@PolySymbol.Property where the declared type
is not assignable to the expected PolySymbol property type.
PolySymbol properties must return a type that is assignable to the type parameter of the corresponding
PolySymbolProperty class.
For example, if JSPropertySignatureProperty expects PropertySignature,
then the annotated property must return PropertySignature or a subtype of it.
Example:
@PolySymbol.Property(JSPropertySignatureProperty::class)
private val propertySignature: TypeScriptTypeMember // Error: TypeScriptTypeMember is not assignable to PropertySignature
get() = source