Why does `defer recover()` not catch panics?

Why does a call to defer func() { recover() }() successfully recover a panicking goroutine, but a call to defer recover() not?

As an minimalistic example, this code doesn't panic

package main

func main() {
    defer func() { recover() }()
    panic("panic")
}

However, replacing the anonymous function with recover directly panics

package main

func main() {
    defer recover()
    panic("panic")
}

Quoting from the documentation of the built-in function recover():

> If recover is called outside the deferred function it will not stop a panicking sequence.

In your second case recover() itself is the deferred function, and obviously recover() does not call itself. So this will not stop the panicking sequence.

If recover() would call recover() in itself, it would stop the panicking sequence (but why would it do that?).

Another Interesting Example:

The following code also doesn't panic (try it on the Go Playground):

package main

func main() {
	var recover = func() { recover() }
	defer recover()
	panic("panic")
}

What happens here is we create a recover variable of function type which has a value of an anonymous function calling the built-in recover() function. And we specify calling the value of the recover variable to be the deferred function, so calling the builtin recover() from that stops the panicing sequence.

The Handling panic section mentions that

> Two built-in functions, panic and recover, assist in reporting and handling run-time panics

> The recover function allows a program to manage behavior of a panicking goroutine.

> Suppose a function G defers a function D that calls recover and a panic occurs in a function on the same goroutine in which G is executing.

> When the running of deferred functions reaches D, the return value of D's call to recover will be the value passed to the call of panic.
If D returns normally, without starting a new panic, the panicking sequence stops.

That illustrates that recover is meant to be called in a deferred function, not directly.
When it panic, the "deferred function" cannot be the built-in recover() one, but one specified in a defer statement.

DeferStmt = "defer" Expression .

> The expression must be a function or method call; it cannot be parenthesized.
Calls of built-in functions are restricted as for expression statements.

> With the exception of specific built-in functions, function and method calls and receive operations can appear in statement context.

An observation is that the real problem here is the design of defer and thus the answer should say that.

Motivating this answer, defer currently needs to take exactly one level of nested stack from a lambda, and the runtime uses a particular side effect of this constraint to make a determination on whether recover() returns nil or not.

Here's an example of this:

func b() {
  defer func() { if recover() != nil { fmt.Printf("bad") } }()
}

func a() {
  defer func() {
    b()
    if recover() != nil {
      fmt.Printf("good")
    }
  }()
  panic("error")
}

The recover() in b() should return nil.

In my opinion, a better choice would have been to say that defer takes a function BODY, or block scope (rather than a function call,) as its argument. At that point, panic and the recover() return value could be tied to a particular stack frame, and any inner stack frame would have a nil pancing context. Thus, it would look like this:

func b() {
  defer { if recover() != nil { fmt.Printf("bad") } }
}

func a() {
  defer {
    b()
    if recover() != nil {
      fmt.Printf("good")
    }
  }
  panic("error")
}

At this point, it's obvious that a() is in a panicking state, but b() is not, and any side effects like "being in the first stack frame of a deferred lambda" aren't necessary to correctly implement the runtime.

So, going against the grain here: The reason this doesn't work as might be expected, is a mistake in the design of the defer keyword in the go language, that was worked around using non-obvious implementation detail side effects and then codified as such.