fix: Ignore empty error codes from 'type: ignore' (#20052)

`type: ignore` comments with leading, trailing, or
repeated commas could lead to empty strings being parsed as valid
error codes.

This resulted in incorrect `[unused-ignore]` warnings when using the
`--warn-unused-ignores` flag, as the empty code '' was considered
a valid but unused ignore rule.

This commit improves the parsing logic to filter out any empty strings
that result from splitting the error codes.

fixes #20038

Author: codenamenam

mypy/fastparse.py

@@ -286,7 +286,7 @@ def parse_type_ignore_tag(tag: str | None) -> list[str] | None:
     if m is None:
         # Invalid "# type: ignore" comment.
         return None
-    return [code.strip() for code in m.group(1).split(",")]
+    return [stripped_code for code in m.group(1).split(",") if (stripped_code := code.strip())]
 
 
 def parse_type_comment(

test-data/unit/check-errorcodes.test

@@ -118,6 +118,20 @@ x              # type: ignore[name-defined]                # E: Unused "type: ig
 "x"  # type: ignore[xyz, unused-ignore]
 x    # type: ignore[name-defined, unused-ignore]
 
+[case testErrorCodeWarnUnusedIgnores9_SloppyFormattingNotEmptyBrackets]
+# flags: --warn-unused-ignores
+1 + "ok" + "ok".foo # type: ignore[  operator,attr-defined]
+1 + "ok" + "ok".foo # type: ignore[  ,operator,attr-defined]
+1 + "ok" + "ok".foo # type: ignore[  operator,attr-defined,]
+1 + "ok" + "ok".foo # type: ignore[  operator,attr-defined,,]
+
+[case testErrorCodeWarnUnusedIgnores10_SloppyFormattingEmptyBrackets]
+# flags: --warn-unused-ignores
+1+1 # type: ignore    # E: Unused "type: ignore" comment  [unused-ignore]
+1+1 # type: ignore[]  # E: Unused "type: ignore" comment  [unused-ignore]
+1+1 # type: ignore[ ] # E: Unused "type: ignore" comment  [unused-ignore]
+1+1 # type: ignore[,,, ] # E: Unused "type: ignore" comment  [unused-ignore]
+
 [case testErrorCodeMissingWhenRequired]
 # flags: --enable-error-code ignore-without-code
 "x" # type: ignore # E: "type: ignore" comment without error code  [ignore-without-code]