Implement more general "apply defaults" functionality for host configurations

libraries/scripting/common/test/kotlin/script/experimental/test/ConfigurationsTest.kt

@@ -0,0 +1,42 @@
+/*
+ * Copyright 2010-2019 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package kotlin.script.experimental.test
+
+import junit.framework.TestCase
+import org.junit.Test
+import kotlin.script.experimental.host.ScriptingHostConfiguration
+import kotlin.script.experimental.host.ScriptingHostConfigurationKeys
+import kotlin.script.experimental.host.withDefaultsFrom
+import kotlin.script.experimental.util.PropertiesCollection
+
+class ConfigurationsTest : TestCase() {
+
+    @Test
+    fun testHostConfigWithDefaultsFrom() {
+        val c0 = ScriptingHostConfiguration()
+        val c1 = ScriptingHostConfiguration {
+            p1(1)
+        }
+        val c2 = ScriptingHostConfiguration {
+            p1(2)
+            p2("yes")
+        }
+
+        assertEquals(c1, c1.withDefaultsFrom(c0))
+        assertEquals(c1, c0.withDefaultsFrom(c1))
+        assertEquals(c1, c1.withDefaultsFrom(c1))
+
+        val c1c2 = c1.withDefaultsFrom(c2)
+        val c2c1 = c2.withDefaultsFrom(c1)
+
+        assertEquals(1, c1c2[ScriptingHostConfiguration.p1])
+        assertEquals("yes", c1c2[ScriptingHostConfiguration.p2])
+        assertEquals(2, c2c1[ScriptingHostConfiguration.p1])
+    }
+}
+
+private val ScriptingHostConfigurationKeys.p1 by PropertiesCollection.key<Int>(-1)
+private val ScriptingHostConfigurationKeys.p2 by PropertiesCollection.key<String>("-")