Always require using Literal for Literal type arguments

Previously we were allowing stuff like `GetArg[T, Base, 0]`.
This requires it to be `GetArg[T, Base, Literal[0]]`.

This is obviously kind of uglier but means we don't need to propose an
expansion of how literal type are treated. (And, importantly, I don't
think it's possible to reliably make *strings* work, because of how
strings are treated as type references...)

Author: msullivan

pre-pep.rst

@@ -214,7 +214,7 @@ grabs the argument to a ``Pointer``).
 
 ::
 
-    type PointerArg[T: Pointer] = GetArg[T, Pointer, 0]
+    type PointerArg[T: Pointer] = GetArg[T, Pointer, Literal[0]]
 
 ``AdjustLink`` sticks a ``list`` around ``MultiLink``, using features
 we've discussed already.

spec-draft.rst

@@ -111,8 +111,6 @@ It's important that there be a clearly specified type language for the type-leve
         # *[... for t in ...] arguments
         | <ident>[<variadic-type-arg(<type>)> +]
 
-        | <string-or-int-literal>  # Only accepted in arguments to new functions?
-
 
    # Type conditional checks are just boolean compositions of
    # subtype checking.

tests/test_fastapilike_2.py

@@ -46,7 +46,9 @@ class Field[T: FieldArgs](InitField[T]):
 ####
 
 # TODO: Should this go into the stdlib?
-type GetFieldItem[T: InitField, K] = GetAttr[GetArg[T, InitField, 0], K]
+type GetFieldItem[T: InitField, K] = GetAttr[
+    GetArg[T, InitField, Literal[0]], K
+]
 
 
 ##

tests/test_qblike.py

@@ -37,7 +37,7 @@ class Link[T]:
 
 # Conditional type alias!
 type FilterLinks[T] = (
-    Link[PropsOnly[GetArg[T, Link, 0]]] if IsSub[T, Link] else T
+    Link[PropsOnly[GetArg[T, Link, Literal[0]]]] if IsSub[T, Link] else T
 )
 
 

tests/test_qblike_2.py

@@ -108,7 +108,7 @@ def select[ModelT, K: BaseTypedDict](
 grabs the argument to a ``Pointer``).
 
 """
-type PointerArg[T: Pointer] = GetArg[T, Pointer, 0]
+type PointerArg[T: Pointer] = GetArg[T, Pointer, Literal[0]]
 
 """
 ``AdjustLink`` sticks a ``list`` around ``MultiLink``, using features

tests/test_type_dir.py

@@ -80,7 +80,7 @@ class Final(Mine, Ordinary, Wrapper[float], AnotherBase[float], Last[int]):
     pass
 
 
-type BaseArg[T] = GetArg[T, Base, 0] if IsSub[T, Base] else Never
+type BaseArg[T] = GetArg[T, Base, Literal[0]] if IsSub[T, Base] else Never
 
 
 type AllOptional[T] = NewProtocol[