Fix crash on walrus assignment shadowing partial type in comprehension

mypy/checker.py

@@ -3710,6 +3710,10 @@ def try_infer_partial_generic_type_from_assignment(
                 return
             rvalue_type = self.expr_checker.accept(rvalue)
             rvalue_type = get_proper_type(rvalue_type)
+            if var not in partial_types:
+                # Evaluating rvalue may have already resolved this partial type
+                # (e.g. a walrus assignment to the same variable inside rvalue).
+                return
             if isinstance(rvalue_type, Instance):
                 if rvalue_type.type == typ.type and is_valid_inferred_type(
                     rvalue_type, self.options

test-data/unit/check-python38.test

@@ -455,6 +455,15 @@ def check_partial_list() -> None:
     reveal_type(z)  # N: Revealed type is "builtins.list[builtins.int]"
 [builtins fixtures/list.pyi]
 
+[case testWalrusPartialTypeShadowedInComprehension]
+# https://github.com/python/mypy/issues/20792
+errors = []
+errors = [
+    i for i in [1, 2, 3] if (errors := [1])
+]
+reveal_type(errors)  # N: Revealed type is "builtins.list[builtins.int]"
+[builtins fixtures/list.pyi]
+
 [case testWalrusAssignmentAndConditionScopeForLiteral]
 # flags: --warn-unreachable