spec: various minor changes (#2013)

- Replace an incorrect use of "subtype" with "assignable"
- Remove discussion of str/bytes Literal types that seems geared towards
  Python 2. Instead, say straightforwardly that strings (str objects) and
  bytes objects are supported.
- Rewrite an example that relies on the int/float/complex special case.
  The example was correct but I think it's confusing to readers if we have
  examples unnecessarily rely on the special case. Instead, use example
  types with more straightforward behavior.

Author: JelleZijlstra

docs/spec/generics.rst

@@ -2000,7 +2000,7 @@ Bound Rules
    T1 = TypeVar("T1", bound=int)
    TypeVar("Ok", default=T1, bound=float)     # Valid
    TypeVar("AlsoOk", default=T1, bound=int)   # Valid
-   TypeVar("Invalid", default=T1, bound=str)  # Invalid: int is not a subtype of str
+   TypeVar("Invalid", default=T1, bound=str)  # Invalid: int is not assignable to str
 
 Constraint Rules
 ^^^^^^^^^^^^^^^^

docs/spec/literal.rst

@@ -98,18 +98,23 @@ expressions, and nothing else.
 Legal parameters for ``Literal`` at type check time
 """""""""""""""""""""""""""""""""""""""""""""""""""
 
-``Literal`` may be parameterized with literal ints, byte and unicode strings,
-bools, Enum values and ``None``. So for example, all of
+``Literal`` may be parameterized with literal ``int``, ``str``, ``bytes``,
+and ``bool`` objects, instances of ``enum.Enum`` subclasses, and ``None``. So for example, all of
 the following would be legal::
 
    Literal[26]
-   Literal[0x1A]  # Exactly equivalent to Literal[26]
+   Literal[0x1A]  # Equivalent to Literal[26]
    Literal[-4]
    Literal["hello world"]
+   Literal[u"hello world"]  # Equivalent to Literal["hello world"]
    Literal[b"hello world"]
-   Literal[u"hello world"]
    Literal[True]
-   Literal[Color.RED]  # Assuming Color is some enum
+
+   class Color(enum.Enum):
+       RED = 1
+       GREEN = 2
+
+   Literal[Color.RED]
    Literal[None]
 
 **Note:** Since the type ``None`` is inhabited by just a single
@@ -143,17 +148,6 @@ This should be exactly equivalent to the following type::
 
     Literal[1, 2, 3, "foo", 5] | None
 
-**Note:** String literal types like ``Literal["foo"]`` should subtype either
-bytes or unicode in the same way regular string literals do at runtime.
-
-For example, in Python 3, the type ``Literal["foo"]`` is equivalent to
-``Literal[u"foo"]``, since ``"foo"`` is equivalent to ``u"foo"`` in Python 3.
-
-Similarly, in Python 2, the type ``Literal["foo"]`` is equivalent to
-``Literal[b"foo"]`` -- unless the file includes a
-``from __future__ import unicode_literals`` import, in which case it would be
-equivalent to ``Literal[u"foo"]``.
-
 Illegal parameters for ``Literal`` at type check time
 """""""""""""""""""""""""""""""""""""""""""""""""""""
 

docs/spec/tuples.rst

@@ -91,7 +91,7 @@ Type Compatibility Rules
 ------------------------
 
 Because tuple contents are immutable, the element types of a tuple are covariant.
-For example, ``tuple[int, int]`` is a subtype of ``tuple[float, complex]``.
+For example, ``tuple[bool, int]`` is a subtype of ``tuple[int, object]``.
 
 As discussed above, a homogeneous tuple of arbitrary length is equivalent
 to a union of tuples of different lengths. That means ``tuple[()]``,