go/types, types2: bad error message for cycles through type instantiation

[mrkfrmn]

I was doing a bit of testing with regards to cycles through instantiated types. If we type check the below:

type a = b[int]
type b[_ any] = a

We get the below error message:

./prog.go:3:6: invalid recursive type a
	./prog.go:3:6: a refers to b
	./prog.go:4:6: b refers to a
./prog.go:3:10: cannot use generic type b[_ any] without instantiation

The first reported error seems reasonable, but the second seems like an erroneous follow-on error. From the user's perspective, the type has been instantiated (b[int]) — it's a coincidence that a cycle error prevents said instantiation. Changing the definition of b to the below solves both errors.

type a = b[int]
type b[_ any] = int

I think it's reasonable to remove the second error here.

[gabyhelp]

Related Issues

• types2, go/types: missing errors for invalid parameterized type declarations #48962 (closed)
• go/types: improve recursive type error message #69574 (closed)
• go/types, cmd/compile: "invalid type loop" depending on declaration order #51244
• cmd/compile: "invalid recursive type" error is not as helpful as go/types #41575 (closed)
• gccgo: internal compiler error with simple recursive type declaration #25320 (closed)
• types2, go/types: need to detect invalid recursive type instantiation #48098 (closed)
• go/types: initialization cycle errors are non-deterministic #71254 (closed)
• cmd/compile: handle alias types in recursive types more consistently #50729 (closed)
• go/types: specious "invalid recursive type" error? #60130

Related Code Changes

• cmd/compile: make invalid recursive type error clearer

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[gopherbot]

Change https://go.dev/cl/695655 mentions this issue: types2: omit redundant generic type instantiation error if type is invalid