diff --git a/mypy/semanal.py b/mypy/semanal.py
index 58e47f14ed94..563e9a580d2b 100644
--- a/mypy/semanal.py
+++ b/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)
diff --git a/mypy/test/data/check-unreachable-code.test b/mypy/test/data/check-unreachable-code.test
index 8aa0fa6eef38..00a67991fbdb 100644
--- a/mypy/test/data/check-unreachable-code.test
+++ b/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