Implement or-ing typemaps for Never-based unreachability

Author: A5rocks

mypy/checker.py

@@ -8136,6 +8136,7 @@ def and_conditional_maps(m1: TypeMap, m2: TypeMap, use_meet: bool = False) -> Ty
     result = m2.copy()
     m2_keys = {literal_hash(n2) for n2 in m2}
     for n1 in m1:
+        # TODO: should this check for UninhabitedType as well as AnyType?
         if literal_hash(n1) not in m2_keys or isinstance(get_proper_type(m1[n1]), AnyType):
             result[n1] = m1[n1]
     if use_meet:
@@ -8156,9 +8157,9 @@ def or_conditional_maps(m1: TypeMap, m2: TypeMap, coalesce_any: bool = False) ->
     joining restrictions.
     """
 
-    if m1 is None:
+    if m1 is None or any(isinstance(get_proper_type(t1), UninhabitedType) for t1 in m1.values()):
         return m2
-    if m2 is None:
+    if m2 is None or any(isinstance(get_proper_type(t2), UninhabitedType) for t2 in m2.values()):
         return m1
     # Both conditions can be true. Combine information about
     # expressions whose type is refined by both conditions. (We do not

test-data/unit/check-narrowing.test

@@ -741,8 +741,6 @@ def test3(switch: FlipFlopEnum) -> None:
     assert switch.state is State.B     # E: Non-overlapping identity check (left operand type: "Literal[State.A]", right operand type: "Literal[State.B]")
     reveal_type(switch.state)          # E: Statement is unreachable \
                                        # N: Revealed type is "Never"
-
-
 [builtins fixtures/primitives.pyi]
 
 [case testNarrowingEqualityRequiresExplicitStrLiteral]
@@ -939,7 +937,7 @@ if a == a == b:
     reveal_type(b)  # N: Revealed type is "Literal[1]"
 else:
     reveal_type(a)  # N: Revealed type is "Literal[1]"
-    reveal_type(b)  # N: Revealed type is "Union[Literal[1], Literal[2]]"
+    reveal_type(b)  # N: Revealed type is "Literal[2]"
 
 # In this case, it's ok for 'b' to narrow down to Literal[1] in the else case
 # since that's the only way 'b == 2' can be false
@@ -1336,6 +1334,7 @@ else:
 [typing fixtures/typing-typeddict.pyi]
 
 [case testNarrowingRuntimeCover]
+# flags: --warn-unreachable
 from typing import Dict, List, Union
 
 def unreachable(x: Union[str, List[str]]) -> None:
@@ -1344,7 +1343,8 @@ def unreachable(x: Union[str, List[str]]) -> None:
     elif isinstance(x, list):
         reveal_type(x)  # N: Revealed type is "builtins.list[builtins.str]"
     else:
-        reveal_type(x)  # No output: this branch is unreachable  # N: Revealed type is "Never"
+        reveal_type(x)  # E: Statement is unreachable \
+                        # N: Revealed type is "Never"
 
 def all_parts_covered(x: Union[str, List[str], List[int], int]) -> None:
     if isinstance(x, str):

test-data/unit/check-python310.test

@@ -2319,7 +2319,7 @@ def f3(e: int | str | bytes) -> int:
             return 0  # E: Statement is unreachable
         case str(x):
             return 0
-    reveal_type(e)  # N: Revealed type is "Union[builtins.int, builtins.bytes]"
+    reveal_type(e)  # N: Revealed type is "builtins.bytes"
     return 0
 
 def f4(e: int | str | bytes) -> int:
@@ -2330,7 +2330,7 @@ def f4(e: int | str | bytes) -> int:
             return 0  # E: Statement is unreachable
         case x if isinstance(x, str):
             return 0
-    reveal_type(e)  # N: Revealed type is "Union[builtins.int, builtins.str, builtins.bytes]"
+    reveal_type(e)  # N: Revealed type is "Union[builtins.int, builtins.bytes]"
     return 0
 
 [builtins fixtures/primitives.pyi]