Put using underscored type arguments under a feature

compiler/testData/diagnostics/tests/inference/annotatedUnderscoredTypeArgument.kt

@@ -1,3 +1,4 @@
+// !LANGUAGE: +PartiallySpecifiedTypeArguments
 // !DIAGNOSTICS: -UNCHECKED_CAST
 // FIR_IDENTICAL
 // WITH_RUNTIME

compiler/testData/diagnostics/tests/inference/underscoredTypeArgument.kt

@@ -0,0 +1,10 @@
+// FIR_IDENTICAL
+// !DIAGNOSTICS: -UNCHECKED_CAST
+// WITH_RUNTIME
+
+fun <K, T> foo(x: (K) -> T): Pair<K, T> = (1 as K) to (1f as T)
+
+fun box(): String {
+    val x = foo<Int, <!UNRESOLVED_REFERENCE!>_<!>> { it.toFloat() } // Pair<Int, Float>
+    return "OK"
+}

compiler/testData/diagnostics/tests/inference/underscoredTypeArgument.txt

@@ -0,0 +1,4 @@
+package
+
+public fun box(): kotlin.String
+public fun </*0*/ K, /*1*/ T> foo(/*0*/ x: (K) -> T): kotlin.Pair<K, T>