diff --git a/mypy/fixup.py b/mypy/fixup.py
index 281f5fc4a0ae..947f3bbb7afc 100644
--- a/mypy/fixup.py
+++ b/mypy/fixup.py
@@ -236,7 +236,10 @@ def lookup_qualified_typeinfo(modules: Dict[str, MypyFile], name: str,
         return node
     else:
         # Looks like a missing TypeInfo in quick mode, put something there
-        assert quick_and_dirty, "Should never get here in normal mode"
+        assert quick_and_dirty, "Should never get here in normal mode," \
+                                " got {}:{} instead of TypeInfo".format(type(node).__name__,
+                                                                        node.fullname() if node
+                                                                        else '')
         return stale_info(modules)
 
 
diff --git a/mypy/semanal.py b/mypy/semanal.py
index 1e24dd3ea1c1..3facb72cb729 100644
--- a/mypy/semanal.py
+++ b/mypy/semanal.py
@@ -1931,6 +1931,10 @@ def check_and_set_up_type_alias(self, s: AssignmentStmt) -> None:
         res, alias_tvars, depends_on, qualified_tvars = self.analyze_alias(rvalue)
         if not res:
             return
+        if (isinstance(res, Instance) and res.type.name() == lvalue.name and
+                res.type.module_name == self.cur_mod_id):
+            # Aliases like C = C is a no-op.
+            return
         s.is_alias_def = True
         node = self.lookup(lvalue.name, lvalue)
         assert node is not None
diff --git a/test-data/unit/check-incremental.test b/test-data/unit/check-incremental.test
index a902a25d5ed3..f386890280b4 100644
--- a/test-data/unit/check-incremental.test
+++ b/test-data/unit/check-incremental.test
@@ -5193,3 +5193,117 @@ def f(x: str) -> None: pass
 [out]
 [out2]
 tmp/main.py:2: error: Argument 1 to "f" has incompatible type "int"; expected "str"
+
+[case testOverrideByIdemAlias]
+import a
+[file a.py]
+import lib
+x = 1
+[file a.py.2]
+import lib
+x = 2
+[file lib.py]
+C = C
+class C:  # type: ignore
+    pass
+[out]
+[out2]
+
+[case testOverrideByIdemAliasReversed]
+import a
+[file a.py]
+import lib
+x = 1
+[file a.py.2]
+import lib
+x = 2
+[file lib.py]
+class C:
+    pass
+C = C  # type: ignore
+x: C
+[out]
+[out2]
+
+[case testOverrideByIdemAliasGeneric]
+import a
+[file a.py]
+import lib
+x = 1
+[file a.py.2]
+import lib
+x = 2
+[file lib.py]
+from typing import Generic, TypeVar
+
+T = TypeVar('T')
+
+class C(Generic[T]):
+    pass
+C = C[int]  # type: ignore
+x: C
+[out]
+[out2]
+
+[case testOverrideByIdemAliasImported]
+import a
+[file a.py]
+import lib
+x = 1
+[file a.py.2]
+import lib
+x = 2
+[file lib.py]
+from other import C
+C = C  # type: ignore
+x: C
+[file other.py]
+class C:
+    pass
+[out]
+[out2]
+
+[case testOverrideByIdemAliasImportedReversed]
+import a
+[file a.py]
+import lib
+x = 1
+[file a.py.2]
+import lib
+x = 2
+[file lib.py]
+C = C  # type: ignore
+from other import C
+[file other.py]
+class C:
+    pass
+[out]
+[out2]
+
+[case testConditionalExceptionAliasOverride]
+import a
+[file a.py]
+import lib
+try:
+    x = 1
+except lib.Exception as e:
+    pass
+[file a.py.2]
+import lib
+try:
+    x = 2
+except lib.Exception as e:
+    pass
+[file lib.py]
+try:
+    Exception = Exception
+except BaseException:
+    class Exception(BaseException): pass  # type: ignore
+
+try:
+    pass
+except Exception as e:
+    pass
+[builtins fixtures/exception.pyi]
+[out]
+[out2]
diff --git a/test-data/unit/check-type-aliases.test b/test-data/unit/check-type-aliases.test
index 44377d584cd8..b8ebe8105213 100644
--- a/test-data/unit/check-type-aliases.test
+++ b/test-data/unit/check-type-aliases.test
@@ -518,3 +518,42 @@ def foo(x: Bogus[int]) -> None:
     reveal_type(x)  # E: Revealed type is 'builtins.int'
 
 [builtins fixtures/dict.pyi]
+
+[case testOverrideByIdemAliasCorrectType]
+C = C
+class C:  # type: ignore
+    pass
+x: C
+reveal_type(x)  # E: Revealed type is '__main__.C'
+[out]
+
+[case testOverrideByIdemAliasCorrectTypeReversed]
+class C:
+    pass
+C = C  # type: ignore
+x: C
+reveal_type(x)  # E: Revealed type is '__main__.C'
+[out]
+
+[case testOverrideByIdemAliasCorrectTypeImported]
+from other import C as B
+C = B
+x: C
+reveal_type(x)  # E: Revealed type is 'other.C'
+[file other.py]
+class C:
+    pass
+[out]
+
+[case testConditionalExceptionAlias]
+try:
+    E = E
+except BaseException:
+    class E(BaseException): pass  # type: ignore
+
+try:
+    pass
+except E as e:
+    reveal_type(e)  # E: Revealed type is '__main__.E'
+[builtins fixtures/exception.pyi]
+[out]
diff --git a/test-data/unit/fixtures/exception.pyi b/test-data/unit/fixtures/exception.pyi
index b6810d41fd1f..f9bda1b2a042 100644
--- a/test-data/unit/fixtures/exception.pyi
+++ b/test-data/unit/fixtures/exception.pyi
@@ -14,3 +14,4 @@ class bool: pass
 class ellipsis: pass
 
 class BaseException: pass
+class Exception(BaseException): pass