c++: add __is_deducible trait [PR105841]

C++20 class template argument deduction for an alias template involves
adding a constraint that the template arguments for the alias template can
be deduced from the return type of the deduction guide for the underlying
class template.  In the standard, this is modeled as defining a class
template with a partial specialization, but it's much more efficient to
implement with a trait that directly tries to perform the deduction.

The first argument to the trait is a template rather than a type, so various
places needed to be adjusted to accommodate that.

	PR c++/105841

gcc/ChangeLog:

	* doc/extend.texi (Type Traits):: Document __is_deducible.

gcc/cp/ChangeLog:

	* cp-trait.def (IS_DEDUCIBLE): New.
	* cxx-pretty-print.cc (pp_cxx_trait): Handle non-type.
	* parser.cc (cp_parser_trait): Likewise.
	* tree.cc (cp_tree_equal): Likewise.
	* pt.cc (tsubst_copy_and_build): Likewise.
	(type_targs_deducible_from): New.
	(alias_ctad_tweaks): Use it.
	* semantics.cc (trait_expr_value): Handle CPTK_IS_DEDUCIBLE.
	(finish_trait_expr): Likewise.
	* constraint.cc (diagnose_trait_expr): Likewise.
	* cp-tree.h (type_targs_deducible_from): Declare.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/is_deducible1.C: New test.

1 files changed, 4 insertions, 0 deletions

diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index c112291..b64a857 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -25213,6 +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.
+
 @item __is_empty (type)
 If @code{__is_class (type)} is @code{false} then the trait is @code{false}.
 Otherwise @code{type} is considered empty if and only if: @code{type}