CC @griesemer

The first error is fixed in the latest (internal, not deployed version). The 2nd error is correct but misleading: Since the type parameter list is [T1, T2] and since type parameter lists are syntactically like regular parameter lists (but for the []'s), it looks like a parameter list with two constraints T1 and T2 and without explicitly named type parameters. For comparison, note that func f(int, int){} is a valid function declaration with unnamed value parameters.

Anyway, agreed that the error message should be better.

A similar case occurs when defining generic types if you omit the constraint on the type parameter, e.g.
https://go2goplay.golang.org/p/GqCc1gE36NL

That is, if you write

type slot[T] ...

instead of

type slot[T any] ...

In the go2go playground, the error currently says "undeclared name: T", rather than "type parameter T is missing type constraint"

For comparison, note that func f(int, int){} is a valid function declaration with unnamed value parameters.

Ugh, I didn't realise that was allowed. If you want to discard arguments, you can also write func f(_, _ int){}, and to me that makes the intention clearer.

"type slot[T] some_type" defines slot as an array of length T of element type some_type. If T is not defined anywhere, then undeclared name: T is a correct error message.

That said, we might want to tweak the compiler error message here to make clear that the type is being interpreted as an array type.

Change https://golang.org/cl/348730 mentions this issue: cmd/compile/internal/syntax: better error message for missing type constraint

Change https://golang.org/cl/348731 mentions this issue: cmd/compile/internal/types2: better error message for invalid array decls

Change https://golang.org/cl/348742 mentions this issue: cmd/compile/internal/syntax: correct follow token for type parameter lists