Nice catch, thank you for reporting this @guangda-hu! As @ianlancetaylor reported in that issue, that code works alright with gccgo.

/cc @griesemer @mdempsky.

Okay now an interesting permutation of this bug passes when the example is NOT in a function i.e

Passing 1

package p
type T string
var _ = []rune(T("T"))

Passing 2

package p
type T string
var t = T("T")
var _ = []rune(t)

Passing 3 based off Passing 1

package p
type T string
func foo() {
	var _ = []rune(T("T"))
}

Failing 1 based off Passing 2, but in function

package p
type T string
func foo() {
        var t = T("T") 
        var _ = []rune(t)
}

$ go tool compile fail.go 
fail.go:7:16: cannot use t (type T) as type string in argument to runtime.stringtoslicerune

My guess was that the problem is only with non-constant (I actually briefly mentioned this at the end of "What did you do?", and the example []rune(MyString("白鵬翔")) provided by the spec works).
In your failing example, it would pass if you change var t to const t.

Edit: Nevermind. Your second example shows it's actually related to whether it's inside a function.

Edit again: Your second example will fail if you don't use '_': https://play.golang.org/p/h4ZAslGQ8eN
Probably the var declaration is just ignored if you use the blank identifier. I don't know.

Change https://golang.org/cl/87695 mentions this issue: cmd/compile: make slice of runes accept to be converted from defined type derived from string

@guangda-hu @odeke-em I've tried to fix the issue. Could you take a look at the CL?

Thank you @namusyaka and Happy New Year! It'll be for Go1.11 :)