Re: [PATCH] c++, v2: Implement CWG2635 - Constrained structured bindings

[Jason Merrill <jason@redhat.com>]

On 11/13/22 01:50, Jakub Jelinek wrote:
> On Sat, Nov 12, 2022 at 12:23:56PM +0100, Jakub Jelinek wrote:
>> The following patch implements CWG2635.
>>
>> So far tested on
>> GXX_TESTSUITE_STDS=98,11,14,17,20,2b make check-g++ RUNTESTFLAGS="dg.exp=decomp*"
>> ok for trunk if it passes full bootstrap/regtest and it is voted in?
> 
> Here is another version of the patch that passed bootstrap/regtest, the only
> change are tweaks to 2 further testcases.
> 
> 2022-11-13  Jakub Jelinek  <jakub@redhat.com>
> 
> 	* decl.cc (grokdeclarator): Implement
> 	CWG2635 - Constrained structured bindings.  Diagnose constrained
> 	auto type.

Let's make the constrained auto case a pedwarn instead of an error.

> 	* g++.dg/cpp2a/decomp5.C: New test.
> 	* g++.dg/cpp2a/concepts-placeholder7.C: Adjust expected diagnostics.
> 	* g++.dg/cpp2a/concepts-placeholder8.C: Likewise.
> 
> --- gcc/cp/decl.cc.jj	2022-11-11 17:14:33.103869977 +0100
> +++ gcc/cp/decl.cc	2022-11-12 12:13:52.217239729 +0100
> @@ -12660,7 +12660,8 @@ grokdeclarator (const cp_declarator *dec
>   	  gcc_unreachable ();
>   	}
>         if (TREE_CODE (type) != TEMPLATE_TYPE_PARM
> -	  || TYPE_IDENTIFIER (type) != auto_identifier)
> +	  || TYPE_IDENTIFIER (type) != auto_identifier
> +	  || PLACEHOLDER_TYPE_CONSTRAINTS_INFO (type))
>   	{
>   	  if (type != error_mark_node)
>   	    {
> --- gcc/testsuite/g++.dg/cpp2a/decomp5.C.jj	2022-11-12 12:17:21.024392082 +0100
> +++ gcc/testsuite/g++.dg/cpp2a/decomp5.C	2022-11-12 12:20:00.700214521 +0100
> @@ -0,0 +1,20 @@
> +// CWG2635 - Constrained structured bindings
> +// { dg-do compile { target c++20 } }
> +
> +namespace std {
> +  template<typename T> struct tuple_size;
> +  template<int, typename> struct tuple_element;
> +}
> +
> +struct A {
> +  int i;
> +  A(int x) : i(x) {}
> +  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<class T> concept C = true;
> +C auto [x, y] = A{1}; // { dg-error "structured binding declaration cannot have type 'auto \\\[requires ::C<<placeholder>, >\\\]'" }
> +		      // { dg-message "type must be cv-qualified 'auto' or reference to cv-qualified 'auto'" "" { target *-*-* } .-1 }
> --- gcc/testsuite/g++.dg/cpp2a/concepts-placeholder7.C.jj	2021-04-06 23:49:08.288882716 +0200
> +++ gcc/testsuite/g++.dg/cpp2a/concepts-placeholder7.C	2022-11-13 12:17:38.570047510 +0100
> @@ -7,16 +7,16 @@ template <class>
>   void f() {
>     int x[] = {1,2};
>     int y[] = {3};
> -  C1 auto [a,b] = x;
> -  C1 auto [c] = y; // { dg-error "constraints" }
> +  C1 auto [a,b] = x;	// { dg-error "structured binding declaration cannot have type 'auto \\\[requires ::C1<<placeholder>, >\\\]'" }
> +  C1 auto [c] = y;	// { dg-error "structured binding declaration cannot have type 'auto \\\[requires ::C1<<placeholder>, >\\\]'" }
>   }
>   
>   template <class T>
>   void g() {
>     T x[] = {1,2};
>     T y[] = {3};
> -  C1 auto [a,b] = x;
> -  C1 auto [c] = y; // { dg-error "constraints" }
> +  C1 auto [a,b] = x;	// { dg-error "structured binding declaration cannot have type 'auto \\\[requires ::C1<<placeholder>, >\\\]'" }
> +  C1 auto [c] = y;	// { dg-error "structured binding declaration cannot have type 'auto \\\[requires ::C1<<placeholder>, >\\\]'" }
>   }
>   template void g<int>();
>   
> @@ -27,6 +27,6 @@ struct S { int a, b; } s;
>   
>   template <class T>
>   void h() {
> -  const C2<T> auto& [a, b] = s;
> +  const C2<T> auto& [a, b] = s;	// { dg-error "structured binding declaration cannot have type 'const auto \\\[requires ::C2<<placeholder>, >\\\]'" }
>   }
>   template void h<int>();
> --- gcc/testsuite/g++.dg/cpp2a/concepts-placeholder8.C.jj	2021-04-06 23:49:08.288882716 +0200
> +++ gcc/testsuite/g++.dg/cpp2a/concepts-placeholder8.C	2022-11-13 12:19:17.034684881 +0100
> @@ -5,6 +5,6 @@ template <class T> concept is_const = __
>   void f() {
>     int x[] = {1,2};
>     const int y[] = {3};
> -  const is_const auto [a,b] = x; // { dg-error "constraints" }
> -  const is_const auto [c] = y;
> +  const is_const auto [a,b] = x;	// { dg-error "structured binding declaration cannot have type 'const auto \\\[requires ::is_const<<placeholder>, >\\\]'" }
> +  const is_const auto [c] = y;		// { dg-error "structured binding declaration cannot have type 'const auto \\\[requires ::is_const<<placeholder>, >\\\]'" }
>   }
> 
> 
> 	Jakub
>