Fix walrus interaction with empty collections

mypy/checkexpr.py

@@ -4057,7 +4057,10 @@ def visit_assignment_expr(self, e: AssignmentExpr) -> Type:
         value = self.accept(e.value)
         self.chk.check_assignment(e.target, e.value)
         self.chk.check_final(e)
-        self.chk.store_type(e.target, value)
+        if not has_uninhabited_component(value):
+            # TODO: can we get rid of this extra store_type()?
+            # Usually, check_assignment() already stores the lvalue type correctly.
+            self.chk.store_type(e.target, value)
         self.find_partial_type_ref_fast_path(e.target)
         return value
 

test-data/unit/check-python38.test

@@ -826,3 +826,11 @@ main:5: error: Dict entry 0 has incompatible type "str": "str"; expected "str":
 main:5: error: Unpacked dict entry 1 has incompatible type "Dict[str, str]"; expected "SupportsKeysAndGetItem[str, int]"
     dct: Dict[str, int] = {"a": "b", **other}
                                        ^~~~~
+
+[case testWalrusAssignmentEmptyCollection]
+from typing import List
+
+y: List[int]
+if (y := []):
+    reveal_type(y)  # N: Revealed type is "builtins.list[builtins.int]"
+[builtins fixtures/list.pyi]