CC @jba @griesemer @jayconrod @matloob

Unfortunately, apidiff may be correct here. The change described here can introduce a compile-time error.

Per https://golang.org/ref/spec#Type_switches:

The types listed in the cases of a type switch must all be different.

And, per https://golang.org/ref/spec#Type_identity:

A defined type is always different from any other type.

So changing T from a defined type to an alias of common.T2 can introduce a compilation error in consumer code by causing two case statements within a type-switch to no longer name different types. For example, compare https://play.golang.org/p/8XvlKYtEOT8 and https://play.golang.org/p/BXfun4_eWkd:

package main

import (
	"fmt"

	"example.com/x/common"
	"example.com/x/v"
)

func main() {
	var x interface{} = v.F
	switch x.(type) {
	case func(common.T2):
		fmt.Println("common")
	case func(v.T):
		fmt.Println("v")
	}
}

After the change, that program fails with the error

# example.com/x
./prog.go:15:2: duplicate case func(common.T2) in type switch
	previous case at ./prog.go:13:2

If common is an entirely new package, then there can be no existing references to common.T2.

But regardless of that question, the bug here is that apidiff cares about the name of the target of the alias. Whether the alias is to common.T or commont.T2 shouldn't be relevant.