FIR: support resolve of implicit Unit types

compiler/fir/tree/src/org/jetbrains/kotlin/fir/FirRenderer.kt

@@ -14,6 +14,7 @@ import org.jetbrains.kotlin.fir.symbols.ConeClassLikeSymbol
 import org.jetbrains.kotlin.fir.symbols.ConeSymbol
 import org.jetbrains.kotlin.fir.symbols.impl.FirTypeParameterSymbol
 import org.jetbrains.kotlin.fir.types.*
+import org.jetbrains.kotlin.fir.types.impl.FirImplicitUnitType
 import org.jetbrains.kotlin.fir.visitors.FirVisitorVoid
 import org.jetbrains.kotlin.name.SpecialNames
 import org.jetbrains.kotlin.types.Variance
@@ -410,7 +411,7 @@ class FirRenderer(builder: StringBuilder) : FirVisitorVoid() {
     }
 
     override fun visitImplicitType(implicitType: FirImplicitType) {
-        print("<implicit>")
+        print(if (implicitType is FirImplicitUnitType) "kotlin.Unit" else "<implicit>")
     }
 
     override fun visitTypeWithNullability(typeWithNullability: FirTypeWithNullability) {

compiler/fir/tree/src/org/jetbrains/kotlin/fir/types/impl/FirImplicitUnitType.kt

@@ -0,0 +1,19 @@
+/*
+ * Copyright 2010-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license
+ * that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.fir.types.impl
+
+import com.intellij.psi.PsiElement
+import org.jetbrains.kotlin.fir.FirSession
+import org.jetbrains.kotlin.fir.expressions.FirAnnotationCall
+import org.jetbrains.kotlin.fir.types.FirImplicitType
+
+class FirImplicitUnitType(
+    override val session: FirSession,
+    override val psi: PsiElement?
+) : FirImplicitType {
+    override val annotations: List<FirAnnotationCall>
+        get() = emptyList()
+}