This is the diagnostic produced for similar example using new diagnostic architecture:
error: binary operator '+' cannot be applied to operands of type 'String' and 'Int'
_ = "a" + 1
~~~ ^ ~
note: overloads for '+' exist with these partially matching parameter lists: (Int, Int), (String, String)_ = "a" + 1
^
note: candidate expects value of type 'String' for parameter #2 @inlinable public static func + (lhs: String, rhs: String) -> String
^
note: candidate expects value of type 'Int' for parameter #1 public static func + (lhs: Int, rhs: Int) -> Int
^