c++: remove coerce_innermost_template_parms

IIUC the only practical difference between coerce_innermost_template_parms
and the main function coerce_template_parms is that the former takes
a multi-level template parameter list and returns a template argument
vector of the same depth, whereas the latter takes a single-level
template parameter vector and returns a single-level template argument
vector.

This patch gets rid of the wrapper function and just overloads the
behavior of the main function according to whether 'parms' is a
multi-level template parameter list or a single-level template argument
vector.  It turns out we can assume parms and args have the same depth
in the multi-level case, which simplifies the overloading logic.

Besides the (subjective) simplificatio benefit, another benefit of this
unification is that it avoids a redundant copy of a multi-level 'args'.
Now, we can return new_args directly from c_t_p.  (And because of this,
we need to turn new_inner_args into a reference so that updating it also
updates new_args.)

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

gcc/cp/ChangeLog:

	* pt.cc (coerce_template_parms): Salvage part of the function
	comment from c_innermost_t_p.  Handle parms being a full
	template parameter list.
	(coerce_innermost_template_parms): Remove.
	(lookup_template_class): Use c_t_p instead of c_innermost_t_p.
	(finish_template_variable): Likewise.
	(tsubst_decl): Likewise.
	(instantiate_alias_template): Likewise.
---
 gcc/cp/pt.cc | 92 +++++++++++++++-------------------------------------
 1 file changed, 27 insertions(+), 65 deletions(-)

On 11/18/22 16:59, Patrick Palka wrote:
> IIUC the only practical difference between coerce_innermost_template_parms
> and the main function coerce_template_parms is that the former takes
> a multi-level template parameter list and returns a template argument
> vector of the same depth, whereas the latter takes a single-level
> template parameter vector and returns a single-level template argument
> vector.
> 
> This patch gets rid of the wrapper function and just overloads the
> behavior of the main function according to whether 'parms' is a
> multi-level template parameter list or a single-level template argument
> vector.  It turns out we can assume parms and args have the same depth
> in the multi-level case, which simplifies the overloading logic.
> 
> Besides the (subjective) simplificatio benefit, another benefit of this
> unification is that it avoids a redundant copy of a multi-level 'args'.
> Now, we can return new_args directly from c_t_p.  (And because of this,
> we need to turn new_inner_args into a reference so that updating it also
> updates new_args.)
> 
> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> trunk?

OK.  But this doesn't really seem like stage 3 material; let's hold off 
on further cleanups like this until next stage 1.

> gcc/cp/ChangeLog:
> 
> 	* pt.cc (coerce_template_parms): Salvage part of the function
> 	comment from c_innermost_t_p.  Handle parms being a full
> 	template parameter list.
> 	(coerce_innermost_template_parms): Remove.
> 	(lookup_template_class): Use c_t_p instead of c_innermost_t_p.
> 	(finish_template_variable): Likewise.
> 	(tsubst_decl): Likewise.
> 	(instantiate_alias_template): Likewise.
> ---
>   gcc/cp/pt.cc | 92 +++++++++++++++-------------------------------------
>   1 file changed, 27 insertions(+), 65 deletions(-)
> 
> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> index 0310e38c9b9..2666e455edf 100644
> --- a/gcc/cp/pt.cc
> +++ b/gcc/cp/pt.cc
> @@ -148,8 +148,6 @@ static void add_pending_template (tree);
>   static tree reopen_tinst_level (struct tinst_level *);
>   static tree tsubst_initializer_list (tree, tree);
>   static tree get_partial_spec_bindings (tree, tree, tree);
> -static tree coerce_innermost_template_parms (tree, tree, tree, tsubst_flags_t,
> -					     bool = true);
>   static void tsubst_enum	(tree, tree, tree);
>   static bool check_instantiated_args (tree, tree, tsubst_flags_t);
>   static int check_non_deducible_conversion (tree, tree, unification_kind_t, int,
> @@ -8827,6 +8825,14 @@ pack_expansion_args_count (tree args)
>      arguments.  If any error occurs, return error_mark_node. Error and
>      warning messages are issued under control of COMPLAIN.
>   
> +   If PARMS represents all template parameters levels, this function
> +   returns a vector of vectors representing all the resulting argument
> +   levels.  Note that in this case, only the innermost arguments are
> +   coerced because the outermost ones are supposed to have been coerced
> +   already.  Otherwise, if PARMS represents only (the innermost) vector
> +   of parameters, this function returns a vector containing just the
> +   innermost resulting arguments.
> +
>      If REQUIRE_ALL_ARGS is false, argument deduction will be performed
>      for arguments not specified in ARGS.  If REQUIRE_ALL_ARGS is true,
>      arguments not specified in ARGS must have default arguments which
> @@ -8842,8 +8848,6 @@ coerce_template_parms (tree parms,
>     int nparms, nargs, parm_idx, arg_idx, lost = 0;
>     tree orig_inner_args;
>     tree inner_args;
> -  tree new_args;
> -  tree new_inner_args;
>   
>     /* When used as a boolean value, indicates whether this is a
>        variadic template parameter list. Since it's an int, we can also
> @@ -8864,6 +8868,17 @@ coerce_template_parms (tree parms,
>     if (args == error_mark_node)
>       return error_mark_node;
>   
> +  bool return_full_args = false;
> +  if (TREE_CODE (parms) == TREE_LIST)
> +    {
> +      if (TMPL_PARMS_DEPTH (parms) > 1)
> +	{
> +	  gcc_assert (TMPL_PARMS_DEPTH (parms) == TMPL_ARGS_DEPTH (args));
> +	  return_full_args = true;
> +	}
> +      parms = INNERMOST_TEMPLATE_PARMS (parms);
> +    }
> +
>     nparms = TREE_VEC_LENGTH (parms);
>   
>     /* Determine if there are any parameter packs or default arguments.  */
> @@ -8961,8 +8976,8 @@ coerce_template_parms (tree parms,
>        template-id may be nested within a "sizeof".  */
>     cp_evaluated ev;
>   
> -  new_inner_args = make_tree_vec (nparms);
> -  new_args = add_outermost_template_args (args, new_inner_args);
> +  tree new_args = add_outermost_template_args (args, make_tree_vec (nparms));
> +  tree& new_inner_args = TMPL_ARGS_LEVEL (new_args, TMPL_ARGS_DEPTH (new_args));
>     int pack_adjust = 0;
>     for (parm_idx = 0, arg_idx = 0; parm_idx < nparms; parm_idx++, arg_idx++)
>       {
> @@ -9164,59 +9179,7 @@ coerce_template_parms (tree parms,
>       SET_NON_DEFAULT_TEMPLATE_ARGS_COUNT (new_inner_args,
>   					 TREE_VEC_LENGTH (new_inner_args));
>   
> -  return new_inner_args;
> -}
> -
> -/* Like coerce_template_parms.  If PARMS represents all template
> -   parameters levels, this function returns a vector of vectors
> -   representing all the resulting argument levels.  Note that in this
> -   case, only the innermost arguments are coerced because the
> -   outermost ones are supposed to have been coerced already.
> -
> -   Otherwise, if PARMS represents only (the innermost) vector of
> -   parameters, this function returns a vector containing just the
> -   innermost resulting arguments.  */
> -
> -static tree
> -coerce_innermost_template_parms (tree parms,
> -				 tree args,
> -				 tree in_decl,
> -				 tsubst_flags_t complain,
> -				 bool require_all_args /* = true */)
> -{
> -  int parms_depth = TMPL_PARMS_DEPTH (parms);
> -  int args_depth = TMPL_ARGS_DEPTH (args);
> -  tree coerced_args;
> -
> -  if (parms_depth > 1)
> -    {
> -      coerced_args = make_tree_vec (parms_depth);
> -      tree level;
> -      int cur_depth;
> -
> -      for (level = parms, cur_depth = parms_depth;
> -	   parms_depth > 0 && level != NULL_TREE;
> -	   level = TREE_CHAIN (level), --cur_depth)
> -	{
> -	  tree l;
> -	  if (cur_depth == args_depth)
> -	    l = coerce_template_parms (TREE_VALUE (level),
> -				       args, in_decl, complain,
> -				       require_all_args);
> -	  else
> -	    l = TMPL_ARGS_LEVEL (args, cur_depth);
> -
> -	  if (l == error_mark_node)
> -	    return error_mark_node;
> -
> -	  SET_TMPL_ARGS_LEVEL (coerced_args, cur_depth, l);
> -	}
> -    }
> -  else
> -    coerced_args = coerce_template_parms (INNERMOST_TEMPLATE_PARMS (parms),
> -					  args, in_decl, complain,
> -					  require_all_args);
> -  return coerced_args;
> +  return return_full_args ? new_args : new_inner_args;
>   }
>   
>   /* Returns true if T is a wrapper to make a C++20 template parameter
> @@ -9909,8 +9872,7 @@ lookup_template_class (tree d1, tree arglist, tree in_decl, tree context,
>         /* Calculate the BOUND_ARGS.  These will be the args that are
>   	 actually tsubst'd into the definition to create the
>   	 instantiation.  */
> -      arglist = coerce_innermost_template_parms (parmlist, arglist, gen_tmpl,
> -						 complain);
> +      arglist = coerce_template_parms (parmlist, arglist, gen_tmpl, complain);
>   
>         if (arglist == error_mark_node)
>   	/* We were unable to bind the arguments.  */
> @@ -10326,7 +10288,7 @@ finish_template_variable (tree var, tsubst_flags_t complain)
>     tree arglist = TREE_OPERAND (var, 1);
>   
>     tree parms = DECL_TEMPLATE_PARMS (templ);
> -  arglist = coerce_innermost_template_parms (parms, arglist, templ, complain);
> +  arglist = coerce_template_parms (parms, arglist, templ, complain);
>     if (arglist == error_mark_node)
>       return error_mark_node;
>   
> @@ -14962,7 +14924,7 @@ tsubst_decl (tree t, tree args, tsubst_flags_t complain)
>   		  /* We're fully specializing a template declaration, so
>   		     we need to coerce the innermost arguments corresponding to
>   		     the template.  */
> -		  argvec = (coerce_innermost_template_parms
> +		  argvec = (coerce_template_parms
>   			    (DECL_TEMPLATE_PARMS (gen_tmpl),
>   			     argvec, t, complain));
>   		if (argvec == error_mark_node)
> @@ -21800,8 +21762,8 @@ instantiate_alias_template (tree tmpl, tree args, tsubst_flags_t complain)
>     if (tmpl == error_mark_node || args == error_mark_node)
>       return error_mark_node;
>   
> -  args = coerce_innermost_template_parms (DECL_TEMPLATE_PARMS (tmpl),
> -					  args, tmpl, complain);
> +  args = coerce_template_parms (DECL_TEMPLATE_PARMS (tmpl),
> +				args, tmpl, complain);
>   
>     /* FIXME check for satisfaction in check_instantiated_args.  */
>     if (flag_concepts