[BUG] fromExpression non-nullable return value handling

[Janiaje]

Describe the bug
Mappie des not recognise if I provided a default for a nullable field when assigning to a non nullable field.

To Reproduce
Mapping:

to::createdAt fromExpression { it.createdAt ?: now() }

Exception

Target ToClass::createdAt of type LocalDateTime cannot be assigned from expression of type @FlexibleNullability LocalDateTime?

Operational workaround:

to::createdAt fromExpression { (it.createdAt ?: now()) as LocalDateTime }

Expected behavior
Mappie recognises if an expression's return value is not nullable

Kotlin version
2.2.10

Mappie version
2.2.10-1.4.2

Mappie and Kotlin compatibility
Is the Mappie version compatible with the Kotlin version?
Yes

Invocation method
The method of invocation, i.e. Gradle, Maven or CLI.
Gradle

Additional context
Add any other context about the problem here.

[stefankoppier]

Hi @Janiaje, thanks for the report. I just created a PR fixing this issue.

Do note, that in the general case, this is an unsafe assignment. As such, a warning will be given instead of an error.
However, this is not completely in line with the design of Kotlin itself. I will create a compiler option and configuration annotation later to disable this warning.

Until this PR is released, I suggest using requireNotNull or !! to force to a non-nullable type instead of a cast. A cast might lead to ClassCastException instead of a NullPointerException which might make it harder to debug the actual cause. Although now() will never be null anyway ;)

[Janiaje]

Hi @stefankoppier, thanks for the quick reply.
I may have misunderstood, but as you mentioned, now() can never be null.
That means if I use the ?: operator as a fallback, the field will either have a value or fall back to the result of now(). In either case, it won’t be null — which makes the whole expression effectively non-nullable, and therefore assignable to a non-nullable field, no?

[stefankoppier]

Yes, so I suspect the runtime value of now() will never be null. But the type actually is nullable, as it's a Java type. This means the result type of the whole expression is LocalDateTime? But as it's a Java type, it has the @FlexibleNullable annotation in the compiler, which means that the compiler normally doesn't complain about it.

[stefankoppier]

Maybe https://kotlinlang.org/docs/java-interop.html#null-safety-and-platform-types explains what I mean bit better :)

[Janiaje]

Oh, understood. I forgot that now() was a Java function hence considered as nullable. Then I think you are right and this is not actually a bug so if you want you can close it.

[stefankoppier]

I do consider it a bug, so I'm glad you reported it. It's not in line with Kotlin, and makes code verbose. The PR should fix it. I will merge later