c++: hide __is_deducible for GCC 13

I want to have more discussion about the interface before claiming the
__is_deducible name, so for GCC 13 make it internal-only.

gcc/ChangeLog:

	* doc/extend.texi: Comment out __is_deducible docs.

gcc/cp/ChangeLog:

	* cp-trait.def (IS_DEDUCIBLE): Add space to name.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/is_deducible1.C: Guard with
	__has_builtin (__is_deducible).

gcc/cp/cp-trait.def

@@ -84,7 +84,8 @@ DEFTRAIT_EXPR (IS_TRIVIALLY_COPYABLE, "__is_trivially_copyable", 1)
 DEFTRAIT_EXPR (IS_UNION, "__is_union", 1)
 DEFTRAIT_EXPR (REF_CONSTRUCTS_FROM_TEMPORARY, "__reference_constructs_from_temporary", 2)
 DEFTRAIT_EXPR (REF_CONVERTS_FROM_TEMPORARY, "__reference_converts_from_temporary", 2)
-DEFTRAIT_EXPR (IS_DEDUCIBLE, "__is_deducible", 2)
+/* FIXME Added space to avoid direct usage in GCC 13.  */
+DEFTRAIT_EXPR (IS_DEDUCIBLE, "__is_deducible ", 2)
 
 DEFTRAIT_TYPE (REMOVE_CV, "__remove_cv", 1)
 DEFTRAIT_TYPE (REMOVE_REFERENCE, "__remove_reference", 1)

gcc/doc/extend.texi

@@ -25213,9 +25213,10 @@ type.  A diagnostic is produced if this requirement is not met.
 If @code{type} is a cv-qualified class type, and not a union type
 ([basic.compound]) the trait is @code{true}, else it is @code{false}.
 
-@item __is_deducible (template, type)
-If template arguments for @code{template} can be deduced from
-@code{type} or obtained from default template arguments.
+@c FIXME Commented out for GCC 13, discuss user interface for GCC 14.
+@c @item __is_deducible (template, type)
+@c If template arguments for @code{template} can be deduced from
+@c @code{type} or obtained from default template arguments.
 
 @item __is_empty (type)
 If @code{__is_class (type)} is @code{false} then the trait is @code{false}.

gcc/testsuite/g++.dg/ext/is_deducible1.C

@@ -1,5 +1,7 @@
 // { dg-do compile { target c++20 } }
 
+#if __has_builtin (__is_deducible)
+
 template <class T> struct A { };
 template <class T> struct B { };
 
@@ -26,6 +28,8 @@ static_assert (__is_deducible (D, D<int>));
 template <class T, T N> using E = int[N];
 static_assert (__is_deducible (E, int[42]));
 
+#endif // __has_builtin (__is_deducible)
+
 // We don't try to support this.
 template <class T> void f(T);
-bool b = __is_deducible (f, void (int)); // { dg-error "class or alias" }
+bool b = __is_deducible (f, void (int)); // { dg-error "" }