[PATCH v14 19/26] c++: Implement __decay built-in trait

[Jason Merrill]

On 2/28/24 11:26, Ken Matsui wrote:
> This patch implements built-in trait for std::decay.

OK.

> gcc/cp/ChangeLog:
> 
> 	* cp-trait.def: Define __decay.
> 	* semantics.cc (finish_trait_type): Handle CPTK_DECAY.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/ext/has-builtin-1.C: Test existence of __decay.
> 	* g++.dg/ext/decay.C: New test.
> 
> Signed-off-by: Ken Matsui <kmatsui@gcc.gnu.org>
> ---
>   gcc/cp/cp-trait.def                      |  1 +
>   gcc/cp/semantics.cc                      | 12 ++++++++++++
>   gcc/testsuite/g++.dg/ext/decay.C         | 22 ++++++++++++++++++++++
>   gcc/testsuite/g++.dg/ext/has-builtin-1.C |  3 +++
>   4 files changed, 38 insertions(+)
>   create mode 100644 gcc/testsuite/g++.dg/ext/decay.C
> 
> diff --git a/gcc/cp/cp-trait.def b/gcc/cp/cp-trait.def
> index 173818adf79..2d1cb7c227c 100644
> --- a/gcc/cp/cp-trait.def
> +++ b/gcc/cp/cp-trait.def
> @@ -51,6 +51,7 @@
>   DEFTRAIT_TYPE (ADD_LVALUE_REFERENCE, "__add_lvalue_reference", 1)
>   DEFTRAIT_TYPE (ADD_POINTER, "__add_pointer", 1)
>   DEFTRAIT_TYPE (ADD_RVALUE_REFERENCE, "__add_rvalue_reference", 1)
> +DEFTRAIT_TYPE (DECAY, "__decay", 1)
>   DEFTRAIT_EXPR (HAS_NOTHROW_ASSIGN, "__has_nothrow_assign", 1)
>   DEFTRAIT_EXPR (HAS_NOTHROW_CONSTRUCTOR, "__has_nothrow_constructor", 1)
>   DEFTRAIT_EXPR (HAS_NOTHROW_COPY, "__has_nothrow_copy", 1)
> diff --git a/gcc/cp/semantics.cc b/gcc/cp/semantics.cc
> index 19d6f87a9ea..45dc509855a 100644
> --- a/gcc/cp/semantics.cc
> +++ b/gcc/cp/semantics.cc
> @@ -12801,6 +12801,18 @@ finish_trait_type (cp_trait_kind kind, tree type1, tree type2,
>   	return type1;
>         return cp_build_reference_type (type1, /*rval=*/true);
>   
> +    case CPTK_DECAY:
> +      if (TYPE_REF_P (type1))
> +	type1 = TREE_TYPE (type1);
> +
> +      if (TREE_CODE (type1) == ARRAY_TYPE)
> +	return finish_trait_type (CPTK_ADD_POINTER, TREE_TYPE (type1), type2,
> +				  complain);
> +      else if (TREE_CODE (type1) == FUNCTION_TYPE)
> +	return finish_trait_type (CPTK_ADD_POINTER, type1, type2, complain);
> +      else
> +	return cv_unqualified (type1);
> +
>       case CPTK_REMOVE_ALL_EXTENTS:
>         return strip_array_types (type1);
>   
> diff --git a/gcc/testsuite/g++.dg/ext/decay.C b/gcc/testsuite/g++.dg/ext/decay.C
> new file mode 100644
> index 00000000000..8adedfeefe6
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/ext/decay.C
> @@ -0,0 +1,22 @@
> +// { dg-do compile { target c++11 } }
> +
> +#define SA(X) static_assert((X),#X)
> +
> +// Positive tests.
> +using test1_type = __decay(bool);
> +SA(__is_same(test1_type, bool));
> +
> +// NB: DR 705.
> +using test2_type = __decay(const int);
> +SA(__is_same(test2_type, int));
> +
> +using test3_type = __decay(int[4]);
> +SA(__is_same(test3_type, __remove_extent(int[4])*));
> +
> +using fn_type = void ();
> +using test4_type = __decay(fn_type);
> +SA(__is_same(test4_type, __add_pointer(fn_type)));
> +
> +using cfn_type = void () const;
> +using test5_type = __decay(cfn_type);
> +SA(__is_same(test5_type, cfn_type));
> diff --git a/gcc/testsuite/g++.dg/ext/has-builtin-1.C b/gcc/testsuite/g++.dg/ext/has-builtin-1.C
> index c2503c5d82b..3aca273aad6 100644
> --- a/gcc/testsuite/g++.dg/ext/has-builtin-1.C
> +++ b/gcc/testsuite/g++.dg/ext/has-builtin-1.C
> @@ -11,6 +11,9 @@
>   #if !__has_builtin (__add_rvalue_reference)
>   # error "__has_builtin (__add_rvalue_reference) failed"
>   #endif
> +#if !__has_builtin (__decay)
> +# error "__has_builtin (__decay) failed"
> +#endif
>   #if !__has_builtin (__builtin_addressof)
>   # error "__has_builtin (__builtin_addressof) failed"
>   #endif