cmd/go2go: improve error messages for missing or superfluous type constraints

[candlerb]

What version of Go are you using (go version)?

go2go playground

Does this issue reproduce with the latest release?

N/A

What operating system and processor architecture are you using (go env)?

N/A

What did you do?

1. https://go2goplay.golang.org/p/m9YUDBTWwpA

   func (m *MyMap[KT,VT comparable]) set(k KT, v VT) { ...
   

2. https://go2goplay.golang.org/p/GDxiXiIIti7

   func blah[T1, T2](x T1) T2 { ...
   

What did you expect to see?

1. error to the effect that a constraint was used in a place where it's not allowed
2. error to the effect that a type constraint is missing

What did you see instead?

1. "missing ',' in type argument list"
2. "all type parameters must be named" (even though they were named!)

[ianlancetaylor]

CC @griesemer

[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.