cmd/compile: incorrect compiler error in for range statement

[randall77]

package main

type T int

const K T = 5

type P struct {
	a [K]*byte
}

func f(p *P) {
	for i := range K {
		p.a[i] = nil
	}
}

Fails to compile with the kinda cryptic error:

tmp1.go:12:17: cannot use len(p.a) - 1 (type int) as type T in assignment

This gets triggered when rewriting that loop to a runtime.memclrHasPointers call.
It is trying to compute the final value for i as len(a)-1, when it should really be T(len(a)-1).

This started happening recently and was noticed inside google. Triggered by CL 659317, not sure exactly why that CL is implicated.

[gopherbot]

Change https://go.dev/cl/667815 mentions this issue: cmd/compile: add cast in range loop final value computation

[cuonglm]

Triggered by CL 659617, not sure exactly why that CL is implicated.

This CL does not seems to be existed.

[randall77]

Sorry, 659317.

[cuonglm]

Sorry, 659317.

Then I think it's reasonable, since this CL added the array clear optimization for the for-range over integer case.