Basic support of partially specified type arguments though a new underscore operator for type arguments

^KT-13394

compiler/psi/src/org/jetbrains/kotlin/psi/KtNameReferenceExpression.kt

@@ -59,6 +59,9 @@ class KtNameReferenceExpression : KtExpressionImplStub<KotlinNameReferenceExpres
         return visitor.visitSimpleNameExpression(this, data)
     }
 
+    val isPlaceholder: Boolean
+        get() = getIdentifier()?.text?.equals("_") == true
+
     companion object {
         private val NAME_REFERENCE_EXPRESSIONS = TokenSet.create(IDENTIFIER, THIS_KEYWORD, SUPER_KEYWORD)
     }

compiler/psi/src/org/jetbrains/kotlin/psi/KtTypeReference.kt

@@ -19,7 +19,7 @@ package org.jetbrains.kotlin.psi
 import com.intellij.lang.ASTNode
 import com.intellij.psi.PsiElement
 import org.jetbrains.kotlin.lexer.KtTokens
-import org.jetbrains.kotlin.psi.psiUtil.collectAnnotationEntriesFromStubOrPsi
+import org.jetbrains.kotlin.psi.psiUtil.findDescendantOfType
 import org.jetbrains.kotlin.psi.stubs.KotlinPlaceHolderStub
 import org.jetbrains.kotlin.psi.stubs.elements.KtStubElementTypes
 
@@ -38,6 +38,9 @@ class KtTypeReference : KtModifierListOwnerStub<KotlinPlaceHolderStub<KtTypeRefe
         return visitor.visitTypeReference(this, data)
     }
 
+    val isPlaceholder: Boolean
+        get() = findDescendantOfType<KtNameReferenceExpression>()?.isPlaceholder == true
+
     val typeElement: KtTypeElement?
         get() = KtStubbedPsiUtil.getStubOrPsiChild(this, KtStubElementTypes.TYPE_ELEMENT_TYPES, KtTypeElement.ARRAY_FACTORY)