Always re-process annotations refering to an alias on update (#13759)

ilevkivskyi · web-flow · commit 0f4e0fb1e55f · 2022-09-29T13:23:40.000+01:00
This should fix a crash discovered by `mypy_primer` in #13516 The logic here is that when type aliases have placeholders they are updated _in place_ (the node gets a new resolved target type). But this means that some variables annotated with type aliases may not get stored types, unless we defer targets where they are defined. This didn't cause troubles before, because we waited for type alias to be complete before putting it into symbol table. Now it is not possible, we need to put something into symbol table for partially complete aliases to support recursive aliases (similar to recursive classes). Also, it was tricky to come up with a repro for this issue, because when it happens, the variable gets a "silent" `Any` type, and only when it appears in a dataclass, it causes a crash.
diff --git a/mypy/semanal.py b/mypy/semanal.py
@@ -3032,6 +3032,7 @@ def process_type_annotation(self, s: AssignmentStmt) -> None:
             analyzed = self.anal_type(s.type, allow_tuple_literal=allow_tuple_literal)
             # Don't store not ready types (including placeholders).
             if analyzed is None or has_placeholder(analyzed):
+                self.defer(s)
                 return
             s.type = analyzed
             if (
diff --git a/test-data/unit/check-dataclasses.test b/test-data/unit/check-dataclasses.test
@@ -1913,3 +1913,23 @@ class MyDataclass:
 
 takes_cp(MyDataclass)
 [builtins fixtures/dataclasses.pyi]
+
+[case testDataclassTypeAnnotationAliasUpdated]
+# flags: --enable-recursive-aliases
+import a
+[file a.py]
+from dataclasses import dataclass
+from b import B
+
+@dataclass
+class D:
+    x: B
+
+reveal_type(D)  # N: Revealed type is "def (x: builtins.list[b.C]) -> a.D"
+[file b.py]
+from typing import List
+import a
+B = List[C]
+class C(CC): ...
+class CC: ...
+[builtins fixtures/dataclasses.pyi]
