fix(modelql): more detailed exception for unsupported cross-query streams

Author: slisson

modelql-core/src/commonMain/kotlin/org/modelix/modelql/core/Query.kt

@@ -492,7 +492,7 @@ class QueryInput<E> : ProducingStep<E>(), IMonoStep<E> {
     }
 
     override fun createStream(context: IStreamInstantiationContext): StepStream<E> {
-        throw RuntimeException("The stream for the query input step is expected to be created by the query")
+        throw IllegalArgumentException("Unsupported cross-query usage of $this in ${owner.query}")
     }
 
     override fun canBeEmpty(): Boolean = false

modelql-core/src/commonTest/kotlin/org/modelix/modelql/core/ModelQLTest.kt

@@ -360,6 +360,19 @@ class ModelQLTest {
         assertEquals("Samsung Universe 9", result)
     }
 
+    @Test
+    fun testIllegalCrossQueryStreams() = runTestWithTimeout {
+        val ex = assertFailsWith(IllegalArgumentException::class) {
+            remoteProductDatabaseQuery<String> { db ->
+                // The `elements` query uses the outside `db` instead of the input `it`.
+                // The query is only allowed to use values from its input, otherwise it wouldn't be cacheable.
+                db.find({ db.products }, { it.id }, 3.asMono()).title
+            }
+        }
+        val expectedMessage = "Unsupported cross-query usage of"
+        assertEquals(expectedMessage, (ex.message ?: "").take(expectedMessage.length))
+    }
+
     @Test
     fun testFindAll() = runTestWithTimeout {
         val result: List<String> = remoteProductDatabaseQuery { db ->