Expression statements

With the exception of specific built-in functions, function and method calls and receive operations can appear in statement context. Such statements may be parenthesized.

ExpressionStmt = Expression .

The following built-in functions are not permitted in statement context:

append cap complex imag len make new real
unsafe.Alignof unsafe.Offsetof unsafe.Sizeof

h(x+y)
f.Close()
<-ch
(<-ch)
len("foo")  // illegal if len is the built-in function