Skip unreachable blocks in ThirdPass. Fixes #1319.

Author: Guido van Rossum

mypy/semanal.py

@@ -2388,6 +2388,11 @@ def visit_file(self, file_node: MypyFile, fnam: str) -> None:
         file_node.accept(self)
         self.errors.set_ignored_lines(set())
 
+    def visit_block(self, b: Block) -> None:
+        if b.is_unreachable:
+            return
+        super().visit_block(b)
+
     def visit_func_def(self, fdef: FuncDef) -> None:
         self.errors.push_function(fdef.name())
         self.analyze(fdef.type)

mypy/test/data/check-unreachable-code.test

@@ -89,3 +89,12 @@ if MYPY:
 else:
     None + ''
 [builtins fixtures/bool.py]
+
+[case testConditionalClassDefPY3]
+def f(): pass
+PY3 = f()
+if PY3:
+    pass
+else:
+    class X(object):
+        pass