go/printer: produces a invalid source, for a successfully parsed AST

[mateusz834]

Consider:

type _[a (b + 3),] struct{}

This is parsed successfully by go/parser and it gets formatted into:

type _[a b + 3] struct{}

This is not valid go syntax. Not sure how this should be handled, should type _[a (b + 3),] struct{} cause a parser error?

For example, the source below causes a parser error (expected ')', found '+')

type _[a ~int, b (a+3)]struct{}

CC @griesemer

[griesemer]

Milestone 1.24 only so it stays on the radar - this is not really urgent in any way as this code won't pass the typecheckers and thus won't appear in real code.

[griesemer]

Looking at this some more, type _[a (b + 3),] struct{} should probably be a parser error, but there's two ways to approach it: since (b + 3) cannot be a type, the entire parameter list is considered an array length, and the , is unexpected. Alternatively, the comma implies a type parameter list, but (b + 3) cannot be a type so there's an error there.

I believe the spec's current wording permits both approaches.

[adonovan]

Looks like the comma causes it to be parsed as a type parameter list, even though the parenexpr holds a term not a type:

[griesemer]

Yes, the comma forces parsing as a type parameter list which is expected in the current implementation. The question is what is the most consistent error to report: is it an invalid type constraint or an invalid array length.