Fix crash with function redefinition (#14064)

Fixes #14027 (issue was surfaced by #13509)

Author: hauntsaninja

mypy/checker.py

@@ -960,7 +960,10 @@ 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:
+                    # If other branch is unreachable, we don't type check it and so we might
+                    # not have a type for the original definition
+                    return
                 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 +1475,20 @@ else:
     @dec
     def f(): pass
 
+[case testConditionalFunctionDefinitionUnreachable]
+def bar() -> None:
+    if False:
+        foo = 1
+    else:
+        def foo(obj): ...
+
+def baz() -> None:
+    if False:
+        foo: int = 1
+    else:
+        def foo(obj): ...  # E: Incompatible redefinition (redefinition with type "Callable[[Any], Any]", original type "int")
+[builtins fixtures/tuple.pyi]
+
 [case testConditionalRedefinitionOfAnUnconditionalFunctionDefinition1]
 from typing import Any
 def f(x: str) -> None: pass