[FIR] Add Any? as default bound for all type parameters

compiler/fir/psi2fir/src/org/jetbrains/kotlin/fir/builder/RawFirBuilder.kt

@@ -444,6 +444,7 @@ class RawFirBuilder(val session: FirSession, val stubMode: Boolean) {
             val typeParameters = firClass.typeParameters.map {
                 FirTypeParameterImpl(session, it.psi, FirTypeParameterSymbol(), it.name, Variance.INVARIANT, false).apply {
                     this.bounds += it.bounds
+                    addDefaultBoundIfNecessary()
                 }
             }
             return FirResolvedTypeRefImpl(
@@ -457,6 +458,12 @@ class RawFirBuilder(val session: FirSession, val stubMode: Boolean) {
             )
         }
 
+        private fun FirTypeParameterImpl.addDefaultBoundIfNecessary() {
+            if (bounds.isEmpty()) {
+                bounds += FirImplicitNullableAnyTypeRef(session, null)
+            }
+        }
+
         override fun visitEnumEntry(enumEntry: KtEnumEntry, data: Unit): FirElement {
             return withChildClassName(enumEntry.nameAsSafeName) {
                 val firEnumEntry = FirEnumEntryImpl(
@@ -917,6 +924,7 @@ class RawFirBuilder(val session: FirSession, val stubMode: Boolean) {
                     firTypeParameter.bounds += typeConstraint.boundTypeReference.toFirOrErrorType()
                 }
             }
+            firTypeParameter.addDefaultBoundIfNecessary()
             return firTypeParameter
         }
 

compiler/fir/tree/src/org/jetbrains/kotlin/fir/declarations/impl/FirTypeParameterImpl.kt

@@ -12,6 +12,7 @@ import org.jetbrains.kotlin.fir.declarations.FirTypeParameter
 import org.jetbrains.kotlin.fir.symbols.impl.FirTypeParameterSymbol
 import org.jetbrains.kotlin.fir.transformInplace
 import org.jetbrains.kotlin.fir.types.FirTypeRef
+import org.jetbrains.kotlin.fir.types.impl.FirImplicitNullableAnyTypeRef
 import org.jetbrains.kotlin.fir.visitors.FirTransformer
 import org.jetbrains.kotlin.name.Name
 import org.jetbrains.kotlin.types.Variance
@@ -28,7 +29,11 @@ class FirTypeParameterImpl(
         symbol.bind(this)
     }
 
-    override val bounds = mutableListOf<FirTypeRef>()
+    /*
+     * Note that each type parameter have to has at least one upper bound (Any? if there is no other bounds)
+     *   so if you create FirTypeParameterImpl you need to guarantee this contract
+     */
+    override val bounds: MutableList<FirTypeRef> = mutableListOf()
 
     override fun <D> transformChildren(transformer: FirTransformer<D>, data: D): FirElement {
         bounds.transformInplace(transformer, data)