add variance

src/main/kotlin/au/com/console/jpaspecificationdsl/JPASpecificationDSL.kt

@@ -60,29 +60,34 @@ fun <T> KProperty1<T, String?>.notLike(x: String): Specifications<T> = spec { no
 fun <T> KProperty1<T, String?>.notLike(x: String, escapeChar: Char): Specifications<T> = spec { notLike(it, x, escapeChar) }
 
 // And
-infix fun <T> Specifications<T>.and(other: Specification<T>) = this.and(other)
-inline fun <reified T> and(vararg specs: Specifications<T>?): Specifications<T> {
+@Suppress("UNCHECKED_CAST")
+infix fun <T> Specifications<T>.and(other: Specification<in T>) = this.and(other as Specification<T>)
+inline fun <reified T> and(vararg specs: Specifications<in T>?): Specifications<T> {
     return and(specs.toList())
 }
-inline fun <reified T> and(specs: Iterable<Specifications<T>?>): Specifications<T> {
+inline fun <reified T> and(specs: Iterable<Specifications<in T>?>): Specifications<T> {
     return combineSpecifications(specs, Specifications<T>::and)
 }
 
 // Or
-infix fun <T> Specifications<T>.or(other: Specification<T>) = this.or(other)
-inline fun <reified T> or(vararg specs: Specifications<T>?): Specifications<T> {
+@Suppress("UNCHECKED_CAST")
+infix fun <T> Specifications<T>.or(other: Specification<in T>) = this.or(other as Specification<T>)
+inline fun <reified T> or(vararg specs: Specifications<in T>?): Specifications<T> {
     return or(specs.toList())
 }
-inline fun <reified T> or(specs: Iterable<Specifications<T>?>): Specifications<T> {
+inline fun <reified T> or(specs: Iterable<Specifications<in T>?>): Specifications<T> {
     return combineSpecifications(specs, Specifications<T>::or)
 }
 
 // Not
 operator fun <T> Specifications<T>.not() = Specifications.not(this)
 
 // Combines Specifications with an operation
-inline fun <reified T> combineSpecifications(specs: Iterable<Specification<T>?>, operation: Specifications<T>.(Specification<T>) -> Specifications<T>): Specifications<T> {
-    return specs.filterNotNull().fold(emptySpecification<T>()) { existing, new -> existing.operation(new) }
+inline fun <reified T> combineSpecifications(specs: Iterable<Specification<in T>?>, operation: Specifications<T>.(Specification<T>) -> Specifications<T>): Specifications<T> {
+    return specs.filterNotNull().fold(emptySpecification()) { existing, new ->
+        @Suppress("UNCHECKED_CAST")
+        existing.operation(new as Specification<T>)
+    }
 }
 
 // Empty Specification

src/test/kotlin/au/com/console/jpaspecificationdsl/TvShowRepository.kt

@@ -12,17 +12,21 @@ import javax.persistence.OneToMany
 @Repository
 interface TvShowRepository : CrudRepository<TvShow, Int>, JpaSpecificationExecutor<TvShow>
 
+interface HasName {
+    val name: String
+}
+
 @Entity
-data class TvShow(
+data class TvShow (
         @Id
         @GeneratedValue
         val id: Int = 0,
-        val name: String = "",
+        override val name: String = "",
         val synopsis: String = "",
         val availableOnNetflix: Boolean = false,
         val releaseDate: String? = null,
         @OneToMany(cascade = kotlin.arrayOf(javax.persistence.CascadeType.ALL))
-        val starRatings: Set<StarRating> = emptySet())
+        val starRatings: Set<StarRating> = emptySet()): HasName
 
 @Entity
 data class StarRating(
@@ -36,8 +40,8 @@ data class StarRating(
 // Note: these functions return null for a null input. This means that when included in
 // and() or or() they will be ignored as if they weren't supplied.
 
-fun hasName(name: String?): Specifications<TvShow>? = name?.let {
-    TvShow::name.equal(it)
+fun hasName(name: String?): Specifications<HasName>? = name?.let {
+    HasName::name.equal(it)
 }
 
 fun availableOnNetflix(available: Boolean?): Specifications<TvShow>? = available?.let {