Remove Never annotations when merging pyi files.

`Never` is a valid type annotation, but it's most likely wrong if it's inferred by pytype, and it has a chain effect that all downstream code starts to get treated as unreachable.

Fixing pytype to not infer `Never` on return types should be better, but this should be dealt as a separate fix.

PiperOrigin-RevId: 684355324

pytype/tools/merge_pyi/merge_pyi.py

@@ -31,20 +31,28 @@ def _merge_csts(*, py_tree, pyi_tree):
   ).transform_module(py_tree)
 
 
-class RemoveAnyTransformer(cst.CSTTransformer):
-  """Transform away every `Any` annotations in function returns and variable assignments."""
+class RemoveAnyNeverTransformer(cst.CSTTransformer):
+  """Transform away every `Any` and `Never` annotations in function returns and variable assignments.
 
-  def _is_any_annotation(self, annotation: expression.Annotation | None):
+  For putting 'Any's, it's basically a no-op, and it doesn't help readability
+  so better not put anything when pytype gives up.
+
+  Having 'Never' annotated on function returns and variables is valid, but
+  they're most likely wrong if it's inferred by pytype, and it has a chain
+  effect that all downstream code starts to get treated as unreachable.
+  """
+
+  def _is_any_or_never(self, annotation: expression.Annotation | None):
     return (
         annotation
         and isinstance(annotation, expression.Name)
-        and annotation.value == "Any"
+        and annotation.value in ("Any", "Never")
     )
 
   def leave_FunctionDef(
       self, original_node: cst.FunctionDef, updated_node: cst.FunctionDef
   ) -> cst.CSTNode:
-    if original_node.returns and self._is_any_annotation(
+    if original_node.returns and self._is_any_or_never(
         original_node.returns.annotation
     ):
       return updated_node.with_changes(returns=None)
@@ -53,7 +61,7 @@ def leave_FunctionDef(
   def leave_AnnAssign(
       self, original_node: cst.AnnAssign, updated_node: cst.AnnAssign
   ) -> cst.CSTNode:
-    if self._is_any_annotation(original_node.annotation):
+    if self._is_any_or_never(original_node.annotation):
       return cst.Assign(
           targets=[cst.AssignTarget(target=updated_node.target)],
           value=updated_node.value,
@@ -100,7 +108,7 @@ def merge_sources(*, py: str, pyi: str) -> str:
     py_cst = cst.parse_module(py)
     pyi_cst = (
         cst.parse_module(pyi)
-        .visit(RemoveAnyTransformer())
+        .visit(RemoveAnyNeverTransformer())
         .visit(RemoveTrivialTypesTransformer())
     )
     merged_cst = _merge_csts(py_tree=py_cst, pyi_tree=pyi_cst)

pytype/tools/merge_pyi/test_data/func_annot.pep484.py

@@ -1,2 +1,6 @@
 def f() -> int:
   return 1
+
+# The Never type in pyi file should not be merged here.
+def g():
+  raise Exception("hi")

pytype/tools/merge_pyi/test_data/func_annot.py

@@ -1,2 +1,6 @@
 def f():
   return 1
+
+# The Never type in pyi file should not be merged here.
+def g():
+  raise Exception("hi")

pytype/tools/merge_pyi/test_data/func_annot.pyi

@@ -1 +1,4 @@
+from typings import Never
+
 def f() -> int: ...
+def g() -> Never: ...