fix crash with long type annotations (#1093)

black.py

@@ -2601,9 +2601,11 @@ def bracket_split_build_line(
             # Since body is a new indent level, remove spurious leading whitespace.
             normalize_prefix(leaves[0], inside_brackets=True)
             # Ensure a trailing comma for imports and standalone function arguments, but
-            # be careful not to add one after any comments.
-            no_commas = original.is_def and not any(
-                l.type == token.COMMA for l in leaves
+            # be careful not to add one after any comments or within type annotations.
+            no_commas = (
+                original.is_def
+                and opening_bracket.value == "("
+                and not any(l.type == token.COMMA for l in leaves)
             )
 
             if original.is_import or no_commas:

tests/data/function_trailing_comma.py

@@ -4,6 +4,11 @@ def f(a,):
 def f(a:int=1,):
     ...
 
+def xxxxxxxxxxxxxxxxxxxxxxxxxxxx() -> Set[
+    "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
+]:
+    pass
+
 # output
 
 def f(a,):
@@ -12,3 +17,9 @@ def f(a,):
 
 def f(a: int = 1,):
     ...
+
+
+def xxxxxxxxxxxxxxxxxxxxxxxxxxxx() -> Set[
+    "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
+]:
+    pass