reflect: type of an interface is nil

[kindermoumoute]

I am trying to understand why this code works:

ioReaderType := reflect.ValueOf(struct{ ioReader io.Reader }{}).FieldByName("ioReader").Type()
fmt.Println(ioReaderType.String()) // prints "io.Reader"

But this one panic

ioReaderType := reflect.TypeOf(io.Reader(nil))
fmt.Println(ioReaderType.String()) // panic

https://play.golang.org/p/F2ndRax1f4E

[randall77]

From the doc of reflect.TypeOf:

TypeOf returns the reflection Type that represents the dynamic type of i. If i is a nil interface value, TypeOf returns nil.

io.Reader(nil) is a nil interface value. Castingnil to an interface type is still nil.

See the example for reflect.TypeOf. You want to instead do:

reflect.TypeOf((*io.Reader)(nil)).Elem()

Casting a nil pointer to an interface type is not nil.