go - Golang underlying types

In this code snippet from the spec

type T1 string

type T2 T1
type T3 []T1
type T4 T3

The spec says:

The underlying type of string, T1, and T2 is string.
The underlying type of []T1, T3, and T4 is []T1.

Why is the underlying type of T2 not T1, but string?
Shouldn't the underlying type of T4 instead be []string and not []T1 if the underlying type of T1 is string?
Confused.

Answer

The spec mentions:

Each type T has an underlying type: If T is one of the predeclared boolean, numeric, or string types, or a type literal, the corresponding underlying type is T itself.
Otherwise, T's underlying type is the underlying type of the type to which T refers in its type declaration.

T2 refers in its type declaration to T1, which has the underlying type string.

It is important for the underlying type of T2 to be string, because it will help for Assignability where

A value x is assignable to a variable of type T ("x is assignable to T")
x's type V and T have identical underlying types and at least one of V or T is not a named type.

This is also detailed in "Golang: Why can I type alias functions and use them without casting?"

---

When it comes to the underlying type of T4, we are talking about an underlying unnamed type []T1.

And again, the assignability rule says you can assign []T1 to T4 (since []T1 is not a named type): its underlying type stops at the first not-named type ([]T1).

See this example on playground

var t3 T3 = []T1{"a", "b"}
fmt.Println("t3='%+v'", t3)
// var t4 T4 = []string{}

// cannot use []string literal (type []string) as type T4 in assignment
var t4 T4 = T4(t3)
fmt.Println("t4='%+v'", t4)
t4 = []T1{T1("c"), T1("d")}
fmt.Println("t4='%+v'", t4)

Output:

t3='%+v' [a b]

t4='%+v' [a b]
t4='%+v' [c d]