FIR: introduce FictitiousFunctionSymbol making it not FIR-based

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/impl/FirLibrarySymbolProviderImpl.kt

@@ -13,6 +13,7 @@ import org.jetbrains.kotlin.fir.deserialization.FirTypeDeserializer
 import org.jetbrains.kotlin.fir.resolve.FirSymbolProvider
 import org.jetbrains.kotlin.fir.symbols.ConeSymbol
 import org.jetbrains.kotlin.fir.symbols.LibraryClassSymbol
+import org.jetbrains.kotlin.fir.symbols.impl.FictitiousFunctionSymbol
 import org.jetbrains.kotlin.fir.symbols.impl.FirClassSymbol
 import org.jetbrains.kotlin.metadata.ProtoBuf
 import org.jetbrains.kotlin.metadata.builtins.BuiltInsBinaryVersion
@@ -105,9 +106,8 @@ class FirLibrarySymbolProviderImpl(val session: FirSession) : FirSymbolProvider
             val prefix = kind.classNamePrefix
             val arity = className.substring(prefix.length).toIntOrNull() ?: return null
             fictitiousFunctionSymbols.getOrPut(arity) {
-                FirClassSymbol(classId)
+                FictitiousFunctionSymbol(relativeClassName.shortName(), arity)
             }
         }
     }
-
 }

compiler/fir/tree/src/org/jetbrains/kotlin/fir/symbols/impl/FictitiousFunctionSymbol.kt

@@ -0,0 +1,32 @@
+/*
+ * 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.symbols.impl
+
+import org.jetbrains.kotlin.builtins.KotlinBuiltIns
+import org.jetbrains.kotlin.descriptors.ClassKind
+import org.jetbrains.kotlin.fir.symbols.ConeClassSymbol
+import org.jetbrains.kotlin.fir.symbols.ConeTypeParameterSymbol
+import org.jetbrains.kotlin.fir.types.ConeClassLikeType
+import org.jetbrains.kotlin.name.ClassId
+import org.jetbrains.kotlin.name.Name
+
+class FictitiousFunctionSymbol(className: Name, arity: Int) : ConeClassSymbol {
+    override val classId = ClassId(KotlinBuiltIns.BUILT_INS_PACKAGE_FQ_NAME, className)
+
+    override val typeParameters: List<ConeTypeParameterSymbol> = (0..arity).map {
+        TypeParameterSymbol(it)
+    }
+
+    class TypeParameterSymbol(index: Int) : ConeTypeParameterSymbol {
+        override val name: Name = Name.identifier("T$index")
+    }
+
+    override val kind: ClassKind
+        get() = ClassKind.INTERFACE
+
+    override val superTypes: List<ConeClassLikeType>
+        get() = emptyList() // TODO: kotlin.Function ???
+}

compiler/testData/fir/resolve/stdlib/functionX.kt

@@ -2,4 +2,8 @@ import kotlin.jvm.functions.Function0
 
 val x: Function0<Int> = { 42 }
 
-val y: Function1<String, String> = { it }
+val y: Function1<String, String> = { it }
+
+class MyFunction : Function2<Int, String, Unit> {
+    override fun invoke(p1: Int, p2: String) {}
+}

compiler/testData/fir/resolve/stdlib/functionX.txt

@@ -3,3 +3,8 @@ FILE: functionX.kt
         public? get(): R|kotlin/jvm/functions/Function0<kotlin/Int>|
     public? final? property y(val): R|kotlin/Function1<kotlin/String, kotlin/String>| = STUB
         public? get(): R|kotlin/Function1<kotlin/String, kotlin/String>|
+    public? final class MyFunction : R|kotlin/Function2<kotlin/Int, kotlin/String, kotlin/Unit>| {
+        public? open? override function invoke(p1: R|kotlin/Int|, p2: R|kotlin/String|): R|error: Not supported: FirImplicitTypeImpl| {
+        }
+
+    }