[C++] PR c++/45114 - Support alias templates

Hello,

This patch adds support for the alias-declaration feature of the c++11
specification, introduced by the paper N2258[1] and voted into in the
standard.  It's a derivative work of a preliminary patch attached by
Jason Merrill to PR C++/45114[2].

alias-declaration introduces a new syntax for declaring an alias for a
type.  Exactly like a typedef.  E.g:

    template<class T> struct Vector {};
    using IntVect = Vector<int>;

The concept generalizes to families of types as well.  That is, you
can define an alias for a family of types -- dubbed alias template.
E.g:

    template<class Elements, class Allocator> struct Vector {};
    template<class T> struct MyAllocactor {};
    template<class T> using Vec = Vector<T, MyAllocator<T> >;

    Vec<int> v; // <-- this is equivalent to
		// Vector<int, MyAllocator<int> > v; Neat heh? :-)

The approach taken by the patch is to represent the alias of a type the
same way we represent a typedef today and mark it as type alias using a
flag.  An alias for a type T is thus a variant of T with a new type
name.

By extension an alias template is represented almost like a class
template except that its pattern is a variant of the type at the right
hand side of the '=' of the alias template declaration.

To represent a specialization of an alias template it's important to
notice that there are two "template info" at stake, unlike with normal
template specializations.  There is the template info of the alias
specialization itself and the one of the underlying type we are
aliasing.  The latter has been historically stored in
CLASSTYPE_TEMPLATE_INFO; now we hang the former off of the type name
of the alias.  Using the right template info helps here.  :-)

Substituting into an alias template means substituting for the
parameters of the alias template itself, but also for the parameters
of the type it aliases.  This part was easy as it's already mostly
handled by the typedef handling code we are based on.

Instantiating an alias template means just instantiating the type it
aliases.  This part is also taken care of by construction as the
alias*ed* type is the main variant of the alias type.  And we already
instantiate only the main variant type of a given type.

[1]: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2258.pdf
[2]: http://gcc.gnu.org/PR45114

Bootstrapped and tested on x86_64-unknown-linux-gnu against trunk.

gcc/cp/

	* cp-tree.h (TYPE_DECL_ALIAS_P, SET_TYPE_DECL_ALIAS_P)
	(TYPE_DECL_NAMES_ALIAS_TEMPLATE_P, TYPE_ALIAS_P)
	(DECL_TYPE_TEMPLATE_P, DECL_ALIAS_TEMPLATE_P): New accessor macros.
	(TYPE_TEMPLATE_INFO): Get template info of an alias template
	specializations from its TYPE_DECL.
	(SET_TYPE_TEMPLATE_INFO): Set template info of alias template
	specializations into its TYPE_DECL.
	(DECL_CLASS_TEMPLATE_P): Re-write using the new
	DECL_TYPE_TEMPLATE_P.
	(enum cp_decl_spec): Add new ds_alias enumerator.
	(alias_type_or_template_p, alias_template_specialization_p):
	Declare new functions.
	* parser.c (cp_parser_alias_declaration): New static function.
	(cp_parser_check_decl_spec): Add "using" name for the `alias'
	declspec.
	(cp_parser_block_declaration): Handle alias-declaration in c++11.
	Update comment.
	(cp_parser_template_id): Handle specializations of alias
	templates.
	(cp_parser_member_declaration): Add alias-declaration production
	to comment.
	(cp_parser_template_declaration_after_export): Handle alias
	templates in c++11.
	* decl.c (grokdeclarator): Set TYPE_DECL_ALIAS_P on alias
	declarations.
	(grokfield): Move template creation after setting up the TYPE_DECL
	of the alias, so that the TEMPLATE_DECL of the alias template
	actually carries the right type-id of the alias declaration.
	* pt.c (get_aliased_type, strip_alias): New static functions.
	(alias_type_or_template_p)
	(alias_template_specialization_p): Define new public functions.
	(maybe_process_partial_specialization): Reject partial
	specializations of alias template.
	(push_template_decl_real): Assert that TYPE_DECLs of alias
	templates are different from those of class template.  Store
	template info onto the TYPE_DECL of the alias template.
	(convert_template_argument): Strip aliases from template
	arguments.
	(lookup_template_class_1): Handle the creation of the
	specialization of an alias template.
	(tsubst_decl): Create a substituted copy of the TYPE_DECL of an
	member alias template.
	(tsubst): Handle substituting into the type of an alias template.
	* semantics.c (finish_template_type): For alias templates, return
	the TYPE_DECL of the actual alias and not the one of the aliased
	type.
	* error.c (dump_type):  Print specialization of alias templates
	like we print specializations of class templates.  Also, handle
	the printing of alias templates.
	(dump_aggr_type): For specialization of alias templates, fetch
	arguments from the right place.
	(dump_decl): Print an alias-declaration like `using decl = type;'

gcc/testsuite/

	* g++.dg/cpp0x/alias-decl-0.C: New test case.
	* g++.dg/cpp0x/alias-decl-1.C: Likewise.
	* g++.dg/cpp0x/alias-decl-3.C: Likewise.
	* g++.dg/cpp0x/alias-decl-4.C: Likewise.
	* g++.dg/cpp0x/alias-decl-5.C: Likewise.
---
 gcc/cp/cp-tree.h                          |   70 ++++++++++++--
 gcc/cp/decl.c                             |    5 +
 gcc/cp/decl2.c                            |    9 +-
 gcc/cp/error.c                            |   31 +++++-
 gcc/cp/parser.c                           |   71 +++++++++++++-
 gcc/cp/pt.c                               |  153 +++++++++++++++++++++++++++--
 gcc/cp/semantics.c                        |   14 ++-
 gcc/testsuite/g++.dg/cpp0x/alias-decl-0.C |   33 ++++++
 gcc/testsuite/g++.dg/cpp0x/alias-decl-1.C |    5 +
 gcc/testsuite/g++.dg/cpp0x/alias-decl-2.C |   33 ++++++
 gcc/testsuite/g++.dg/cpp0x/alias-decl-3.C |   29 ++++++
 gcc/testsuite/g++.dg/cpp0x/alias-decl-4.C |   14 +++
 gcc/testsuite/g++.dg/cpp0x/alias-decl-5.C |   34 +++++++
 13 files changed, 470 insertions(+), 31 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/alias-decl-0.C
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/alias-decl-1.C
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/alias-decl-2.C
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/alias-decl-3.C
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/alias-decl-4.C
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/alias-decl-5.C

[C++] PR c++/45114 - Support alias templates

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