Re: [PATCH] c++, v2: Fix up decltype of non-dependent structured binding decl in template [PR92687]

[Jason Merrill <jason@redhat.com>]

On 3/1/24 02:55, Jakub Jelinek wrote:
> On Thu, Feb 29, 2024 at 12:50:47PM +0100, Jakub Jelinek wrote:
>> finish_decltype_type uses DECL_HAS_VALUE_EXPR_P (expr) check for
>> DECL_DECOMPOSITION_P (expr) to determine if it is
>> array/struct/vector/complex etc. subobject proxy case vs. structured
>> binding using std::tuple_{size,element}.
>> For non-templates or when templates are already instantiated, that works
>> correctly, finalized DECL_DECOMPOSITION_P non-base vars indeed have
>> DECL_VALUE_EXPR in the former case and don't have it in the latter.
>> It works fine for dependent structured bindings as well, cp_finish_decomp in
>> that case creates DECLTYPE_TYPE tree and defers the handling until
>> instantiation.
>> As the testcase shows, this doesn't work for the non-dependent structured
>> binding case in templates, because DECL_HAS_VALUE_EXPR_P is set in that case
>> always; cp_finish_decomp ends with:
>>    if (processing_template_decl)
>>      {
>>        for (unsigned int i = 0; i < count; i++)
>>          if (!DECL_HAS_VALUE_EXPR_P (v[i]))
>>            {
>>              tree a = build_nt (ARRAY_REF, decl, size_int (i),
>>                                 NULL_TREE, NULL_TREE);
>>              SET_DECL_VALUE_EXPR (v[i], a);
>>              DECL_HAS_VALUE_EXPR_P (v[i]) = 1;
>>            }
>>      }
>> and those artificial ARRAY_REFs are used in various places during
>> instantiation to find out what base the DECL_DECOMPOSITION_P VAR_DECLs
>> have and their positions.
> 
>> Another option would be to change
>>   tree
>>   lookup_decomp_type (tree v)
>>   {
>> -  return *decomp_type_table->get (v);
>> +  if (tree *slot = decomp_type_table->get (v))
>> +    return *slot;
>> +  return NULL_TREE;
>>   }
>>
>> and in finish_decl_decomp either just in the ptds.saved case or always
>> try to lookup_decomp_type, if it returns non-NULL, return what it returned,
>> otherwise return unlowered_expr_type (expr).  I guess it would be cleaner,
>> I thought it would be more costly due to the hash table lookup, but now that
>> I think about it again, DECL_VALUE_EXPR is a hash table lookup as well.
>> So maybe then
>> +	  if (ptds.saved)
>> +	    {
>> +	      gcc_checking_assert (DECL_HAS_VALUE_EXPR_P (expr));
>> +	      /* DECL_HAS_VALUE_EXPR_P is always set if
>> +		 processing_template_decl.  If lookup_decomp_type
>> +		 returns non-NULL, it is the tuple case.  */
>> +	      if (tree ret = lookup_decomp_type (expr))
>> +		return ret;
>> +	    }
>> 	  if (DECL_HAS_VALUE_EXPR_P (expr))
>> 	    /* Expr is an array or struct subobject proxy, handle
>> 	       bit-fields properly.  */
>> 	    return unlowered_expr_type (expr);
>> 	  else
>> 	    /* Expr is a reference variable for the tuple case.  */
>> 	    return lookup_decomp_type (expr);
> 
> Here is a variant of the patch which does that.
> 
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

OK.

> Or the other version, or adding some flag to the DECL_DECOMPOSITION_P
> decls?
> 
> 2024-03-01  Jakub Jelinek  <jakub@redhat.com>
> 
> 	PR c++/92687
> 	* decl.cc (lookup_decomp_type): Return NULL_TREE if decomp_type_table
> 	doesn't have entry for V.
> 	* semantics.cc (finish_decltype_type): If ptds.saved, assert
> 	DECL_HAS_VALUE_EXPR_P is true and decide on tuple vs. non-tuple based
> 	on if lookup_decomp_type is NULL or not.
> 
> 	* g++.dg/cpp1z/decomp59.C: New test.
> 
> --- gcc/cp/decl.cc.jj	2024-02-28 23:20:01.004751204 +0100
> +++ gcc/cp/decl.cc	2024-02-29 20:03:11.087218176 +0100
> @@ -9262,7 +9262,9 @@ static GTY((cache)) decl_tree_cache_map
>   tree
>   lookup_decomp_type (tree v)
>   {
> -  return *decomp_type_table->get (v);
> +  if (tree *slot = decomp_type_table->get (v))
> +    return *slot;
> +  return NULL_TREE;
>   }
>   
>   /* Mangle a decomposition declaration if needed.  Arguments like
> --- gcc/cp/semantics.cc.jj	2024-02-28 22:57:08.101800588 +0100
> +++ gcc/cp/semantics.cc	2024-02-29 20:04:51.936880622 +0100
> @@ -11804,6 +11804,15 @@ finish_decltype_type (tree expr, bool id
>   	 access expression).  */
>         if (DECL_DECOMPOSITION_P (expr))
>   	{
> +	  if (ptds.saved)
> +	    {
> +	      gcc_checking_assert (DECL_HAS_VALUE_EXPR_P (expr));
> +	      /* DECL_HAS_VALUE_EXPR_P is always set if
> +		 processing_template_decl.  If lookup_decomp_type
> +		 returns non-NULL, it is the tuple case.  */
> +	      if (tree ret = lookup_decomp_type (expr))
> +		return ret;
> +	    }
>   	  if (DECL_HAS_VALUE_EXPR_P (expr))
>   	    /* Expr is an array or struct subobject proxy, handle
>   	       bit-fields properly.  */
> --- gcc/testsuite/g++.dg/cpp1z/decomp59.C.jj	2024-02-29 20:02:17.467929327 +0100
> +++ gcc/testsuite/g++.dg/cpp1z/decomp59.C	2024-02-29 20:02:17.467929327 +0100
> @@ -0,0 +1,63 @@
> +// PR c++/92687
> +// { dg-do compile { target c++11 } }
> +// { dg-options "" }
> +
> +namespace std {
> +  template<typename T> struct tuple_size;
> +  template<int, typename> struct tuple_element;
> +}
> +
> +struct A {
> +  int i;
> +  template <int I> int& get() { return i; }
> +};
> +
> +template<> struct std::tuple_size<A> { static const int value = 2; };
> +template<int I> struct std::tuple_element<I,A> { using type = int; };
> +
> +template<typename T>
> +struct is_reference {
> +  static const bool value = false;
> +};
> +
> +template<typename T>
> +struct is_reference<T&>
> +{
> +  static const bool value = true;
> +};
> +
> +template<typename T>
> +struct is_reference<T&&>
> +{
> +  static const bool value = true;
> +};
> +
> +template<int N>
> +void
> +foo ()
> +{
> +  auto [x, y] = A {};	// { dg-warning "structured bindings only available with" "" { target c++14_down } }
> +  static_assert (!is_reference<decltype (x)>::value, "");
> +}
> +
> +void
> +bar ()
> +{
> +  auto [x, y] = A {};	// { dg-warning "structured bindings only available with" "" { target c++14_down } }
> +  static_assert (!is_reference<decltype (x)>::value, "");
> +}
> +
> +template<typename T>
> +void
> +baz ()
> +{
> +  auto [x, y] = T {};	// { dg-warning "structured bindings only available with" "" { target c++14_down } }
> +  static_assert (!is_reference<decltype (x)>::value, "");
> +}
> +
> +void
> +qux ()
> +{
> +  foo<0> ();
> +  baz<A> ();
> +}
> 
> 
> 	Jakub
>