I think a simple statements inside "if" is a bad idea. It too complex to understand that code.I never use that language feature.
So I think this one is a bad idea too.

I think a simple statements inside "if" is a bad idea. It too complex to understand that code.I never use that language feature.

if err := fn(); err != nil {
       // handle error
}

Is quite popular.

It Is quite popular indeed.
And we should do independent thinking.So I hate this language feature("if" simple statements).
It just cost a lot energy to understand that code.
Because I need to think is whether this err in a scope, it also too easy to just skip that fn function call.
By the way I hate "if"/"switch"/"for" variable scope language feature too.The php function scope stuff does the best.

I prefer:

err := fn()
if err != nil {
       // handle error
}

I'd like to add: case already supports simple statements under select:

select {
case <-ch1:
    // ...
case x := <-ch2
    // ...use x
    ...
}

which is widely used in go stdlib itself:

kirr@deco:~/src/tools/go/go/src$ git grep -w case |grep ':=' | wc -l
142

For me this is simply language consistency fix, not go2 matter...

case already supports simple statements under select:

Send statement is the only simple statement a CommCase is specified to accept.

This makes the switch statement harder to understand when switching on a value. Consider

switch v {
case 0:
case v++; 3:
case 4:
}

While we can define exactly what this means, it is just confusing. The advantage of being able to write the statement does not seem worth the complexity cost.