Fix crash with function redefinition

Fixes python#14027

mypy/checker.py

@@ -960,7 +960,11 @@ def _visit_func_def(self, defn: FuncDef) -> None:
                 # Function definition overrides a variable initialized via assignment or a
                 # decorated function.
                 orig_type = defn.original_def.type
-                assert orig_type is not None, f"Error checking function redefinition {defn}"
+                if orig_type is None:
+                    # I'm not sure why this happens, probably a bug elsewhere
+                    # See testConditionalFunctionDefinitionWithTuple
+                    # Setting orig_type to UninhabitedType ensures an error is reported
+                    orig_type = UninhabitedType()
                 if isinstance(orig_type, PartialType):
                     if orig_type.type is None:
                         # Ah this is a partial type. Give it the type of the function.

test-data/unit/check-functions.test

@@ -1475,6 +1475,17 @@ else:
     @dec
     def f(): pass
 
+[case testConditionalFunctionDefinitionWithTuple]
+def bar() -> None:
+    # weirdly, needs to be an empty tuple. empty lists and various other special assignments
+    # don't trigger the bug
+    cond = ()
+    if cond:
+        foo = 1
+    else:
+        def foo(obj): ...  # E: Incompatible redefinition (redefinition with type "Callable[[Any], Any]", original type <nothing>)
+[builtins fixtures/tuple.pyi]
+
 [case testConditionalRedefinitionOfAnUnconditionalFunctionDefinition1]
 from typing import Any
 def f(x: str) -> None: pass