solidity/test/libsolidity/syntaxTests/modifiers/use_on_interface_function.sol

interface I {
  function f() external m pure returns (uint);
  modifier m() { _; }
}
// ----
// SyntaxError 5842: (16-60): Functions in interfaces cannot have modifiers.
// TypeError 6408: (63-82='modifier m() { _; }'): Modifiers cannot be defined or declared in interfaces.