It turns out that we cannot make this work as expected (i.e., to be interpreted as a type parameter followed by a parameterized constraint):

In general, an expression of the form T(x) may be a conversion or a function call, both of which may potentially result in a constant result (there are constant conversions, and some built-in functions like len can produce constant results). Thus, when we see [T (x)] we cannot assume a type parameter T followed by a parameterized constraint x without type checking; it could potentially be an array length. Here's a valid example:

package main

type A [len(*p)]struct{} // array declaration, not a parameterized type

var p *[10]byte

func main() {
	println(len(A{}))
}

Closing as working as expected.

Another intended case:

package main

type A [len (S)]struct{} // array declaration, not a parameterized type

const S = "Go"

func main() {
	println(len(A{})) // 2
}