cmd/compile: inconsistent acceptance of cyclic variable declaration (esoteric)

[griesemer]

Another test case for handling of cycle detection:
https://play.golang.org/p/DAftfsYBsrX
is accepted and appears to run correctly. The similar (and functionally identical) program
https://play.golang.org/p/AChHOplAHzp
is not accepted with an incorrect type alias cycle error. And the simplified version eliminating the type name T
https://play.golang.org/p/ozYsLzyDax8
is not accepted with a type checking loop error.

Yet all of them are functionally the same. Either all of them should be accepted or none of them should be accepted.

[griesemer]

For reference: gccgo accepts and runs the first two versions. It crashes with the 3rd version (issue #25679).

[go101]

some related? #23823

[griesemer]

@go101 Related in the sense that there are cycles. But there may be different code paths executed.