</p>

<p>
This fact invalidates a few common idioms.
</p>

<p>
Double-checked locking is an attempt to avoid the overhead of synchronization.
For example, the <code>twoprint</code> program might be
incorrectly written as:
</p>

<pre>
var a string
var done bool

func setup() {
	a = "hello, world"
	done = true
}

func doprint() {
	if !done {
		once.Do(setup)
	}
	print(a)

a <a href="#Run_time_panics">run-time panic</a> occurs.
In other words, even though the dynamic type of <code>x</code>
is known only at run time, the type of <code>x.(T)</code> is
known to be <code>T</code> in a correct program.
</p>

<pre>
var x interface{} = 7          // x has dynamic type int and value 7
i := x.(int)                   // i has type int and value 7

type I interface { m() }

func f(y I) {

a struct with a <code>_size</code> field), <code>#include
"go_asm.h"</code> will fail with a "redefinition of macro" error.
</p>

<h3 id="runtime">Runtime Coordination</h3>

<p>
For garbage collection to run correctly, the runtime must know the
location of pointers in all global data and in most stack frames.
The Go compiler emits this information when compiling Go source files,
but assembly programs must define it explicitly.
</p>

<h3 id="Language_versions">Language versions</h3>

<p>
The <a href="/doc/go1compat">Go 1 compatibility guarantee</a> ensures that
programs written to the Go 1 specification will continue to compile and run
correctly, unchanged, over the lifetime of that specification.
More generally, as adjustments are made and features added to the language,
the compatibility guarantee ensures that a Go program that works with a