E096: An error variant's payload is declared or used wrongly

error IOError {    NotFound(path: string),    Denied(path: string, mode: int),    Timeout}IOError::NotFound("a.txt")             // okIOError::NotFound("a.txt", "message")  // ok: fields, then a string messageIOError::Denied("a.txt")               // ERROR: declares 2 fields, given 1IOError::Denied("a.txt", "rw")         // ERROR: 'mode' is an intIOError::NotFound                      // ERROR: has fields, so it isn't a value//   Hint: Pass one argument per field, in order: 'IOError::Denied(...)'//   takes 2, or 3 with a trailing 'string' message.

A variant may declare typed fields, written Name(field: Type, ...). A variant with N fields is built with N arguments, or N+1 when the extra trailing argument is the string message (err.message still reads it). A variant with no fields is written without parentheses and takes no arguments, or exactly one, its message – as it always did.

A payload is read only by destructuring it in a compare:

compare err {    matches IOError::NotFound(path):   ...    matches IOError::Denied(path, _):  ...    otherwise:                         ...}

What this code reports:

• Empty parentheses in a declaration (NotFound()), a field with no type (NotFound(path)), or a field name used twice in one variant.
• A field whose type is an error type, a !T or void: an error value carries data about what went wrong, not another failure.
• A constructor call with the wrong number of arguments, or an argument whose type is not its field’s (or, past the fields, not a string message).
• A bare IOError::NotFound where a value is wanted, for a variant that has fields: without its payload there is no value. It is still fine as an == or != operand (err == IOError::NotFound, which compares the variant only) and as the head of a pattern.

An unknown field type is the ordinary unknown-type error, and a pattern that does not fit its variant is E092.

Run turf --explain E096 in a terminal to read this again.