cmd/compile/internal/syntax: allow eliding interface in constraint literals
This CL permits an arbitrary type as well as the type sets ~T and A|B
in constraint position, without the need of a surrrounding interface.
For instance, the type parameter list
[P interface{ ~map[K]V }, K comparable, V interface{ ~string }]
may be written as
[P ~map[K]V, K comparable, V ~string]
The feature must be enabled explicitly with the AllowTypeSets mode
and is only available if AllowGenerics is set as well.
For #48424.
Change-Id: Ic70bb97a49ff75e67e040853eac10e6aed0fef1a
Reviewed-on: https://go-review.googlesource.com/c/go/+/353133
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Robert Findley <rfindley@google.com>