How to see source code of a fun in Kotlin in Android Studio 3.1.3?

I hope to see the source code of the fun let, but I get the following content after I ctrl + click data?.let.
How to see the source code of a fun in Kotlin in Android Studio 3.1.3?

@kotlin.internal.InlineOnly public inline fun <T, R> T.let(block: (T) -> R): R
 { /* compiled code */ }

Added Content


And More
The image when I click on choose sources



from How to see source code of a fun in Kotlin in Android Studio 3.1.3?