Allow unary + in Literal (#16729)

Implements python/typing#1550

Fixes #16727 (a trivial bug I found while implementing this feature)

mypy/fastparse.py

@@ -126,7 +126,7 @@
 import ast as ast3
 
 # TODO: Index, ExtSlice are deprecated in 3.9.
-from ast import AST, Attribute, Call, FunctionType, Index, Name, Starred, UnaryOp, USub
+from ast import AST, Attribute, Call, FunctionType, Index, Name, Starred, UAdd, UnaryOp, USub
 
 
 def ast3_parse(
@@ -1940,13 +1940,19 @@ def visit_Constant(self, n: Constant) -> Type:
 
     # UnaryOp(op, operand)
     def visit_UnaryOp(self, n: UnaryOp) -> Type:
-        # We support specifically Literal[-4] and nothing else.
-        # For example, Literal[+4] or Literal[~6] is not supported.
+        # We support specifically Literal[-4], Literal[+4], and nothing else.
+        # For example, Literal[~6] or Literal[not False] is not supported.
         typ = self.visit(n.operand)
-        if isinstance(typ, RawExpressionType) and isinstance(n.op, USub):
-            if isinstance(typ.literal_value, int):
+        if (
+            isinstance(typ, RawExpressionType)
+            # Use type() because we do not want to allow bools.
+            and type(typ.literal_value) is int  # noqa: E721
+        ):
+            if isinstance(n.op, USub):
                 typ.literal_value *= -1
                 return typ
+            if isinstance(n.op, UAdd):
+                return typ
         return self.invalid_type(n)
 
     def numeric_type(self, value: object, n: AST) -> Type:

test-data/unit/check-literal.test

@@ -591,7 +591,6 @@ from typing_extensions import Literal
 def dummy() -> int: return 3
 a: Literal[3 + 4]               # E: Invalid type: Literal[...] cannot contain arbitrary expressions
 b: Literal["  foo  ".trim()]    # E: Invalid type: Literal[...] cannot contain arbitrary expressions
-c: Literal[+42]                 # E: Invalid type: Literal[...] cannot contain arbitrary expressions
 d: Literal[~12]                 # E: Invalid type: Literal[...] cannot contain arbitrary expressions
 e: Literal[dummy()]             # E: Invalid type: Literal[...] cannot contain arbitrary expressions
 [builtins fixtures/tuple.pyi]
@@ -2739,16 +2738,22 @@ def test() -> None:
         ...
 [builtins fixtures/bool.pyi]
 
-[case testNegativeIntLiteral]
+[case testUnaryOpLiteral]
 from typing_extensions import Literal
 
 a: Literal[-2] = -2
 b: Literal[-1] = -1
 c: Literal[0] = 0
 d: Literal[1] = 1
 e: Literal[2] = 2
+f: Literal[+1] = 1
+g: Literal[+2] = 2
+
+x: Literal[+True] = True  # E: Invalid type: Literal[...] cannot contain arbitrary expressions
+y: Literal[-True] = -1  # E: Invalid type: Literal[...] cannot contain arbitrary expressions
+z: Literal[~0] = 0  # E: Invalid type: Literal[...] cannot contain arbitrary expressions
 [out]
-[builtins fixtures/float.pyi]
+[builtins fixtures/ops.pyi]
 
 [case testNegativeIntLiteralWithFinal]
 from typing_extensions import Literal, Final

test-data/unit/fixtures/ops.pyi

@@ -24,8 +24,6 @@ class tuple(Sequence[Tco]):
 
 class function: pass
 
-class bool: pass
-
 class str:
     def __init__(self, x: 'int') -> None: pass
     def __add__(self, x: 'str') -> 'str': pass
@@ -54,6 +52,8 @@ class int:
     def __gt__(self, x: 'int') -> bool: pass
     def __ge__(self, x: 'int') -> bool: pass
 
+class bool(int): pass
+
 class float:
     def __add__(self, x: 'float') -> 'float': pass
     def __radd__(self, x: 'float') -> 'float': pass