FIR type enhancement: fold flexible types when it's possible

Related to KT-29937

compiler/fir/java/src/org/jetbrains/kotlin/fir/java/enhancement/javaTypeUtils.kt

@@ -23,6 +23,7 @@ import org.jetbrains.kotlin.fir.service
 import org.jetbrains.kotlin.fir.symbols.ConeClassifierSymbol
 import org.jetbrains.kotlin.fir.symbols.FirBasedSymbol
 import org.jetbrains.kotlin.fir.symbols.impl.FirClassSymbol
+import org.jetbrains.kotlin.fir.typeContext
 import org.jetbrains.kotlin.fir.types.*
 import org.jetbrains.kotlin.fir.types.impl.FirResolvedTypeRefImpl
 import org.jetbrains.kotlin.load.java.JvmAnnotationNames
@@ -33,6 +34,7 @@ import org.jetbrains.kotlin.load.java.structure.*
 import org.jetbrains.kotlin.load.java.typeEnhancement.*
 import org.jetbrains.kotlin.name.ClassId
 import org.jetbrains.kotlin.name.FqName
+import org.jetbrains.kotlin.types.AbstractStrictEqualityTypeChecker
 import org.jetbrains.kotlin.types.Variance
 import org.jetbrains.kotlin.utils.addToStdlib.safeAs
 
@@ -65,7 +67,7 @@ private fun JavaType?.enhancePossiblyFlexible(
 
             FirResolvedTypeRefImpl(
                 session, psi = null,
-                type = ConeFlexibleType(lowerResult, upperResult),
+                type = coneFlexibleOrSimpleType(session, lowerResult, upperResult),
                 isMarkedNullable = false, annotations = annotations
             )
         }
@@ -81,6 +83,17 @@ private fun JavaType?.subtreeSize(): Int {
     return 1 + typeArguments.sumBy { it?.subtreeSize() ?: 0 }
 }
 
+private fun coneFlexibleOrSimpleType(
+    session: FirSession,
+    lowerBound: ConeLookupTagBasedType,
+    upperBound: ConeLookupTagBasedType
+): ConeKotlinType {
+    if (AbstractStrictEqualityTypeChecker.strictEqualTypes(session.typeContext, lowerBound, upperBound)) {
+        return lowerBound
+    }
+    return ConeFlexibleType(lowerBound, upperBound)
+}
+
 private val KOTLIN_COLLECTIONS = FqName("kotlin.collections")
 
 private val KOTLIN_COLLECTIONS_PREFIX_LENGTH = KOTLIN_COLLECTIONS.asString().length + 1

compiler/fir/java/src/org/jetbrains/kotlin/fir/java/scopes/JavaClassUseSiteScope.kt

@@ -33,15 +33,6 @@ class JavaClassUseSiteScope(
     //base symbol as key, overridden as value
     private val overriddenByBase = mutableMapOf<ConeFunctionSymbol, ConeFunctionSymbol?>()
 
-    @Suppress("UNUSED_PARAMETER")
-    private fun isSubtypeOf(subType: ConeKotlinType, superType: ConeKotlinType): Boolean {
-        // TODO: introduce normal sub-typing
-        return true
-    }
-
-    private fun isSubtypeOf(subType: FirTypeRef, superType: FirTypeRef) =
-        isSubtypeOf(subType.toNotNullConeKotlinType(), superType.toNotNullConeKotlinType())
-
     @Suppress("UNUSED_PARAMETER")
     private fun isEqualTypes(a: ConeKotlinType, b: ConeKotlinType): Boolean {
         // TODO: introduce normal type comparison
@@ -73,7 +64,6 @@ class JavaClassUseSiteScope(
             val member = (it as FirFunctionSymbol).fir as FirNamedFunction
             self.modality != Modality.FINAL
                     && sameReceivers(member.receiverTypeRef, self.receiverTypeRef)
-                    && isSubtypeOf(member.returnTypeRef, self.returnTypeRef)
                     && isOverriddenFunCheck(member, self)
         } // TODO: two or more overrides for one fun?
         overriddenByBase[this] = overriding