tl;dr
You can perform a type assertion only on interface values. So you must convert x
to a valid interface type first, any
/ interface{}
in this case:
func isInt[T any](x T) (ok bool) {
_, ok = any(x).(int) // convert, then assert
return
}
So why does this fail to compile?
_, ok = x.(int) // ... cannot use type assertion on type parameter value ...
x
‘s type T
is a type parameter, not an interface. It is only constrained by an interface. The Go (revised 1.18
) language spec explicitly states type parameters are not allowed in a type assertion:
For an expression
x
of interface type, but not a type
parameter, and a typeT
… the notationx.(T)
is called a type assertion.
Also from from the generics tutorial on why parameter types need to be resolved at compile-time:
While a type parameter’s constraint typically represents a set of
types, at compile time the type parameter stands for a single type –
the type provided as a type argument by the calling code. If the type
argument’s type isn’t allowed by the type parameter’s constraint, the
code won’t compile.