Fix crash when a dataclass with a no init InitVar is inherited

mypy/plugins/dataclasses.py

@@ -83,7 +83,12 @@ def transform(self) -> None:
             # Some definitions are not ready, defer() should be already called.
             return
         for attr in attributes:
-            if info[attr.name].type is None:
+            node = info.get(attr.name)
+            if node is None:
+                # Nodes of superclass InitVars not used in __init__ cannot be reached.
+                assert attr.is_init_var and not attr.is_in_init
+                continue
+            if node.type is None:
                 ctx.api.defer()
                 return
         decorator_arguments = {
@@ -184,7 +189,11 @@ def reset_init_only_vars(self, info: TypeInfo, attributes: List[DataclassAttribu
         """Remove init-only vars from the class and reset init var declarations."""
         for attr in attributes:
             if attr.is_init_var:
-                del info.names[attr.name]
+                if attr.name in info.names:
+                    del info.names[attr.name]
+                else:
+                    # Nodes of superclass InitVars not used in __init__ cannot be reached.
+                    assert attr.is_init_var and not attr.is_in_init
                 for stmt in info.defn.defs.body:
                     if isinstance(stmt, AssignmentStmt) and stmt.unanalyzed_type:
                         lvalue = stmt.lvalues[0]

test-data/unit/check-dataclasses.test

@@ -555,6 +555,22 @@ app.database_name  # E: "Application" has no attribute "database_name"
 class Yes: ...
 [builtins fixtures/list.pyi]
 
+[case testDataclassesNoInitInitVarInheritance]
+from dataclasses import dataclass, field, InitVar
+
+@dataclass
+class Super:
+    foo: InitVar = field(init=False)
+
+@dataclass
+class Sub(Super):
+    bar: int
+
+sub = Sub(5)
+sub.foo  # E: "Sub" has no attribute "foo"
+sub.bar
+[builtins fixtures/bool.pyi]
+
 [case testDataclassFactory]
 from typing import Type, TypeVar
 from dataclasses import dataclass