Re: [PATCH] c++: Relax rules for non-type arguments in partial specs [CWG1315]

[Jason Merrill <jason@redhat.com>]

On 5/26/21 7:36 PM, Patrick Palka wrote:
> This implements the wording changes of CWG 1315, which permits non-type
> template arguments in a partial specialization to use template
> parameters more freely.  Delightfully, it seems the only change needed
> is to remove a few checks from process_partial_specialization.
> 
> But that change alone revealed a latent problem with
> for_each_template_parm: it ends up looking into some non-deduced
> contexts even when include_nondeduced_p is false.  This causes us to
> silently accept some partial specializations within the testsuite that
> contain non-deducible non-type template parameters (and that were
> previously rejected due to the rule that CWG 1315 removed).  For now
> this patch makes a minimal amount of changes to for_each_template_parm_r
> so we continue to reject existing ill-formed partial specializations
> within the testsuite.
> 
> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> trunk?  Patch generated with -w to hide noisy whitespace changes.

OK.

> 	DR 1315
> 	PR c++/96555
> 	PR c++/100779
> 
> gcc/cp/ChangeLog:
> 
> 	* pt.c (process_partial_specialization): Don't error on a
> 	non-simple non-type template argument that involves template
> 	parameters.
> 	(for_each_template_parm_r): Don't walk TRAIT_EXPR, PLUS_EXPR,
> 	MULT_EXPR, or SCOPE_REF when include_nondeduced_p is unset.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/template/partial16.C: New test.
> 	* g++.dg/template/partial17.C: New test.
> 	* g++.dg/template/partial18.C: New test.
> 	* g++.dg/cpp0x/pr68724.C: Adjust expected diagnostic for
> 	  ill-formed partial specialization.
> 	* g++.dg/cpp0x/variadic38.C: Likewise.
> 	* g++.dg/cpp1z/pr81016.C: Likewise.
> 	* g++.dg/template/partial5.C: Likewise.
> 	* g++.old-deja/g++.pt/spec21.C: Likewise.
> ---
>   gcc/cp/pt.c                                | 28 ++++++++--------------
>   gcc/testsuite/g++.dg/cpp0x/pr68724.C       |  2 +-
>   gcc/testsuite/g++.dg/cpp0x/variadic38.C    |  3 ++-
>   gcc/testsuite/g++.dg/cpp1z/pr81016.C       |  2 +-
>   gcc/testsuite/g++.dg/template/partial16.C  |  8 +++++++
>   gcc/testsuite/g++.dg/template/partial17.C  | 14 +++++++++++
>   gcc/testsuite/g++.dg/template/partial18.C  | 19 +++++++++++++++
>   gcc/testsuite/g++.dg/template/partial5.C   |  2 +-
>   gcc/testsuite/g++.old-deja/g++.pt/spec21.C |  3 +--
>   9 files changed, 57 insertions(+), 24 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/template/partial16.C
>   create mode 100644 gcc/testsuite/g++.dg/template/partial17.C
>   create mode 100644 gcc/testsuite/g++.dg/template/partial18.C
> 
> diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
> index f3fa9c192ad..6b0bc815404 100644
> --- a/gcc/cp/pt.c
> +++ b/gcc/cp/pt.c
> @@ -5157,11 +5157,7 @@ process_partial_specialization (tree decl)
>   		maintmpl);
>       }
>   
> -  /* [temp.class.spec]
> -
> -     A partially specialized non-type argument expression shall not
> -     involve template parameters of the partial specialization except
> -     when the argument expression is a simple identifier.
> +  /* [temp.spec.partial]
>   
>        The type of a template parameter corresponding to a specialized
>        non-type argument shall not be dependent on a parameter of the
> @@ -5221,13 +5217,6 @@ process_partial_specialization (tree decl)
>   	      && !((REFERENCE_REF_P (arg)
>   		    || TREE_CODE (arg) == VIEW_CONVERT_EXPR)
>   		   && TREE_CODE (TREE_OPERAND (arg, 0)) == TEMPLATE_PARM_INDEX))
> -            {
> -              if ((!packed_args && tpd.arg_uses_template_parms[i])
> -                  || (packed_args && uses_template_parms (arg)))
> -		error_at (cp_expr_loc_or_input_loc (arg),
> -			  "template argument %qE involves template "
> -			  "parameter(s)", arg);
> -              else
>               {
>   	      /* Look at the corresponding template parameter,
>   		 marking which template parameters its type depends
> @@ -5281,7 +5270,6 @@ process_partial_specialization (tree decl)
>               }
>           }
>       }
> -    }
>   
>     /* We should only get here once.  */
>     if (TREE_CODE (decl) == TYPE_DECL)
> @@ -10502,6 +10490,15 @@ for_each_template_parm_r (tree *tp, int *walk_subtrees, void *d)
>   	return error_mark_node;
>         break;
>   
> +    case TRAIT_EXPR:
> +    case PLUS_EXPR:
> +    case MULT_EXPR:
> +    case SCOPE_REF:
> +      /* These are always non-deduced contexts.  */
> +      if (!pfd->include_nondeduced_p)
> +	*walk_subtrees = 0;
> +      break;
> +
>       case MODOP_EXPR:
>       case CAST_EXPR:
>       case IMPLICIT_CONV_EXPR:
> @@ -10517,11 +10514,6 @@ for_each_template_parm_r (tree *tp, int *walk_subtrees, void *d)
>   	return error_mark_node;
>         break;
>   
> -    case SCOPE_REF:
> -      if (pfd->include_nondeduced_p)
> -	WALK_SUBTREE (TREE_OPERAND (t, 0));
> -      break;
> -
>       case REQUIRES_EXPR:
>         {
>   	if (!fn)
> diff --git a/gcc/testsuite/g++.dg/cpp0x/pr68724.C b/gcc/testsuite/g++.dg/cpp0x/pr68724.C
> index 4e99d53d5a9..6df7f718a7e 100644
> --- a/gcc/testsuite/g++.dg/cpp0x/pr68724.C
> +++ b/gcc/testsuite/g++.dg/cpp0x/pr68724.C
> @@ -9,7 +9,7 @@ struct integral_constant
>   integral_constant<bool, true> inst;
>   
>   template <typename _Tp>
> -struct integral_constant<bool, __is_enum(_Tp)> // { dg-error "32:template argument" }
> +struct integral_constant<bool, __is_enum(_Tp)> // { dg-error "not deducible" }
>   {
>   };
>   
> diff --git a/gcc/testsuite/g++.dg/cpp0x/variadic38.C b/gcc/testsuite/g++.dg/cpp0x/variadic38.C
> index b569404cdf2..9155445dce1 100644
> --- a/gcc/testsuite/g++.dg/cpp0x/variadic38.C
> +++ b/gcc/testsuite/g++.dg/cpp0x/variadic38.C
> @@ -3,4 +3,5 @@ template<int... Values>
>   struct int_vec {};
>   
>   template<int... Values>
> -struct int_vec<0, (Values+1)...> {}; // { dg-error "26:template argument" }
> +struct int_vec<0, (Values+1)...> {}; // { dg-error "not deducible" }
> +
> diff --git a/gcc/testsuite/g++.dg/cpp1z/pr81016.C b/gcc/testsuite/g++.dg/cpp1z/pr81016.C
> index 358b12056c0..a17afcc6b65 100644
> --- a/gcc/testsuite/g++.dg/cpp1z/pr81016.C
> +++ b/gcc/testsuite/g++.dg/cpp1z/pr81016.C
> @@ -1,4 +1,4 @@
>   // { dg-do compile { target c++17 } }
>   
>   template <typename a, a> struct b;
> -template <typename c> struct b<bool, c::d>; // { dg-error "template parameter" }
> +template <typename c> struct b<bool, c::d>; // { dg-error "not deducible" }
> diff --git a/gcc/testsuite/g++.dg/template/partial16.C b/gcc/testsuite/g++.dg/template/partial16.C
> new file mode 100644
> index 00000000000..30c34c3e0d9
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/template/partial16.C
> @@ -0,0 +1,8 @@
> +// [temp.spec.partial.general]/9
> +
> +template <class T, T t> struct C {};
> +template <class T> struct C<T, 1>;              // { dg-error "depends on a template parameter" }
> +
> +template< int X, int (*array_ptr)[X] > class A {};
> +int array[5];
> +template< int X > class A<X,&array> { };        // { dg-error "depends on a template parameter" }
> diff --git a/gcc/testsuite/g++.dg/template/partial17.C b/gcc/testsuite/g++.dg/template/partial17.C
> new file mode 100644
> index 00000000000..d5c82d26db3
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/template/partial17.C
> @@ -0,0 +1,14 @@
> +// [temp.spec.partial.match]/3
> +
> +template <int I, int J> struct A;
> +template <int I> struct A<I+5, I*2> {};     // { dg-error "not deducible" }
> +
> +template <int I> struct A<I, I> {};         // OK
> +
> +template <int I, int J, int K> struct B;
> +template <int I> struct B<I, I*2, I> {};    // OK
> +template <int I> struct B<I, I*2, 2> { typedef int type; };    // OK
> +
> +B<1, 2, 1> b1;
> +B<1, 2, 2>::type b2;
> +B<1, 2, 3> b3; // { dg-error "incomplete" }
> diff --git a/gcc/testsuite/g++.dg/template/partial18.C b/gcc/testsuite/g++.dg/template/partial18.C
> new file mode 100644
> index 00000000000..7b7614ebc6b
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/template/partial18.C
> @@ -0,0 +1,19 @@
> +// PR c++/96555
> +
> +template<class T, int i>
> +struct X;
> +
> +template<class T>
> +struct X<T, sizeof(T)> {};
> +
> +X<int, sizeof(int)> x1;
> +X<int, sizeof(int)+1> x2; // { dg-error "incomplete" }
> +
> +
> +struct A { int x; } a;
> +template<int, int> struct B;
> +template<int y>
> +struct B<y, sizeof(a.x)> { };
> +
> +B<0, sizeof(int)> b1;
> +B<0, sizeof(int)+1> b2; // { dg-error "incomplete" }
> diff --git a/gcc/testsuite/g++.dg/template/partial5.C b/gcc/testsuite/g++.dg/template/partial5.C
> index 40d8c45b087..037f684cbd2 100644
> --- a/gcc/testsuite/g++.dg/template/partial5.C
> +++ b/gcc/testsuite/g++.dg/template/partial5.C
> @@ -21,4 +21,4 @@ template<typename T, T V>
>   struct Z { };
>   
>   template<typename T>
> -struct Z<T, (T)0> { }; // { dg-error "13:template argument" }
> +struct Z<T, (T)0> { }; // { dg-error "depends on a template parameter" }
> diff --git a/gcc/testsuite/g++.old-deja/g++.pt/spec21.C b/gcc/testsuite/g++.old-deja/g++.pt/spec21.C
> index cf89d6b325a..bf25c0ebb39 100644
> --- a/gcc/testsuite/g++.old-deja/g++.pt/spec21.C
> +++ b/gcc/testsuite/g++.old-deja/g++.pt/spec21.C
> @@ -4,8 +4,7 @@ template <class T> struct S {};
>   template <class T = int> struct S<T*> {}; // { dg-error "" } default argument
>   
>   template <int I, int J> struct A {};
> -template <int I> struct A<I+5, I*2> {}; // { dg-error "28:template argument" }
> -// { dg-error "33:template argument" "" { target *-*-* } .-1 }
> +template <int I> struct A<I+5, I*2> {}; // { dg-error "not deducible" }
>   template <class T, T t> struct C {};
>   template <class T> struct C<T, 1>;  // { dg-error "" } type depends on parameter
>   int i;
>