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# kotest
r

Rob Elliot

03/24/2022, 10:44 AM
Is there a way to assert the compile time type of an expression? (I don't want a test that passes because the runtime type of the result of the expression is a subtype of the requested type - I want to assert exactly what type the compiler decided the expression had. When doing overloading with subtypes and overriding with more specific return types it's important.)
c

christophsturm

03/24/2022, 10:51 AM
why not 
val x: ExpectedType = expression
? that will fail at compile time
r

Rob Elliot

03/24/2022, 10:53 AM
If 
expression
returns a subtype of 
ExpectedType
that will pass. So in order to prove the compile time return type is 
ExpectedType
, not a subtype of 
ExpectedType
, I need to use 
shouldNotCompile
with every single subtype of 
ExpectedType
. Which is a bit of a pain.
I'm experimenting with 
io.kotest.matchers.reflection.shouldHaveReturnType
- might give me what I need.
Yup, that works. Needs kotlin-reflect on the classpath, but then you can do:
Copy code
import io.kotest.matchers.reflection.shouldHaveReturnType
import kotlin.reflect.jvm.reflect

{ expression }.reflect().shouldNotBeNull().shouldHaveReturnType<ExpectedType>()
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