Fix standalone comment before binary op causing unnecessary split

src/black/linegen.py

@@ -13,6 +13,7 @@
 from black.brackets import (
     COMMA_PRIORITY,
     DOT_PRIORITY,
+    LOGIC_PRIORITY,
     STRING_PRIORITY,
     get_leaves_inside_matching_brackets,
     max_delimiter_priority_in_atom,
@@ -1384,6 +1385,21 @@ def delimiter_split(
     ):
         raise CannotSplit("Splitting a single attribute from its owner looks wrong")
 
+    # When a standalone comment is the first token on the line and precedes an
+    # arithmetic/comparator expression, let standalone_comment_split handle it so
+    # the expression can stay on one line if it fits (issue #3713). Only applies
+    # when the comment leads the line (not trailing) and the delimiter has lower
+    # priority than logical operators to avoid changing `or`/`and` behavior.
+    if (
+        line.leaves
+        and line.leaves[0].type == STANDALONE_COMMENT
+        and delimiter_priority < LOGIC_PRIORITY
+    ):
+        raise CannotSplit(
+            "Standalone comment leads line before arithmetic delimiter; deferring to"
+            " standalone_comment_split"
+        )
+
     current_line = Line(
         mode=line.mode, depth=line.depth, inside_brackets=line.inside_brackets
     )

tests/data/cases/comment_before_binary_op.py

@@ -0,0 +1,42 @@
+my_func(
+    arg1=1,
+    arg2=[
+        func_call(
+            # Comment.
+            [MyClass(arg1, arg2)] * 10000
+        ),
+    ],
+)
+
+result = func_call(
+    # Comment.
+    value * 10000
+)
+
+result = func_call(
+    # Comment.
+    value + other_value
+)
+
+
+# output
+
+my_func(
+    arg1=1,
+    arg2=[
+        func_call(
+            # Comment.
+            [MyClass(arg1, arg2)] * 10000
+        ),
+    ],
+)
+
+result = func_call(
+    # Comment.
+    value * 10000
+)
+
+result = func_call(
+    # Comment.
+    value + other_value
+)

tests/data/cases/function_trailing_comma.py

@@ -252,9 +252,7 @@ def foo() -> (
 
 def foo() -> (
     # comment inside parenthesised new union return type
-    int
-    | str
-    | bytes
+    int | str | bytes
 ): ...