Fix crash in attrs_plugin when using a converter

mypy/plugin.py

@@ -93,6 +93,10 @@ def class_type(self, info: TypeInfo) -> Type:
     def lookup_fully_qualified(self, name: str) -> SymbolTableNode:
         raise NotImplementedError
 
+    @abstractmethod
+    def lookup_fully_qualified_or_none(self, name: str) -> Optional[SymbolTableNode]:
+        raise NotImplementedError
+
 
 # A context for a function hook that infers the return type of a function with
 # a special signature.

mypy/plugins/attrs.py

@@ -54,7 +54,7 @@ def argument(self, ctx: 'mypy.plugin.ClassDefContext') -> Argument:
         if self.converter_name:
             # When a converter is set the init_type is overriden by the first argument
             # of the converter method.
-            converter = ctx.api.lookup_fully_qualified(self.converter_name)
+            converter = ctx.api.lookup_fully_qualified_or_none(self.converter_name)
             if (converter
                     and converter.type
                     and isinstance(converter.type, CallableType)

test-data/unit/check-incremental.test

@@ -3745,6 +3745,28 @@ class C(A, B):
 [out1]
 [out2]
 
+[case testAttrsIncrementalConverterInSubmodule]
+# This test is currently broken (type of x should not be Any). But at least it doesn't crash.
+from a.a import A
+reveal_type(A)
+[file a/__init__.py]
+[file a/a.py]
+from typing import Optional
+def converter(s:Optional[int]) -> int:
+    ...
+
+import attr
+@attr.s
+class A:
+    x: int = attr.ib(converter=converter)
+
+[builtins fixtures/list.pyi]
+[out1]
+main:3: error: Revealed type is 'def (x: Any) -> a.a.A'
+[out2]
+main:3: error: Revealed type is 'def (x: Any) -> a.a.A'
+
+
 [case testAttrsIncrementalThreeRuns]
 from a import A
 A(5)