introduce llnl.util.compat to remove sys.version_info checks (#21720)

- also split typing.py into typing_extensions and add py2 shims

lib/spack/external/py2/typing.py

@@ -6,79 +6,77 @@
 This is a fake set of symbols to allow spack to import typing in python
 versions where we do not support type checking (<3)
 """
-Annotated = None
-Any = None
-Callable = None
-ForwardRef = None
-Generic = None
-Literal = None
-Optional = None
-Tuple = None
-TypeVar = None
-Union = None
-AbstractSet = None
-ByteString = None
-Container = None
-Hashable = None
-ItemsView = None
-Iterable = None
-Iterator = None
-KeysView = None
-Mapping = None
-MappingView = None
-MutableMapping = None
-MutableSequence = None
-MutableSet = None
-Sequence = None
-Sized = None
-ValuesView = None
-Awaitable = None
-AsyncIterator = None
-AsyncIterable = None
-Coroutine = None
-Collection = None
-AsyncGenerator = None
-AsyncContextManager = None
-Reversible = None
-SupportsAbs = None
-SupportsBytes = None
-SupportsComplex = None
-SupportsFloat = None
-SupportsInt = None
-SupportsRound = None
-ChainMap = None
-Dict = None
-List = None
-OrderedDict = None
-Set = None
-FrozenSet = None
-NamedTuple = None
-Generator = None
-AnyStr = None
-cast = None
+from collections import defaultdict
+
+# (1) Unparameterized types.
+Annotated = object
+Any = object
+AnyStr = object
+ByteString = object
+Counter = object
+Final = object
+Hashable = object
+NoReturn = object
+Sized = object
+SupportsAbs = object
+SupportsBytes = object
+SupportsComplex = object
+SupportsFloat = object
+SupportsIndex = object
+SupportsInt = object
+SupportsRound = object
+
+# (2) Parameterized types.
+AbstractSet = defaultdict(lambda: object)
+AsyncContextManager = defaultdict(lambda: object)
+AsyncGenerator = defaultdict(lambda: object)
+AsyncIterable = defaultdict(lambda: object)
+AsyncIterator = defaultdict(lambda: object)
+Awaitable = defaultdict(lambda: object)
+Callable = defaultdict(lambda: object)
+ChainMap = defaultdict(lambda: object)
+ClassVar = defaultdict(lambda: object)
+Collection = defaultdict(lambda: object)
+Container = defaultdict(lambda: object)
+ContextManager = defaultdict(lambda: object)
+Coroutine = defaultdict(lambda: object)
+DefaultDict = defaultdict(lambda: object)
+Deque = defaultdict(lambda: object)
+Dict = defaultdict(lambda: object)
+ForwardRef = defaultdict(lambda: object)
+FrozenSet = defaultdict(lambda: object)
+Generator = defaultdict(lambda: object)
+Generic = defaultdict(lambda: object)
+ItemsView = defaultdict(lambda: object)
+Iterable = defaultdict(lambda: object)
+Iterator = defaultdict(lambda: object)
+KeysView = defaultdict(lambda: object)
+List = defaultdict(lambda: object)
+Literal = defaultdict(lambda: object)
+Mapping = defaultdict(lambda: object)
+MappingView = defaultdict(lambda: object)
+MutableMapping = defaultdict(lambda: object)
+MutableSequence = defaultdict(lambda: object)
+MutableSet = defaultdict(lambda: object)
+NamedTuple = defaultdict(lambda: object)
+Optional = defaultdict(lambda: object)
+OrderedDict = defaultdict(lambda: object)
+Reversible = defaultdict(lambda: object)
+Sequence = defaultdict(lambda: object)
+Set = defaultdict(lambda: object)
+Tuple = defaultdict(lambda: object)
+Type = defaultdict(lambda: object)
+TypedDict = defaultdict(lambda: object)
+Union = defaultdict(lambda: object)
+ValuesView = defaultdict(lambda: object)
+
+# (3) Type variable declarations.
+TypeVar = lambda *args, **kwargs: None
+
+# (4) Functions.
+cast = lambda _type, x: x
 get_args = None
 get_origin = None
 get_type_hints = None
 no_type_check = None
 no_type_check_decorator = None
-NoReturn = None
-
-# these are the typing extension symbols
-ClassVar = None
-Final = None
-Protocol = None
-Type = None
-TypedDict = None
-ContextManager = None
-Counter = None
-Deque = None
-DefaultDict = None
-SupportsIndex = None
-final = None
-IntVar = None
-Literal = None
-NewType = None
-overload = None
-runtime_checkable = None
-Text = None
-TYPE_CHECKING = None

lib/spack/external/py2/typing_extensions.py

@@ -0,0 +1,26 @@
+# Copyright 2013-2021 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+"""
+This is a fake set of symbols to allow spack to import typing in python
+versions where we do not support type checking (<3)
+"""
+from collections import defaultdict
+
+# (1) Unparameterized types.
+IntVar = object
+Literal = object
+NewType = object
+Text = object
+
+# (2) Parameterized types.
+Protocol = defaultdict(lambda: object)
+
+# (3) Macro for avoiding evaluation except during type checking.
+TYPE_CHECKING = False
+
+# (4) Decorators.
+final = lambda x: x
+overload = lambda x: x
+runtime_checkable = lambda x: x