cmd/compile: cannot parse certain constraint literals in generic functions

[griesemer]

package p

func _[_ []int | int]() {}  // cannot parse: syntax error: unexpected |, expecting comma or ]
func _[_ int | []int]() {}  // ok

Both constraints are identical and both are valid. Should be able to parse both of them.

[gopherbot]

Change https://golang.org/cl/359014 mentions this issue: cmd/compile/internal/syntax: fix constraint literal parsing for generic functions

[findleyr]

This does not affect go/parser.

[griesemer]

Fix is pending but this is also not a release blocker. There is a work-around:

func _[_ ([]int) | int]() {} // work-around: use parentheses

and this case is uncommon; most of the time we want

func _[_ ~[]int | ~int]() {}

[gopherbot]

Change https://golang.org/cl/359134 mentions this issue: go/parser: fix parsing of array or slice constraint types