Warn about unused type: ignore comments when error code is disabled (#18849)

Fixes #11059

Author: brianschubert

mypy/errors.py

@@ -506,10 +506,13 @@ def add_error_info(self, info: ErrorInfo) -> None:
                 # line == end_line for most nodes, so we only loop once.
                 for scope_line in lines:
                     if self.is_ignored_error(scope_line, info, self.ignored_lines[file]):
+                        err_code = info.code or codes.MISC
+                        if not self.is_error_code_enabled(err_code):
+                            # Error code is disabled - don't mark the current
+                            # "type: ignore" comment as used.
+                            return
                         # Annotation requests us to ignore all errors on this line.
-                        self.used_ignored_lines[file][scope_line].append(
-                            (info.code or codes.MISC).code
-                        )
+                        self.used_ignored_lines[file][scope_line].append(err_code.code)
                         return
             if file in self.ignored_files:
                 return

test-data/unit/check-errorcodes.test

@@ -105,6 +105,19 @@ x # type: ignore[name-defined, attr-defined] # E: Unused "type: ignore[attr-defi
 # flags: --warn-unused-ignores
 "x" # type: ignore[name-defined] # E: Unused "type: ignore" comment  [unused-ignore]
 
+[case testErrorCodeWarnUnusedIgnores7_WarnWhenErrorCodeDisabled]
+# flags: --warn-unused-ignores --disable-error-code name-defined
+x              # type: ignore                              # E: Unused "type: ignore" comment  [unused-ignore]
+x              # type: ignore[name-defined]                # E: Unused "type: ignore" comment  [unused-ignore]
+"x".foobar(y)  # type: ignore[name-defined, attr-defined]  # E: Unused "type: ignore[name-defined]" comment  [unused-ignore]
+
+[case testErrorCodeWarnUnusedIgnores8_IgnoreUnusedIgnore]
+# flags: --warn-unused-ignores --disable-error-code name-defined
+"x"  # type: ignore[unused-ignore]
+"x"  # type: ignore[name-defined, unused-ignore]
+"x"  # type: ignore[xyz, unused-ignore]
+x    # type: ignore[name-defined, unused-ignore]
+
 [case testErrorCodeMissingWhenRequired]
 # flags: --enable-error-code ignore-without-code
 "x" # type: ignore # E: "type: ignore" comment without error code  [ignore-without-code]