The Go Programming Language Specification
Composite literals
A parsing ambiguity arises when a composite literal using the TypeName
form of the LiteralType appears as an operand between the keyword and
the opening brace of the block of an "if", "for", or "switch"
statement, and the composite literal is not enclosed in parentheses,
square brackets, or curly braces. In this rare case, the opening brace
of the literal is erroneously parsed as the one introducing the block
of statements. To resolve the ambiguity, the composite literal must
appear within parentheses.
if x == (T{a,b,c}[i]) { … }
if (x == T{a,b,c}[i]) { … }
An ambiguous composite literal common.Address{}
before an if
block { … }
.
if etherbase != common.Address{} {
return etherbase, nil
}
An unambiguous composite literal (common.Address{})
before an if
block { … }.
if etherbase != (common.Address{}) {
return etherbase, nil
}
For example,
package main
const AddressLength = 20
type Address [AddressLength]byte
func f(etherbase Address) (Address, error) {
// Unambiguous
if etherbase != (Address{}) {
return etherbase, nil
}
return Address{}, nil
}
func g(etherbase Address) (Address, error) {
// Ambiguous
if etherbase != Address{} {
return etherbase, nil
}
return Address{}, nil
}
func main() {}
Playground: https://play.golang.org/p/G5-40eONgmD