cmd/compile: a[1.0 << s] should be accepted

[griesemer]

Per the fix for #14844, https://play.golang.org/p/fWbHLgkJ7Z should now be accepted by the compiler.

[gopherbot]

Change https://golang.org/cl/60230 mentions this issue: spec: clarify context type for certain non-constant shifts

[bradfitz]

Ping Robert.

I assume this is a release-blocker, given the spec update?

[griesemer]

Will look into this tomorrow. It's not spec compliant anymore but I don't think it's a release blocker - after all there's a trivial work around (just write a[1 << s] instead). Leaving as is for now in case there's an easy fix.

[odeke-em]

I had started working on this at some ungodly hours of Saturday morning but I couldn't get a proper cast/conversion to int in

go/src/cmd/compile/internal/gc/const.go

Line 262 in 992ce90

if t != nil && !t.IsInteger() {

so I'll defer to you @griesemer :)

[gopherbot]

Change https://golang.org/cl/81277 mentions this issue: cmd/compile: permit indices of certain non-constant shifts