Re: [PATCH] c++: Reject pack expansion of assume attribute [PR109756]

[Jason Merrill <jason@redhat.com>]

On 5/9/23 04:22, Jakub Jelinek wrote:
> Hi!
> 
> http://eel.is/c++draft/dcl.attr#grammar-4 says
> "In an attribute-list, an ellipsis may appear only if that attribute's
> specification permits it."
> and doesn't explicitly permit it on any standard attribute.
> The https://wg21.link/p1774r8 paper which introduced assume attribute says
> "We could therefore hypothetically permit the assume attribute to directly
> support pack expansion:
> template <int... args>
> void f() {
> [[assume(args >= 0)...]];
> }
> However, we do not propose this. It would require substantial additional work

I question "substantial"; it could easily be treated as a 
fold-expression internally.  But the patch is OK.

> for a very rare use case. Note that this can instead be expressed with a fold
> expression, which is equivalent to the above and works out of the box without
> any extra effort:
> template <int... args>
> void f() {
> [[assume(((args >= 0) && ...))]];
> }
> ", but as the testcase shows, GCC 13+ ICEs on assume attribute followed by
> ... if it contains packs.
> The following patch rejects those instead of ICE and for C++17 or later
> suggests using fold expressions instead (it doesn't make sense to suggest
> it for C++14 and earlier when we'd error on the fold expressions).
> 
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk/13.2?
> 
> 2023-05-09  Jakub Jelinek  <jakub@redhat.com>
> 
> 	PR c++/109756
> 	* gp-gimplify.cc (process_stmt_assume_attribute): Diagnose pack
> 	expansion of assume attribute.
> 
> 	* g++.dg/cpp23/attr-assume11.C: New test.
> 
> --- gcc/cp/cp-gimplify.cc.jj	2023-05-04 12:13:50.791647593 +0200
> +++ gcc/cp/cp-gimplify.cc	2023-05-08 13:20:33.191070530 +0200
> @@ -3267,6 +3267,16 @@ process_stmt_assume_attribute (tree std_
>     for (; attr; attr = lookup_attribute ("gnu", "assume", TREE_CHAIN (attr)))
>       {
>         tree args = TREE_VALUE (attr);
> +      if (args && PACK_EXPANSION_P (args))
> +	{
> +	  auto_diagnostic_group d;
> +	  error_at (attrs_loc, "pack expansion of %qE attribute",
> +		    get_attribute_name (attr));
> +	  if (cxx_dialect >= cxx17)
> +	    inform (attrs_loc, "use fold expression in the attribute "
> +			       "argument instead");
> +	  continue;
> +	}
>         int nargs = list_length (args);
>         if (nargs != 1)
>   	{
> --- gcc/testsuite/g++.dg/cpp23/attr-assume11.C.jj	2023-05-08 13:19:07.812290213 +0200
> +++ gcc/testsuite/g++.dg/cpp23/attr-assume11.C	2023-05-08 13:22:07.746719746 +0200
> @@ -0,0 +1,22 @@
> +// PR c++/109756
> +// { dg-do compile { target c++11 } }
> +
> +template <int ...args>
> +void
> +foo ()
> +{
> +  [[assume (1 > 0)...]];		// { dg-error "expansion pattern '\\\(1 > 0\\\)' contains no parameter packs" }
> +					// { dg-warning "attributes at the beginning of statement are ignored" "" { target *-*-* } .-1 }
> +  [[assume (args > 0)...]];		// { dg-error "pack expansion of 'assume' attribute" }
> +					// { dg-message "use fold expression in the attribute argument instead" "" { target c++17 } .-1 }
> +#if __cpp_fold_expressions >= 201603L
> +  [[assume (((args > 0) && ...))]];
> +#endif
> +  [[gnu::assume (1 > 0)...]];		// { dg-error "expansion pattern '\\\(1 > 0\\\)' contains no parameter packs" }
> +					// { dg-warning "attributes at the beginning of statement are ignored" "" { target *-*-* } .-1 }
> +  [[gnu::assume (args > 0)...]];	// { dg-error "pack expansion of 'assume' attribute" }
> +					// { dg-message "use fold expression in the attribute argument instead" "" { target c++17 } .-1  }
> +#if __cpp_fold_expressions >= 201603L
> +  [[gnu::assume (((args > 0) && ...))]];
> +#endif
> +}
> 
> 	Jakub
>