Re: [PATCH] c++, tree: walk TREE_VEC (and VECTOR_CST) in natural order [PR101886]

[Jason Merrill <jason@redhat.com>]

On 12/20/22 10:30, Patrick Palka wrote:
> Unfortunately the extract_autos_r fix in r13-4799-ga7c8036b26082d is
> derailed by the fact that walk_tree_1 currently walks the elements of a
> TREE_VEC in reverse, which means for A<auto, auto> in the below testcase
> extract_autos_r ends up adjusting the TEMPLATE_TYPE_IDX of the first
> auto rather than the second one, and the first auto is the canonical
> auto of level 2 and index 0, so we rightfully trigger the sanity check
> added in that commit.
> 
> It seems TREE_VEC and VECTOR_CST are the only trees that we walk in
> reverse, and this has been the case since r21884 whence the original
> version of walk_tree_1 was introduced.  But that's arguably unnatural
> and inconsistent with how we walk other compound trees such as
> CONSTRUCTORs and EXPR_P trees.  So this patch makes walk_tree_1 walk
> these trees in forward order, which fixes the below testcase.  This in
> turn revealed that keep_template_parm builds up the list of found
> template parameters in reverse, which previously compensated for the
> TREE_VEC behavior, but now we should be building the list in the natural
> order as well for sake of pretty printing parameter mappings.
> 
> Bootstrapped and regtested on x86_64-pc-linux-gnu with
> --enable-languages=all,ada,go does this look OK for trunk?

OK.

> 	PR c++/101886
> 
> gcc/cp/ChangeLog:
> 
> 	* pt.cc (find_template_parameter_info::parm_list_tail):
> 	New data member.
> 	(keep_template_parm): Use parm_list_tail to append rather
> 	than prepend to parm_list.
> 
> gcc/ChangeLog:
> 
> 	* tree.cc (walk_tree_1) <case TREE_VEC>: Walk the elements
> 	in the natural order.
> 	<case VECTOR_CST>: Likewise.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/concepts/diagnostic12.C: Adjust expected order of
> 	template parameters within parameter mapping.
> 	* g++.dg/concepts/auto6.C: New test.
> ---
>   gcc/cp/pt.cc                                 | 13 +++++++++----
>   gcc/testsuite/g++.dg/concepts/auto6.C        | 14 ++++++++++++++
>   gcc/testsuite/g++.dg/concepts/diagnostic12.C |  2 +-
>   gcc/tree.cc                                  | 20 ++++++++++----------
>   4 files changed, 34 insertions(+), 15 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/concepts/auto6.C
> 
> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> index 2b7b3756b68..df125a63785 100644
> --- a/gcc/cp/pt.cc
> +++ b/gcc/cp/pt.cc
> @@ -10640,14 +10640,14 @@ for_each_template_parm (tree t, tree_fn_t fn, void* data,
>   struct find_template_parameter_info
>   {
>     explicit find_template_parameter_info (tree ctx_parms)
> -    : parm_list (NULL_TREE),
> -      ctx_parms (ctx_parms),
> +    : ctx_parms (ctx_parms),
>         max_depth (TMPL_PARMS_DEPTH (ctx_parms))
>     {}
>   
>     hash_set<tree> visited;
>     hash_set<tree> parms;
> -  tree parm_list;
> +  tree parm_list = NULL_TREE;
> +  tree *parm_list_tail = &parm_list;
>     tree ctx_parms;
>     int max_depth;
>   };
> @@ -10693,7 +10693,12 @@ keep_template_parm (tree t, void* data)
>     if (TYPE_P (t))
>       t = TYPE_MAIN_VARIANT (t);
>     if (!ftpi->parms.add (t))
> -    ftpi->parm_list = tree_cons (NULL_TREE, t, ftpi->parm_list);
> +    {
> +      /* Append T to PARM_LIST.  */
> +      tree node = build_tree_list (NULL_TREE, t);
> +      *ftpi->parm_list_tail = node;
> +      ftpi->parm_list_tail = &TREE_CHAIN (node);
> +    }
>   
>     /* Verify the parameter we found has a valid index.  */
>     if (flag_checking)
> diff --git a/gcc/testsuite/g++.dg/concepts/auto6.C b/gcc/testsuite/g++.dg/concepts/auto6.C
> new file mode 100644
> index 00000000000..1f6d72e54cc
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/concepts/auto6.C
> @@ -0,0 +1,14 @@
> +// PR c++/101886
> +// { dg-do compile { target c++17_only } }
> +// { dg-options "-fconcepts-ts" }
> +
> +template<typename, typename> struct A { };
> +
> +template<class T>
> +void f() {
> +  A<int, int> a;
> +  A<auto, auto> b1 = a;
> +  A<auto, auto> b2 = a;
> +}
> +
> +template void f<int>();
> diff --git a/gcc/testsuite/g++.dg/concepts/diagnostic12.C b/gcc/testsuite/g++.dg/concepts/diagnostic12.C
> index 548ba9c1b3d..8086500760b 100644
> --- a/gcc/testsuite/g++.dg/concepts/diagnostic12.C
> +++ b/gcc/testsuite/g++.dg/concepts/diagnostic12.C
> @@ -3,7 +3,7 @@
>   
>   template<typename T, typename... Args>
>     concept c1 = requires (T t, Args... args) { *t; };
> -// { dg-message "in requirements with .T t., .Args ... args. .with Args = \{\}; T = int" "" { target *-*-* } .-1 }
> +// { dg-message "in requirements with .T t., .Args ... args. .with T = int; Args = \{\}" "" { target *-*-* } .-1 }
>   
>   static_assert(c1<int>); // { dg-error "failed" }
>   
> diff --git a/gcc/tree.cc b/gcc/tree.cc
> index 0a51f9ddb4d..92199bb6503 100644
> --- a/gcc/tree.cc
> +++ b/gcc/tree.cc
> @@ -11310,12 +11310,12 @@ walk_tree_1 (tree *tp, walk_tree_fn func, void *data,
>   	if (len == 0)
>   	  break;
>   
> -	/* Walk all elements but the first.  */
> -	while (--len)
> -	  WALK_SUBTREE (TREE_VEC_ELT (*tp, len));
> +	/* Walk all elements but the last.  */
> +	for (int i = 0; i < len - 1; ++i)
> +	  WALK_SUBTREE (TREE_VEC_ELT (*tp, i));
>   
> -	/* Now walk the first one as a tail call.  */
> -	WALK_SUBTREE_TAIL (TREE_VEC_ELT (*tp, 0));
> +	/* Now walk the last one as a tail call.  */
> +	WALK_SUBTREE_TAIL (TREE_VEC_ELT (*tp, len - 1));
>         }
>   
>       case VECTOR_CST:
> @@ -11323,11 +11323,11 @@ walk_tree_1 (tree *tp, walk_tree_fn func, void *data,
>   	unsigned len = vector_cst_encoded_nelts (*tp);
>   	if (len == 0)
>   	  break;
> -	/* Walk all elements but the first.  */
> -	while (--len)
> -	  WALK_SUBTREE (VECTOR_CST_ENCODED_ELT (*tp, len));
> -	/* Now walk the first one as a tail call.  */
> -	WALK_SUBTREE_TAIL (VECTOR_CST_ENCODED_ELT (*tp, 0));
> +	/* Walk all elements but the last.  */
> +	for (unsigned i = 0; i < len - 1; ++i)
> +	  WALK_SUBTREE (VECTOR_CST_ENCODED_ELT (*tp, i));
> +	/* Now walk the last one as a tail call.  */
> +	WALK_SUBTREE_TAIL (VECTOR_CST_ENCODED_ELT (*tp, len - 1));
>         }
>   
>       case COMPLEX_CST: