Fix for issue #10514 and #17118

Fixes type narrowing for negative integer literals

mypy/plugins/default.py

@@ -489,7 +489,7 @@ def int_neg_callback(ctx: MethodContext, multiplier: int = -1) -> Type:
                 return ctx.type.copy_modified(
                     last_known_value=LiteralType(
                         value=multiplier * value,
-                        fallback=ctx.type,
+                        fallback=fallback,
                         line=ctx.type.line,
                         column=ctx.type.column,
                     )

test-data/unit/check-narrowing.test

@@ -2089,3 +2089,28 @@ if isinstance(x, (Z, NoneType)):  # E: Subclass of "X" and "Z" cannot exist: "Z"
     reveal_type(x)  # E: Statement is unreachable
 
 [builtins fixtures/isinstance.pyi]
+
+[case testTypeNarrowingReachableNegative]
+# flags: --warn-unreachable
+from typing import Literal
+
+x: Literal[-1]
+
+if x == -1:
+    assert True
+
+[typing fixtures/typing-medium.pyi]
+[builtins fixtures/ops.pyi]
+
+[case testTypeNarrowingReachableNegativeUnion]
+from typing import Literal
+
+x: Literal[-1, 1]
+
+if x == -1:
+    reveal_type(x)  # N: Revealed type is "Literal[-1]"
+else:
+    reveal_type(x)  # N: Revealed type is "Literal[1]"
+
+[typing fixtures/typing-medium.pyi]
+[builtins fixtures/ops.pyi]