Stop changing return type annotations to tuples

This fixes a bug where a trailing comma would be added to a
parenthesized return type annotation changing its type to a tuple.
Here's one case where this bug shows up:

```
def spam() -> (
    this_is_a_long_type_annotation_which_should_NOT_get_a_trailing_comma
):
    pass
```

The root problem was that the type annotation was treated as if it was
a collection (is_body=True to linegen::bracket_split_build_line) where
a trailing comma is usually fine. Now there's another check in the
aforementioned function to make sure the body it's operating on isn't
a parenthesized return type annotation before truly adding a trailing
comma.

CHANGES.md

@@ -2,7 +2,11 @@
 
 ## Unreleased
 
+### _Black_
+
 - Moved from `appdirs` dependency to `platformdirs` (#2375)
+- Avoid changing a function return type annotation's type to a tuple by adding a
+  trailing comma (#2384)
 
 ## 21.7b0
 

src/black/linegen.py

@@ -7,7 +7,7 @@
 
 from dataclasses import dataclass, field
 
-from black.nodes import WHITESPACE, STATEMENT, STANDALONE_COMMENT
+from black.nodes import WHITESPACE, RARROW, STATEMENT, STANDALONE_COMMENT
 from black.nodes import ASSIGNMENTS, OPENING_BRACKETS, CLOSING_BRACKETS
 from black.nodes import Visitor, syms, first_child_is_arith, ensure_visible
 from black.nodes import is_docstring, is_empty_tuple, is_one_tuple, is_one_tuple_between
@@ -575,8 +575,20 @@ def bracket_split_build_line(
                 and opening_bracket.value == "("
                 and not any(leaf.type == token.COMMA for leaf in leaves)
             )
+            # Enables us to avoid putting a comma after a parenthesesized (return) type
+            # annotation (and changing its type to a tuple) in a case like this one:
+            #
+            # def spam() -> (
+            #     this_is_a_long_type_annotation_which_should_NOT_get_a_trailing_comma
+            # ):
+            #     pass
+            type_annotation = (
+                isinstance(leaves[0].parent, Node)
+                and leaves[0].parent.prev_sibling
+                and leaves[0].parent.prev_sibling.type == RARROW
+            )
 
-            if original.is_import or no_commas:
+            if (original.is_import or no_commas) and not type_annotation:
                 for i in range(len(leaves) - 1, -1, -1):
                     if leaves[i].type == STANDALONE_COMMENT:
                         continue

src/black/nodes.py

@@ -135,6 +135,8 @@
 BRACKETS = OPENING_BRACKETS | CLOSING_BRACKETS
 ALWAYS_NO_SPACE = CLOSING_BRACKETS | {token.COMMA, STANDALONE_COMMENT}
 
+RARROW = 55
+
 
 class Visitor(Generic[T]):
     """Basic lib2to3 visitor that yields things of type `T` on `visit()`."""

tests/data/function_trailing_comma.py

@@ -21,6 +21,22 @@ def xxxxxxxxxxxxxxxxxxxxxxxxxxxx() -> Set[
 ]:
     json = {"k": {"k2": {"k3": [1,]}}}
 
+
+
+# The type annotation shouldn't get a trailing comma since that would change its type.
+# Relevant bug report: https://github.com/psf/black/issues/2381.
+def some_function_with_a_really_long_name() -> (
+    returning_a_deeply_nested_import_of_a_type_i_suppose
+):
+    pass
+
+
+def some_method_with_a_really_long_name(very_long_parameter_so_yeah: str, another_long_parameter: int) -> (
+    another_case_of_returning_a_deeply_nested_import_of_a_type_i_suppose_cause_why_not
+):
+    pass
+
+
 # output
 
 def f(
@@ -85,4 +101,20 @@ def xxxxxxxxxxxxxxxxxxxxxxxxxxxx() -> Set[
                 ]
             }
         }
-    }
+    }
+
+
+# The type annotation shouldn't get a trailing comma since that would change its type.
+# Relevant bug report: https://github.com/psf/black/issues/2381.
+def some_function_with_a_really_long_name() -> (
+    returning_a_deeply_nested_import_of_a_type_i_suppose
+):
+    pass
+
+
+def some_method_with_a_really_long_name(
+    very_long_parameter_so_yeah: str, another_long_parameter: int
+) -> (
+    another_case_of_returning_a_deeply_nested_import_of_a_type_i_suppose_cause_why_not
+):
+    pass