The Go spec disallows duplicate map key constants and allows implementations to restrict duplicate switch case constants. However, in both cases, these must be Go language constants (as the Go spec doesn't have any other notion of "constant".)

This code should be valid, but cmd/compile currently rejects it for having duplicate keys and cases:

package p

import "unsafe"

func f() {
	_ = map[uintptr]int {
		0: 0,
		uintptr(unsafe.Pointer(nil)): 0,
	}

	switch uintptr(0) {
	case 0:
	case uintptr(unsafe.Pointer(nil)):
	}

	switch interface{}(nil) {
	case nil:
	case nil:
	}

	_ = map[interface{}]int {
		nil: 0,
		nil: 0,
	}
}

gotype accepts it.

(This is yet another bug relating to gc's overly eager constant folding in the frontend.)