Fix handling of name collisions

mypy/stubutil.py

@@ -635,6 +635,8 @@ def add_name(self, fullname: str, require: bool = True) -> str:
         """
         module, name = fullname.rsplit(".", 1)
         alias = "_" + name if name in self.defined_names else None
+        while alias in self.defined_names:
+            alias = "_" + alias
         if module != "builtins" or alias:  # don't import from builtins unless needed
             self.import_tracker.add_import_from(module, [(name, alias)], require=require)
         return alias or name

test-data/unit/stubgen.test

@@ -4292,6 +4292,21 @@ z_alias = _dict[str, float]
 v_alias = _set[int]
 w_alias = _list[_dict[str, _tuple[int, ...]]]
 
+[case testHandlingNameCollisions]
+# flags: --include-private
+from typing import Tuple
+tuple = int
+_tuple = range
+__tuple = map
+x: Tuple[int, str]
+[out]
+from builtins import tuple as ___tuple
+
+tuple = int
+_tuple = range
+__tuple = map
+x: ___tuple[int, str]
+
 [case testPEP570PosOnlyParams]
 def f(x=0, /): ...
 def f1(x: int, /): ...