Fix single item enum match type exhaustion (python#16966)

Fixes python#14109

---------

Co-authored-by: Shantanu Jain <[email protected]>

Author: superosku

mypy/checkpattern.py

@@ -202,7 +202,7 @@ def visit_value_pattern(self, o: ValuePattern) -> PatternType:
         typ = self.chk.expr_checker.accept(o.expr)
         typ = coerce_to_literal(typ)
         narrowed_type, rest_type = self.chk.conditional_types_with_intersection(
-            current_type, [get_type_range(typ)], o, default=current_type
+            current_type, [get_type_range(typ)], o, default=get_proper_type(typ)
         )
         if not isinstance(get_proper_type(narrowed_type), (LiteralType, UninhabitedType)):
             return PatternType(narrowed_type, UnionType.make_union([narrowed_type, rest_type]), {})

test-data/unit/check-python310.test

@@ -1369,6 +1369,27 @@ match m3:
         reveal_type(m3)  # N: Revealed type is "Tuple[Union[builtins.int, builtins.str]]"
 [builtins fixtures/tuple.pyi]
 
+[case testMatchEnumSingleChoice]
+from enum import Enum
+from typing import NoReturn
+
+def assert_never(x: NoReturn) -> None: ...
+
+class Medal(Enum):
+    gold = 1
+
+def f(m: Medal) -> None:
+    always_assigned: int | None = None
+    match m:
+        case Medal.gold:
+            always_assigned = 1
+            reveal_type(m)  # N: Revealed type is "Literal[__main__.Medal.gold]"
+        case _:
+            assert_never(m)
+
+    reveal_type(always_assigned)  # N: Revealed type is "builtins.int"
+[builtins fixtures/bool.pyi]
+
 [case testMatchLiteralPatternEnumNegativeNarrowing]
 from enum import Enum
 class Medal(Enum):
@@ -1388,10 +1409,13 @@ def f(m: Medal) -> int:
 def g(m: Medal) -> int:
     match m:
         case Medal.gold:
+            reveal_type(m)  # N: Revealed type is "Literal[__main__.Medal.gold]"
             return 0
         case Medal.silver:
+            reveal_type(m)  # N: Revealed type is "Literal[__main__.Medal.silver]"
             return 1
         case Medal.bronze:
+            reveal_type(m)  # N: Revealed type is "Literal[__main__.Medal.bronze]"
             return 2
 
 [case testMatchLiteralPatternEnumCustomEquals-skip]