Fine-grained: Fix crash caused by unreachable class

Fixes #4596.

Author: JukkaL

mypy/server/astmerge.py

@@ -254,7 +254,9 @@ def fixup(self, node: SN) -> SN:
     def fixup_type(self, typ: Type) -> None:
         typ.accept(TypeReplaceVisitor(self.replacements))
 
-    def process_type_info(self, info: TypeInfo) -> None:
+    def process_type_info(self, info: Optional[TypeInfo]) -> None:
+        if info is None:
+            return
         # TODO: Additional things:
         # - declared_metaclass
         # - metaclass_type

test-data/unit/fine-grained.test

@@ -1861,3 +1861,59 @@ def f() -> None:
 [out]
 a.py:1: error: Need type annotation for 'y'
 ==
+
+[case testSkippedClass1]
+import a
+[file a.py]
+class A: pass
+[file a.py.2]
+import sys
+if sys.platform == 'xyz':
+    class A: pass
+[builtins fixtures/ops.pyi]
+[out]
+==
+
+[case testSkippedClass2]
+import a
+[file a.py]
+import sys
+if sys.platform == 'xyz':
+    class A: pass
+[file a.py.2]
+import sys
+if sys.platform == 'xyz':
+    class A: pass
+[builtins fixtures/ops.pyi]
+[out]
+==
+
+[case testSkippedClass3]
+import a
+[file a.py]
+import sys
+if sys.platform == 'xyz':
+    class A: pass
+[file a.py.2]
+class A: pass
+[builtins fixtures/ops.pyi]
+[out]
+==
+
+[case testSkippedClass4]
+import a
+[file a.py]
+import sys
+if sys.platform == 'xyz':
+    class A: pass
+else:
+    class A: pass
+[file a.py.2]
+import sys
+if sys.platform == 'xyz':
+    class A: pass
+else:
+    class A: pass
+[builtins fixtures/ops.pyi]
+[out]
+==