Two final items with regard to type switches, before we move on to the next topic:

Type switches

…

The type switch guard may be preceded by a simple statement, which executes before the guard is evaluated.

This is pretty straight forward. It works just like the simple statements in if statements and normal switch statements. A simple statement can be used both with or without a temporary variable. In other words, both of these are valid:

switch x := someFunc(); x.(type) {

switch x := someFunc(); t := x.(type) {

The “fallthrough” statement is not permitted in a type switch.

And finally, fallthrough isn’t valid with a type switch. As much as you might like to do something like:

switch x.(type) {
case int8:
	fallthrough
case int16:
	fallthrough
case int:
	/* handle integer types */
}

This isn’t possible. It would wreak all kinds of havoc if it were allowed—what is the actual type of the variable when a case int falls through to a case error, for example?

Of course you can group like types in a single case:

switch x.(type) {
case int, int8, int16, ...:

With the caveat previously mentioned that it won’t work with temporary variables.

Quotes from The Go Programming Language Specification Language version go1.22 (Feb 6, 2024)