARMS), but rewrite_tparm_list ends up giving
it a complete parameter list.  In the new testcase below, this causes
infinite recursion from structural_comptypes when comparing Tmpl<char>
with Tmpl<long> (here both 'Tmpl's are rewritten).

This patch fixes this by making rewrite_template_parm give a rewritten
template template parm an empty parameter list at its own level, thereby
avoiding the tree cycle.  Testing the alias CTAD case revealed that
we're not setting current_template_parms in alias_ctad_tweaks, which
this patch also fixes.  Also, the change to use TMPL_ARGS_LEVEL instead
of TREE_VEC_ELT is needed because alias_ctad_tweaks passes only a single
level of targs to rewrite_tparm_list.

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

	PR c++/102479

gcc/cp/ChangeLog:

	* pt.c (rewrite_template_parm): Use TMPL_ARGS_LEVEL instead of
	TREE_VEC_ELT directly to properly handle one-level tsubst_args.
	Avoid a tree cycle when assigning the DECL_TEMPLATE_PARMS for a
	rewritten ttp.
	(alias_ctad_tweaks): Set current_template_parms accordingly.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/class-deduction12.C: Also test alias CTAD in the
	same way.
	* g++.dg/cpp1z/class-deduction99.C: New test.
---
 gcc/cp/pt.c                                   | 20 +++++++++--
 .../g++.dg/cpp1z/class-deduction12.C          |  6 ++++
 .../g++.dg/cpp1z/class-deduction99.C          | 35 +++++++++++++++++++
 3 files changed, 58 insertions(+), 3 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp1z/class-deduction99.C
  

On 9/27/21 10:44, Patrick Palka wrote:
> The problem here is ultimately that rewrite_tparm_list when rewriting a
> TEMPLATE_TEMPLATE_PARM introduces a tree cycle in the rewritten
> ttp that structural_comptypes can't cope with.  In particular the
> DECL_TEMPLATE_PARMS of a ttp's TEMPLATE_DECL normally captures an empty
> parameter list at its own level (and so the TEMPLATE_DECL doesn't appear
> in its own DECL_TEMPLATE_PARMS), but rewrite_tparm_list ends up giving
> it a complete parameter list.  In the new testcase below, this causes
> infinite recursion from structural_comptypes when comparing Tmpl<char>
> with Tmpl<long> (here both 'Tmpl's are rewritten).
> 
> This patch fixes this by making rewrite_template_parm give a rewritten
> template template parm an empty parameter list at its own level, thereby
> avoiding the tree cycle.  Testing the alias CTAD case revealed that
> we're not setting current_template_parms in alias_ctad_tweaks, which
> this patch also fixes.  Also, the change to use TMPL_ARGS_LEVEL instead
> of TREE_VEC_ELT is needed because alias_ctad_tweaks passes only a single
> level of targs to rewrite_tparm_list.
> 
> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> trunk?

OK.

> 	PR c++/102479
> 
> gcc/cp/ChangeLog:
> 
> 	* pt.c (rewrite_template_parm): Use TMPL_ARGS_LEVEL instead of
> 	TREE_VEC_ELT directly to properly handle one-level tsubst_args.
> 	Avoid a tree cycle when assigning the DECL_TEMPLATE_PARMS for a
> 	rewritten ttp.
> 	(alias_ctad_tweaks): Set current_template_parms accordingly.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp1z/class-deduction12.C: Also test alias CTAD in the
> 	same way.
> 	* g++.dg/cpp1z/class-deduction99.C: New test.
> ---
>   gcc/cp/pt.c                                   | 20 +++++++++--
>   .../g++.dg/cpp1z/class-deduction12.C          |  6 ++++
>   .../g++.dg/cpp1z/class-deduction99.C          | 35 +++++++++++++++++++
>   3 files changed, 58 insertions(+), 3 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp1z/class-deduction99.C
> 
> diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
> index 6bd6ceb29be..cba0f5c8279 100644
> --- a/gcc/cp/pt.c
> +++ b/gcc/cp/pt.c
> @@ -28754,7 +28754,7 @@ rewrite_template_parm (tree olddecl, unsigned index, unsigned level,
>   	  const int depth = TMPL_ARGS_DEPTH (tsubst_args);
>   	  tree ttargs = make_tree_vec (depth + 1);
>   	  for (int i = 0; i < depth; ++i)
> -	    TREE_VEC_ELT (ttargs, i) = TREE_VEC_ELT (tsubst_args, i);
> +	    TREE_VEC_ELT (ttargs, i) = TMPL_ARGS_LEVEL (tsubst_args, i + 1);
>   	  TREE_VEC_ELT (ttargs, depth)
>   	    = template_parms_level_to_args (ttparms);
>   	  // Substitute ttargs into ttparms to fix references to
> @@ -28767,8 +28767,17 @@ rewrite_template_parm (tree olddecl, unsigned index, unsigned level,
>   	  ttparms = tsubst_template_parms_level (ttparms, ttargs,
>   						 complain);
>   	  // Finally, tack the adjusted parms onto tparms.
> -	  ttparms = tree_cons (size_int (depth), ttparms,
> -			       current_template_parms);
> +	  ttparms = tree_cons (size_int (level + 1), ttparms,
> +			       copy_node (current_template_parms));
> +	  // As with all template template parms, the parameter list captured
> +	  // by this template template parm that corresponds to its own level
> +	  // should be empty.  This avoids infinite recursion when structurally
> +	  // comparing two such rewritten template template parms (102479).
> +	  gcc_assert (!TREE_VEC_LENGTH
> +		      (TREE_VALUE (TREE_CHAIN (DECL_TEMPLATE_PARMS (olddecl)))));
> +	  gcc_assert (TMPL_PARMS_DEPTH (TREE_CHAIN (ttparms)) == level);
> +	  TREE_VALUE (TREE_CHAIN (ttparms)) = make_tree_vec (0);
> +	  // All done.
>   	  DECL_TEMPLATE_PARMS (newdecl) = ttparms;
>   	}
>       }
> @@ -29266,6 +29275,11 @@ alias_ctad_tweaks (tree tmpl, tree uguides)
>   	      ++ndlen;
>   	  tree gtparms = make_tree_vec (natparms + ndlen);
>   
> +	  /* Set current_template_parms as in build_deduction_guide.  */
> +	  auto ctp = make_temp_override (current_template_parms);
> +	  current_template_parms = copy_node (DECL_TEMPLATE_PARMS (tmpl));
> +	  TREE_VALUE (current_template_parms) = gtparms;
> +
>   	  /* First copy over the parms of A.  */
>   	  for (j = 0; j < natparms; ++j)
>   	    TREE_VEC_ELT (gtparms, j) = TREE_VEC_ELT (atparms, j);
> diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction12.C b/gcc/testsuite/g++.dg/cpp1z/class-deduction12.C
> index a31cc1526db..f0d7ea0e16b 100644
> --- a/gcc/testsuite/g++.dg/cpp1z/class-deduction12.C
> +++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction12.C
> @@ -15,3 +15,9 @@ A a(&i,2,B<42>());
>   template <class,class> class same;
>   template <class T> class same<T,T> {};
>   same<decltype(a), A<int*>> s;
> +
> +#if __cpp_deduction_guides >= 201907
> +template <class T> using C = A<const T*>;
> +
> +same<decltype(C(&i, 2, B<42>())), A<const int*>> t;
> +#endif
> diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction99.C b/gcc/testsuite/g++.dg/cpp1z/class-deduction99.C
> new file mode 100644
> index 00000000000..6daa4b7c55c
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction99.C
> @@ -0,0 +1,35 @@
> +// PR c++/102479
> +// { dg-do compile { target c++17 } }
> +
> +template<class T> struct A;
> +
> +template<class T>
> +struct tuple {
> +  tuple(T);
> +
> +  template<template<class> class Tmpl>
> +  tuple(Tmpl<T>);
> +
> +  template<template<class> class Tmpl, typename A<Tmpl<char>>::type = 0>
> +  tuple(Tmpl<T>);
> +
> +  template<template<class> class Tmpl, typename A<Tmpl<long>>::type = 0>
> +  tuple(Tmpl<T>);
> +};
> +
> +template<class T> struct B { };
> +
> +using ty1 = tuple<int>;
> +using ty1 = decltype(tuple(0));
> +using ty1 = decltype(tuple(B<int>{}));
> +
> +#if __cpp_deduction_guides >= 201907
> +template<class T> using const_tuple = tuple<const T>;
> +
> +using ty2 = const_tuple<int>;
> +using ty2 = decltype(const_tuple(0));
> +using ty2 = decltype(const_tuple(B<const int>{}));
> +
> +using ty3 = const_tuple<B<int>>;
> +using ty3 = decltype(const_tuple(B<int>{}));
> +#endif
>