[FIR] Add CANNOT_CHECK_FOR_ERASED

analysis/analysis-api-fir/src/org/jetbrains/kotlin/analysis/api/fir/diagnostics/KtFirDataClassConverters.kt

@@ -3097,6 +3097,13 @@ internal val KT_DIAGNOSTIC_CONVERTER = KtDiagnosticConverterBuilder.buildConvert
             token,
         )
     }
+    add(FirErrors.CANNOT_CHECK_FOR_ERASED) { firDiagnostic ->
+        CannotCheckForErasedImpl(
+            firSymbolBuilder.typeBuilder.buildKtType(firDiagnostic.a),
+            firDiagnostic as FirPsiDiagnostic,
+            token,
+        )
+    }
     add(FirErrors.USELESS_CAST) { firDiagnostic ->
         UselessCastImpl(
             firDiagnostic as KtPsiDiagnostic,

analysis/analysis-api-fir/src/org/jetbrains/kotlin/analysis/api/fir/diagnostics/KtFirDiagnostics.kt

@@ -2168,6 +2168,11 @@ sealed class KtFirDiagnostic<PSI : PsiElement> : KtDiagnosticWithPsi<PSI> {
         override val diagnosticClass get() = UselessElvisRightIsNull::class
     }
 
+    abstract class CannotCheckForErased : KtFirDiagnostic<PsiElement>() {
+        override val diagnosticClass get() = CannotCheckForErased::class
+        abstract val type: KtType
+    }
+
     abstract class UselessCast : KtFirDiagnostic<KtBinaryExpressionWithTypeRHS>() {
         override val diagnosticClass get() = UselessCast::class
     }

analysis/analysis-api-fir/src/org/jetbrains/kotlin/analysis/api/fir/diagnostics/KtFirDiagnosticsImpl.kt

@@ -2610,6 +2610,14 @@ internal class UselessElvisRightIsNullImpl(
     override val token: ValidityToken,
 ) : KtFirDiagnostic.UselessElvisRightIsNull(), KtAbstractFirDiagnostic<KtBinaryExpression>
 
+internal class CannotCheckForErasedImpl(
+    override val type: KtType,
+    firDiagnostic: FirPsiDiagnostic,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.CannotCheckForErased(), KtAbstractFirDiagnostic<PsiElement> {
+    override val firDiagnostic: FirPsiDiagnostic by weakRef(firDiagnostic)
+}
+
 internal class UselessCastImpl(
     override val firDiagnostic: KtPsiDiagnostic,
     override val token: ValidityToken,

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/CommonExpressionCheckers.kt

@@ -132,6 +132,7 @@ object CommonExpressionCheckers : ExpressionCheckers() {
     override val typeOperatorCallCheckers: Set<FirTypeOperatorCallChecker>
         get() = setOf(
             FirUselessTypeOperationCallChecker,
+            FirCannotCheckForErasedChecker
         )
 
     override val resolvedQualifierCheckers: Set<FirResolvedQualifierChecker>

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/FirCastDiagnosticsHelpers.kt

@@ -0,0 +1,213 @@
+/*
+ * Copyright 2010-2021 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * 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.analysis.checkers
+
+import org.jetbrains.kotlin.fir.analysis.checkers.context.CheckerContext
+import org.jetbrains.kotlin.fir.expressions.*
+import org.jetbrains.kotlin.fir.resolve.defaultType
+import org.jetbrains.kotlin.fir.resolve.platformClassMapper
+import org.jetbrains.kotlin.fir.resolve.substitution.ConeSubstitutorByMap
+import org.jetbrains.kotlin.fir.symbols.impl.FirFunctionSymbol
+import org.jetbrains.kotlin.fir.symbols.impl.FirRegularClassSymbol
+import org.jetbrains.kotlin.fir.typeContext
+import org.jetbrains.kotlin.fir.types.*
+import org.jetbrains.kotlin.types.AbstractTypeChecker
+import org.jetbrains.kotlin.types.AbstractTypeChecker.findCorrespondingSupertypes
+import org.jetbrains.kotlin.types.model.typeConstructor
+
+fun isCastErased(supertype: ConeKotlinType, subtype: ConeKotlinType, context: CheckerContext): Boolean {
+    val typeContext = context.session.typeContext
+
+    val isNonReifiedTypeParameter = subtype.isNonReifiedTypeParameter()
+    val isUpcast = isUpcast(context, supertype, subtype)
+
+    // here we want to restrict cases such as `x is T` for x = T?, when T might have nullable upper bound
+    if (isNonReifiedTypeParameter && !isUpcast) {
+        // hack to save previous behavior in case when `x is T`, where T is not nullable, see IsErasedNullableTasT.kt
+        val nullableToDefinitelyNotNull = !subtype.canBeNull && supertype.withNullability(ConeNullability.NOT_NULL, typeContext) == subtype
+        if (!nullableToDefinitelyNotNull) {
+            return true
+        }
+    }
+
+    // cast between T and T? is always OK
+    if (supertype.isMarkedNullable || subtype.isMarkedNullable) {
+        return isCastErased(
+            supertype.withNullability(ConeNullability.NOT_NULL, typeContext),
+            subtype.withNullability(ConeNullability.NOT_NULL, typeContext),
+            context
+        )
+    }
+
+    // if it is a upcast, it's never erased
+    if (isUpcast) return false
+
+    // downcasting to a non-reified type parameter is always erased
+    if (isNonReifiedTypeParameter) return true
+
+    // Check that we are actually casting to a generic type
+    // NOTE: this does not account for 'as Array<List<T>>'
+    if (subtype.allParameterReified()) return false
+
+    val staticallyKnownSubtype = findStaticallyKnownSubtype(supertype, subtype, context).first ?: return true
+
+    // If the substitution failed, it means that the result is an impossible type, e.g. something like Out<in Foo>
+    // In this case, we can't guarantee anything, so the cast is considered to be erased
+
+    // If the type we calculated is a subtype of the cast target, it's OK to use the cast target instead.
+    // If not, it's wrong to use it
+    return !AbstractTypeChecker.isSubtypeOf(context.session.typeContext, staticallyKnownSubtype, subtype)
+}
+
+private fun ConeKotlinType.allParameterReified(): Boolean {
+    return typeArguments.all { (it.type as? ConeTypeParameterType)?.lookupTag?.typeParameterSymbol?.isReified == true }
+}
+
+/**
+ * Remember that we are trying to cast something of type `supertype` to `subtype`.
+
+ * Since at runtime we can only check the class (type constructor), the rest of the subtype should be known statically, from supertype.
+ * This method reconstructs all static information that can be obtained from supertype.
+
+ * Example 1:
+ * supertype = Collection
+ * subtype = List<...>
+ * result = List, all arguments are inferred
+
+ * Example 2:
+ * supertype = Any
+ * subtype = List<...>
+ * result = List<*>, some arguments were not inferred, replaced with '*'
+ */
+fun findStaticallyKnownSubtype(
+    supertype: ConeKotlinType,
+    subtype: ConeKotlinType,
+    context: CheckerContext
+): Pair<ConeKotlinType?, Boolean> {
+    assert(!supertype.isMarkedNullable) { "This method only makes sense for non-nullable types" }
+
+    val session = context.session
+    val typeContext = session.typeContext
+
+    // Assume we are casting an expression of type Collection<Foo> to List<Bar>
+    // First, let's make List<T>, where T is a type variable
+    val subtypeWithVariables = subtype.toRegularClassSymbol(session)!!
+    val subtypeWithVariablesType = subtypeWithVariables.defaultType()
+
+    // Now, let's find a supertype of List<T> that is a Collection of something,
+    // in this case it will be Collection<T>
+    val typeCheckerState = context.session.typeContext.newTypeCheckerState(
+        errorTypesEqualToAnything = false,
+        stubTypesEqualToAnything = true
+    )
+
+    fun getFirstNotIntersectedType(type: ConeKotlinType): ConeKotlinType? {
+        if (type is ConeIntersectionType) {
+            for (intersectionType in type.intersectedTypes) {
+                val result = getFirstNotIntersectedType(intersectionType)
+                if (result != null) {
+                    return result
+                }
+            }
+            return null
+        }
+        return type
+    }
+
+    // Obtaining not intersected type to get not null typeConstructor.
+    // Not sure if it's correct
+    val notIntersectedSupertype = getFirstNotIntersectedType(supertype) ?: supertype
+    val supertypeWithVariables =
+        findCorrespondingSupertypes(
+            typeCheckerState,
+            subtypeWithVariablesType,
+            notIntersectedSupertype.typeConstructor(typeContext)
+        ).firstOrNull()
+
+    val variables = subtypeWithVariables.typeParameterSymbols
+
+    val substitution = if (supertypeWithVariables != null) {
+        // Now, let's try to unify Collection<T> and Collection<Foo> solution is a map from T to Foo
+        val typeUnifier = TypeUnifier(session, variables)
+        val unificationResult = typeUnifier.unify(supertype, supertypeWithVariables as ConeKotlinTypeProjection)
+        unificationResult.substitution.toMutableMap()
+    } else {
+        mutableMapOf()
+    }
+
+    // If some of the parameters are not determined by unification, it means that these parameters are lost,
+    // let's put stars instead, so that we can only cast to something like List<*>, e.g. (a: Any) as List<*>
+    var allArgumentsInferred = true
+    for (variable in variables) {
+        val value = substitution[variable]
+        if (value == null) {
+            substitution[variable] = context.session.builtinTypes.nullableAnyType.type
+            allArgumentsInferred = false
+        }
+    }
+
+    // At this point we have values for all type parameters of List
+    // Let's make a type by substituting them: List<T> -> List<Foo>
+    val substitutor = ConeSubstitutorByMap(substitution, session)
+    val substituted = substitutor.substituteOrSelf(subtypeWithVariablesType)
+
+    return Pair(substituted, allArgumentsInferred)
+}
+
+fun ConeKotlinType.isNonReifiedTypeParameter(): Boolean {
+    return this is ConeTypeParameterType && !this.lookupTag.typeParameterSymbol.isReified
+}
+
+@Suppress("UNUSED_PARAMETER")
+fun shouldCheckForExactType(expression: FirTypeOperatorCall, context: CheckerContext): Boolean {
+    return when (expression.operation) {
+        FirOperation.IS, FirOperation.NOT_IS -> false
+        // TODO: differentiate if this expression defines the enclosing thing's type
+        //   e.g.,
+        //   val c1 get() = 1 as Number
+        //   val c2: Number get() = 1 <!USELESS_CAST!>as Number<!>
+        FirOperation.AS, FirOperation.SAFE_AS -> true
+        else -> throw AssertionError("Should not be here: ${expression.operation}")
+    }
+}
+
+fun isRefinementUseless(
+    context: CheckerContext,
+    candidateType: ConeKotlinType,
+    targetType: ConeKotlinType,
+    shouldCheckForExactType: Boolean,
+    arg: FirExpression,
+): Boolean {
+    return if (shouldCheckForExactType) {
+        if (arg is FirFunctionCall) {
+            val functionSymbol = arg.toResolvedCallableSymbol() as? FirFunctionSymbol<*>
+            if (functionSymbol != null && functionSymbol.isFunctionForExpectTypeFromCastFeature()) return false
+        }
+
+        isExactTypeCast(context, candidateType, targetType)
+    } else {
+        isUpcast(context, candidateType, targetType)
+    }
+}
+
+private fun isExactTypeCast(context: CheckerContext, candidateType: ConeKotlinType, targetType: ConeKotlinType): Boolean {
+    if (!AbstractTypeChecker.equalTypes(context.session.typeContext, candidateType, targetType, stubTypesEqualToAnything = false))
+        return false
+    // See comments at [isUpcast] why we need to check the existence of @ExtensionFunctionType
+    return candidateType.isExtensionFunctionType == targetType.isExtensionFunctionType
+}
+
+private fun isUpcast(context: CheckerContext, candidateType: ConeKotlinType, targetType: ConeKotlinType): Boolean {
+    if (!AbstractTypeChecker.isSubtypeOf(context.session.typeContext, candidateType, targetType, stubTypesEqualToAnything = false))
+        return false
+
+    // E.g., foo(p1: (X) -> Y), where p1 has a functional type whose receiver type is X and return type is Y.
+    // For bar(p2: X.() -> Y), p2 has the same functional type (with same receiver and return types).
+    // The only difference is the existence of type annotation, @ExtensionFunctionType,
+    //   which indicates that the annotated type represents an extension function.
+    // If one casts p1 to p2 (or vice versa), it is _not_ up cast, i.e., not redundant, yet meaningful.
+    return candidateType.isExtensionFunctionType == targetType.isExtensionFunctionType
+}

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/TypeUnifier.kt

@@ -0,0 +1,157 @@
+/*
+ * Copyright 2010-2021 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * 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.analysis.checkers
+
+import org.jetbrains.kotlin.fir.FirSession
+import org.jetbrains.kotlin.fir.symbols.impl.FirTypeParameterSymbol
+import org.jetbrains.kotlin.fir.types.*
+import org.jetbrains.kotlin.types.AbstractTypeChecker
+
+class TypeUnifier(private val session: FirSession, private val typeParameterSymbols: List<FirTypeParameterSymbol>) {
+    /**
+     * Finds a substitution S that turns {@code projectWithVariables} to {@code knownProjection}.
+     *
+     * Example:
+     *      known = List<String>
+     *      withVariables = List<X>
+     *      variables = {X}
+     *
+     *      result = X -> String
+     *
+     * Only types accepted by {@code isVariable} are considered variables.
+     */
+    fun unify(knownProjection: ConeKotlinTypeProjection, projectWithVariables: ConeKotlinTypeProjection): UnificationResult {
+        val result = UnificationResult()
+        doUnify(knownProjection, projectWithVariables, result)
+        return result
+    }
+
+    fun doUnify(knownProjection: ConeTypeProjection, projectWithVariables: ConeTypeProjection, result: UnificationResult) {
+        val firstType = knownProjection.type
+
+        if (firstType is ConeIntersectionType) {
+            val intersectionResult = mutableMapOf<FirTypeParameterSymbol, ConeKotlinType>()
+
+            for (intersectedType in firstType.intersectedTypes) {
+                val localResult = UnificationResult()
+                doUnify(intersectedType, projectWithVariables, localResult)
+
+                for ((typeParameterSymbol, typeParameterType) in localResult.substitution) {
+                    val existingTypeParameterType = intersectionResult[typeParameterSymbol]
+                    if (existingTypeParameterType == null ||
+                        AbstractTypeChecker.isSubtypeOf(session.typeContext, typeParameterType, existingTypeParameterType)
+                    ) {
+                        intersectionResult[typeParameterSymbol] = typeParameterType
+                    }
+                }
+            }
+
+            for ((typeParameterSymbol, typeParameterType) in intersectionResult) {
+                result.put(typeParameterSymbol, typeParameterType)
+            }
+
+            return
+        }
+
+        val known = firstType?.lowerBoundIfFlexible() ?: session.builtinTypes.nullableAnyType.type
+        val withVariables = projectWithVariables.type?.lowerBoundIfFlexible() ?: session.builtinTypes.nullableAnyType.type
+
+        // in Foo ~ in X  =>  Foo ~ X
+        val knownProjectionKind = knownProjection.kind
+        val withVariablesProjectionKind = projectWithVariables.kind
+        if (knownProjectionKind == withVariablesProjectionKind && knownProjectionKind != ProjectionKind.INVARIANT) {
+            doUnify(known, withVariables, result)
+            return
+        }
+
+        // Foo? ~ X?  =>  Foo ~ X
+        if (known.isMarkedNullable && withVariables.isMarkedNullable) {
+            doUnify(
+                known.withNullability(ConeNullability.NOT_NULL, session.typeContext).toTypeProjection(
+                    knownProjectionKind
+                ) as ConeKotlinTypeProjection,
+                withVariables.withNullability(ConeNullability.NOT_NULL, session.typeContext).toTypeProjection(
+                    withVariablesProjectionKind
+                ) as ConeKotlinTypeProjection,
+                result
+            )
+        }
+
+        // in Foo ~ out X  => fail
+        // in Foo ~ X  =>  may be OK
+        if (knownProjectionKind != withVariablesProjectionKind && withVariablesProjectionKind != ProjectionKind.INVARIANT) {
+            result.fail()
+            return
+        }
+
+        // Foo ~ X? => fail
+        if (!known.isMarkedNullable && withVariables.isMarkedNullable) {
+            result.fail()
+            return
+        }
+
+        // Foo ~ X  =>  x |-> Foo
+        // * ~ X => x |-> *
+        val maybeVariable = withVariables.toSymbol(session)
+        if (maybeVariable is FirTypeParameterSymbol && typeParameterSymbols.contains(maybeVariable)) {
+            result.put(maybeVariable, known)
+            return
+        }
+
+        // Foo? ~ Foo || in Foo ~ Foo || Foo ~ Bar
+        val structuralMismatch = known.isMarkedNullable != withVariables.isMarkedNullable ||
+                knownProjectionKind != withVariablesProjectionKind ||
+                known.toSymbol(session) != maybeVariable
+        if (structuralMismatch) {
+            result.fail()
+            return
+        }
+
+        // Foo<A> ~ Foo<B, C>
+        if (known.typeArguments.size != withVariables.typeArguments.size) {
+            result.fail()
+            return
+        }
+
+        // Foo ~ Foo
+        if (known.typeArguments.isEmpty()) {
+            return
+        }
+
+        // Foo<...> ~ Foo<...>
+        val knownArguments = known.typeArguments
+        val withVariablesArguments = withVariables.typeArguments
+        for (index in knownArguments.indices) {
+            val knownArg = knownArguments[index]
+            val withVariablesArg = withVariablesArguments[index]
+            doUnify(knownArg, withVariablesArg, result)
+        }
+    }
+}
+
+class UnificationResult {
+    private var success: Boolean = true
+    private var failedVariables: MutableSet<FirTypeParameterSymbol> = mutableSetOf()
+    private val _substitution: MutableMap<FirTypeParameterSymbol, ConeKotlinType> = mutableMapOf()
+    val substitution: Map<FirTypeParameterSymbol, ConeKotlinType>
+        get() = _substitution
+
+    fun fail() {
+        success = false
+    }
+
+    fun put(key: FirTypeParameterSymbol, value: ConeKotlinType) {
+        if (failedVariables.contains(key)) return
+
+        if (substitution.containsKey(key)) {
+            _substitution.remove(key)
+            failedVariables.add(key)
+            fail()
+        } else {
+            _substitution[key] = value
+        }
+    }
+}

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/expression/FirCannotCheckForErasedChecker.kt

@@ -0,0 +1,31 @@
+/*
+ * Copyright 2010-2021 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * 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.analysis.checkers.expression
+
+import org.jetbrains.kotlin.fir.analysis.checkers.context.CheckerContext
+import org.jetbrains.kotlin.fir.analysis.checkers.isCastErased
+import org.jetbrains.kotlin.fir.analysis.diagnostics.DiagnosticReporter
+import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors
+import org.jetbrains.kotlin.fir.analysis.diagnostics.reportOn
+import org.jetbrains.kotlin.fir.expressions.FirOperation
+import org.jetbrains.kotlin.fir.expressions.FirTypeOperatorCall
+import org.jetbrains.kotlin.fir.resolve.fullyExpandedType
+import org.jetbrains.kotlin.fir.types.coneType
+
+object FirCannotCheckForErasedChecker : FirTypeOperatorCallChecker() {
+    override fun check(expression: FirTypeOperatorCall, context: CheckerContext, reporter: DiagnosticReporter) {
+        if (expression.operation != FirOperation.IS) return
+
+        val session = context.session
+        val subjectType = expression.argumentList.arguments[0].typeRef.coneType.fullyExpandedType(session)
+        val conversionTypeRef = expression.conversionTypeRef
+        val targetType = conversionTypeRef.coneType.fullyExpandedType(session)
+
+        if (isCastErased(subjectType, targetType, context)) {
+            reporter.reportOn(conversionTypeRef.source, FirErrors.CANNOT_CHECK_FOR_ERASED, targetType, context)
+        }
+    }
+}

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/expression/FirCastOperatorsChecker.kt

@@ -0,0 +1,49 @@
+/*
+ * Copyright 2010-2021 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * 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.analysis.checkers.expression
+
+import org.jetbrains.kotlin.fir.analysis.checkers.CastingType
+import org.jetbrains.kotlin.fir.analysis.checkers.context.CheckerContext
+import org.jetbrains.kotlin.fir.analysis.checkers.isCastErased
+import org.jetbrains.kotlin.fir.analysis.checkers.checkCasting
+import org.jetbrains.kotlin.fir.analysis.diagnostics.DiagnosticReporter
+import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors
+import org.jetbrains.kotlin.fir.analysis.diagnostics.reportOn
+import org.jetbrains.kotlin.fir.expressions.FirExpressionWithSmartcast
+import org.jetbrains.kotlin.fir.expressions.FirOperation
+import org.jetbrains.kotlin.fir.expressions.FirTypeOperatorCall
+import org.jetbrains.kotlin.fir.resolve.fullyExpandedType
+import org.jetbrains.kotlin.fir.types.coneType
+
+object FirCastOperatorsChecker : FirTypeOperatorCallChecker() {
+    override fun check(expression: FirTypeOperatorCall, context: CheckerContext, reporter: DiagnosticReporter) {
+        val session = context.session
+        val firstArgument = expression.argumentList.arguments[0]
+        val actualType = (if (firstArgument is FirExpressionWithSmartcast) {
+            firstArgument.originalType.coneType
+        } else {
+            firstArgument.typeRef.coneType
+        }).fullyExpandedType(session)
+        val conversionTypeRef = expression.conversionTypeRef
+        val targetType = conversionTypeRef.coneType.fullyExpandedType(session)
+
+        val isSafeAs = expression.operation == FirOperation.SAFE_AS
+        if (expression.operation == FirOperation.AS || isSafeAs) {
+            val castType = checkCasting(actualType, targetType, isSafeAs, context)
+            if (castType == CastingType.Impossible) {
+                reporter.reportOn(expression.source, FirErrors.CAST_NEVER_SUCCEEDS, context)
+            } else if (castType == CastingType.Always) {
+                reporter.reportOn(expression.source, FirErrors.USELESS_CAST, context)
+            } else if (isCastErased(actualType, targetType, context)) {
+                reporter.reportOn(expression.source, FirErrors.UNCHECKED_CAST, actualType, targetType, context)
+            }
+        } else if (expression.operation == FirOperation.IS) {
+            if (isCastErased(actualType, targetType, context)) {
+                reporter.reportOn(conversionTypeRef.source, FirErrors.CANNOT_CHECK_FOR_ERASED, targetType, context)
+            }
+        }
+    }
+}

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/expression/FirUselessTypeOperationCallChecker.kt

@@ -6,15 +6,14 @@
 package org.jetbrains.kotlin.fir.analysis.checkers.expression
 
 import org.jetbrains.kotlin.fir.analysis.checkers.context.CheckerContext
-import org.jetbrains.kotlin.fir.analysis.checkers.isFunctionForExpectTypeFromCastFeature
 import org.jetbrains.kotlin.diagnostics.DiagnosticReporter
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors
 import org.jetbrains.kotlin.diagnostics.reportOn
+import org.jetbrains.kotlin.fir.analysis.checkers.isRefinementUseless
+import org.jetbrains.kotlin.fir.analysis.checkers.shouldCheckForExactType
 import org.jetbrains.kotlin.fir.expressions.*
 import org.jetbrains.kotlin.fir.resolve.fullyExpandedType
-import org.jetbrains.kotlin.fir.symbols.impl.FirFunctionSymbol
 import org.jetbrains.kotlin.fir.types.*
-import org.jetbrains.kotlin.types.AbstractTypeChecker
 
 // See .../types/CastDiagnosticsUtil.kt for counterparts, including isRefinementUseless, isExactTypeCast, isUpcast.
 object FirUselessTypeOperationCallChecker : FirTypeOperatorCallChecker() {
@@ -44,55 +43,4 @@ object FirUselessTypeOperationCallChecker : FirTypeOperatorCallChecker() {
             }
         }
     }
-
-    @Suppress("UNUSED_PARAMETER")
-    private fun shouldCheckForExactType(expression: FirTypeOperatorCall, context: CheckerContext): Boolean {
-        return when (expression.operation) {
-            FirOperation.IS, FirOperation.NOT_IS -> false
-            // TODO: differentiate if this expression defines the enclosing thing's type
-            //   e.g.,
-            //   val c1 get() = 1 as Number
-            //   val c2: Number get() = 1 <!USELESS_CAST!>as Number<!>
-            FirOperation.AS, FirOperation.SAFE_AS -> true
-            else -> throw AssertionError("Should not be here: ${expression.operation}")
-        }
-    }
-
-    private fun isRefinementUseless(
-        context: CheckerContext,
-        candidateType: ConeKotlinType,
-        targetType: ConeKotlinType,
-        shouldCheckForExactType: Boolean,
-        arg: FirExpression,
-    ): Boolean {
-        return if (shouldCheckForExactType) {
-            if (arg is FirFunctionCall) {
-                val functionSymbol = arg.toResolvedCallableSymbol() as? FirFunctionSymbol<*>
-                if (functionSymbol != null && functionSymbol.isFunctionForExpectTypeFromCastFeature()) return false
-            }
-
-            isExactTypeCast(context, candidateType, targetType)
-        } else {
-            isUpcast(context, candidateType, targetType)
-        }
-    }
-
-    private fun isExactTypeCast(context: CheckerContext, candidateType: ConeKotlinType, targetType: ConeKotlinType): Boolean {
-        if (!AbstractTypeChecker.equalTypes(context.session.typeContext, candidateType, targetType, stubTypesEqualToAnything = false))
-            return false
-        // See comments at [isUpcast] why we need to check the existence of @ExtensionFunctionType
-        return candidateType.isExtensionFunctionType == targetType.isExtensionFunctionType
-    }
-
-    private fun isUpcast(context: CheckerContext, candidateType: ConeKotlinType, targetType: ConeKotlinType): Boolean {
-        if (!AbstractTypeChecker.isSubtypeOf(context.session.typeContext, candidateType, targetType, stubTypesEqualToAnything = false))
-            return false
-
-        // E.g., foo(p1: (X) -> Y), where p1 has a functional type whose receiver type is X and return type is Y.
-        // For bar(p2: X.() -> Y), p2 has the same functional type (with same receiver and return types).
-        // The only difference is the existence of type annotation, @ExtensionFunctionType,
-        //   which indicates that the annotated type represents an extension function.
-        // If one casts p1 to p2 (or vice versa), it is _not_ up cast, i.e., not redundant, yet meaningful.
-        return candidateType.isExtensionFunctionType == targetType.isExtensionFunctionType
-    }
 }

compiler/fir/cones/src/org/jetbrains/kotlin/fir/types/ConeTypeUtils.kt

@@ -65,6 +65,15 @@ fun ConeKotlinType.toTypeProjection(variance: Variance): ConeTypeProjection =
         Variance.OUT_VARIANCE -> ConeKotlinTypeProjectionOut(this)
     }
 
+fun ConeKotlinType.toTypeProjection(projectionKind: ProjectionKind): ConeTypeProjection {
+    return when (projectionKind) {
+        ProjectionKind.INVARIANT -> this
+        ProjectionKind.IN -> ConeKotlinTypeProjectionIn(this)
+        ProjectionKind.OUT -> ConeKotlinTypeProjectionOut(this)
+        ProjectionKind.STAR -> ConeStarProjection
+    }
+}
+
 fun ConeClassLikeType.replaceArgumentsWithStarProjections(): ConeClassLikeType {
     if (typeArguments.isEmpty()) return this
     val newArguments = Array(typeArguments.size) { ConeStarProjection }

compiler/testData/diagnostics/tests/cast/IsErasedAllowForTypeWithIrrelevantMixin.fir.kt

@@ -0,0 +1,9 @@
+interface A
+interface B: A
+interface D
+
+interface BaseSuper<out T>
+interface BaseImpl: BaseSuper<D>
+interface DerivedSuper<out S>: <!INCONSISTENT_TYPE_PARAMETER_VALUES!>BaseSuper<S>, BaseImpl<!>
+
+fun test(t: BaseSuper<B>) = t is <!CANNOT_CHECK_FOR_ERASED!>DerivedSuper<A><!>

compiler/testData/diagnostics/tests/cast/IsErasedAllowForTypeWithIrrelevantMixin.kt

@@ -1,4 +1,3 @@
-// FIR_IDENTICAL
 interface A
 interface B: A
 interface D

compiler/testData/diagnostics/tests/cast/IsErasedDisallowDifferentArgInvariantPosition.fir.kt

@@ -1 +0,0 @@
-fun f(a: MutableList<String>) = a is MutableList<CharSequence>

compiler/testData/diagnostics/tests/cast/IsErasedDisallowDifferentArgInvariantPosition.kt

@@ -1 +1,2 @@
+// FIR_IDENTICAL
 fun f(a: MutableList<String>) = a is <!CANNOT_CHECK_FOR_ERASED!>MutableList<CharSequence><!>

compiler/testData/diagnostics/tests/cast/IsErasedDisallowForOverridenVariance.fir.kt

@@ -1,8 +0,0 @@
-open class A
-open class B: A()
-
-open class Base<out T>
-open class SubBase<T> : Base<T>()
-
-
-fun ff(l: Base<B>) = l is SubBase<A>

compiler/testData/diagnostics/tests/cast/IsErasedDisallowForOverridenVariance.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 open class A
 open class B: A()
 

compiler/testData/diagnostics/tests/cast/IsErasedDisallowForTypeWithConstraints.fir.kt

@@ -1,6 +0,0 @@
-interface A
-interface B: A
-
-interface Base<T>
-
-fun <T> test(a: Base<B>) where T: Base<A> = a is T

compiler/testData/diagnostics/tests/cast/IsErasedDisallowForTypeWithConstraints.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 interface A
 interface B: A
 

compiler/testData/diagnostics/tests/cast/IsErasedDisallowFromAny.fir.kt

@@ -1,2 +0,0 @@
-
-fun ff(l: Any) = l is MutableList<String>

compiler/testData/diagnostics/tests/cast/IsErasedDisallowFromAny.kt

@@ -1,2 +1,3 @@
+// FIR_IDENTICAL
 
 fun ff(l: Any) = l is <!CANNOT_CHECK_FOR_ERASED!>MutableList<String><!>

compiler/testData/diagnostics/tests/cast/IsErasedDisallowFromIn.fir.kt

@@ -1 +0,0 @@
-fun f(a: MutableList<in String>) = a is MutableList<CharSequence>

compiler/testData/diagnostics/tests/cast/IsErasedDisallowFromIn.kt

@@ -1 +1,2 @@
+// FIR_IDENTICAL
 fun f(a: MutableList<in String>) = a is <!CANNOT_CHECK_FOR_ERASED!>MutableList<CharSequence><!>

compiler/testData/diagnostics/tests/cast/IsErasedDisallowFromOut.fir.kt

@@ -1,2 +0,0 @@
-
-fun f(a : MutableList<out Any>) = a is MutableList<out Int>

compiler/testData/diagnostics/tests/cast/IsErasedDisallowFromOut.kt

@@ -1,2 +1,3 @@
+// FIR_IDENTICAL
 
 fun f(a : MutableList<out Any>) = a is <!CANNOT_CHECK_FOR_ERASED!>MutableList<out Int><!>

compiler/testData/diagnostics/tests/cast/IsErasedDisallowFromOutAtClass.fir.kt

@@ -1 +0,0 @@
-fun f(a: List<Any>) = a is List<Number>

compiler/testData/diagnostics/tests/cast/IsErasedDisallowFromOutAtClass.kt

@@ -1 +1,2 @@
+// FIR_IDENTICAL
 fun f(a: List<Any>) = a is <!CANNOT_CHECK_FOR_ERASED!>List<Number><!>

compiler/testData/diagnostics/tests/cast/IsErasedDissallowForSubtypeMappedToTwoParamsWithFirstInvalid.fir.kt

@@ -1,8 +0,0 @@
-open class A
-open class B: A()
-open class D
-
-open class Base<out T, out U>
-open class Derived<out S>: Base<S, S>()
-
-fun test(a: Base<D, B>) = a is Derived<A>

compiler/testData/diagnostics/tests/cast/IsErasedDissallowForSubtypeMappedToTwoParamsWithFirstInvalid.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 open class A
 open class B: A()
 open class D

compiler/testData/diagnostics/tests/cast/IsErasedDissallowForSubtypeMappedToTwoParamsWithSecondInvalid.fir.kt

@@ -1,8 +0,0 @@
-open class A
-open class B: A()
-open class D
-
-open class Base<out T, out U>
-open class Derived<out S>: Base<S, S>()
-
-fun test(a: Base<B, D>) = a is Derived<A>

compiler/testData/diagnostics/tests/cast/IsErasedDissallowForSubtypeMappedToTwoParamsWithSecondInvalid.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 open class A
 open class B: A()
 open class D

compiler/testData/diagnostics/tests/cast/IsErasedUpcastToNonReified.fir.kt

@@ -1,5 +1,5 @@
 fun <T, S : T> test(x: T?, y: S, z: T) {
-    x is T
+    x is <!CANNOT_CHECK_FOR_ERASED!>T<!>
     <!USELESS_IS_CHECK!>x is T?<!>
 
     <!USELESS_IS_CHECK!>y is T<!>

compiler/testData/diagnostics/tests/cast/IsRecursionSustainable.fir.kt

@@ -2,5 +2,5 @@ open class RecA<T>: <!CYCLIC_INHERITANCE_HIERARCHY!>RecB<T><!>()
 open class RecB<T>: <!CYCLIC_INHERITANCE_HIERARCHY!>RecA<T><!>()
 open class SelfR<T>: <!CYCLIC_INHERITANCE_HIERARCHY!>SelfR<T><!>()
 
-fun test(f: SelfR<String>) = f is RecA<String>
-fun test(f: RecB<String>) = f is RecA<String>
+fun test(f: SelfR<String>) = f is <!CANNOT_CHECK_FOR_ERASED!>RecA<String><!>
+fun test(f: RecB<String>) = f is <!CANNOT_CHECK_FOR_ERASED!>RecA<String><!>

compiler/testData/diagnostics/tests/cast/WhenErasedDisallowFromAny.fir.kt

@@ -1,5 +0,0 @@
-
-fun ff(l: Any) = when(l) {
-    is MutableList<String> -> 1
-    else <!SYNTAX!>2<!>
-}

compiler/testData/diagnostics/tests/cast/WhenErasedDisallowFromAny.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 
 fun ff(l: Any) = when(l) {
     is <!CANNOT_CHECK_FOR_ERASED!>MutableList<String><!> -> 1

compiler/testData/diagnostics/tests/platformTypes/genericVarianceViolation/smartCast.fir.kt

@@ -9,7 +9,7 @@ public class A {
 // FILE: main.kt
 
 fun main(a: A, ml: Any) {
-    if (ml is MutableList<String>) {
+    if (ml is <!CANNOT_CHECK_FOR_ERASED!>MutableList<String><!>) {
         a.foo(<!JAVA_TYPE_MISMATCH!>ml<!>)
         a.foo(ml as List<Any>)
     }

compiler/testData/diagnostics/tests/regressions/kt398.fir.kt

@@ -1,11 +0,0 @@
-// KT-398 Internal error when property initializes with function
-
-class X<T>() {
-    val check = { a : Any -> a is T }
-}
-
-fun box() : String {
-    if(X<String>().check(10)) return "fail"
-    if(!X<String>().check("lala")) return "fail"
-    return "OK"
-}

compiler/testData/diagnostics/tests/regressions/kt398.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 // KT-398 Internal error when property initializes with function
 
 class X<T>() {

compiler/testData/diagnostics/tests/regressions/kt399.fir.kt

@@ -1,11 +0,0 @@
-// KT-399 Type argument inference not implemented for CALL_EXPRESSION
-
-fun <T> getSameTypeChecker(obj: T) : Function1<Any,Boolean> {
-    return { a : Any -> a is T }
-}
-
-fun box() : String {
-    if(getSameTypeChecker<String>("lala")(10)) return "fail"
-    if(!getSameTypeChecker<String>("mama")("lala")) return "fail"
-    return "OK"
-}

compiler/testData/diagnostics/tests/regressions/kt399.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 // KT-399 Type argument inference not implemented for CALL_EXPRESSION
 
 fun <T> getSameTypeChecker(obj: T) : Function1<Any,Boolean> {

compiler/testData/diagnostics/testsWithStdLib/cast/IsArray.fir.kt

@@ -1,5 +0,0 @@
-fun f(a: Array<out Number>) = a.isArrayOf<Int>()
-
-fun f1(a: Array<out Number>) = <!USELESS_IS_CHECK!>a is Array<*><!>
-
-fun f2(a: Array<out Number>) = a is Array<Int>

compiler/testData/diagnostics/testsWithStdLib/cast/IsArray.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 fun f(a: Array<out Number>) = a.isArrayOf<Int>()
 
 fun f1(a: Array<out Number>) = <!USELESS_IS_CHECK!>a is Array<*><!>

compiler/tests-spec/testData/diagnostics/linked/type-system/type-contexts-and-scopes/inner-and-nested-type-contexts/p-1/neg/1.1.fir.kt

@@ -9,14 +9,14 @@ class Case1<AT>(val x: AT) {
 
     inner class C() {
         fun case1a(x: Any) {
-            if (x is AT) {
+            if (x is <!CANNOT_CHECK_FOR_ERASED!>AT<!>) {
                 ""
             }
         }
 
         fun case1b(x: Any) {
             when (x) {
-                is AT -> println("at")
+                is <!CANNOT_CHECK_FOR_ERASED!>AT<!> -> println("at")
             }
         }
     }

compiler/tests-spec/testData/diagnostics/linked/type-system/type-contexts-and-scopes/inner-and-nested-type-contexts/p-1/neg/1.2.fir.kt

@@ -9,13 +9,13 @@ class Case1<AT: CharSequence>(val x: AT) {
 
     inner class C() {
         fun case1a(x: Any) {
-            if (x is AT) {
+            if (x is <!CANNOT_CHECK_FOR_ERASED!>AT<!>) {
                 ""
             }
         }
 
         fun case1b(x: Any) = when (x) {
-                is AT -> println("at")
+                is <!CANNOT_CHECK_FOR_ERASED!>AT<!> -> println("at")
                 else -> ""
             }
 
@@ -28,14 +28,14 @@ class Case2<AT: CharSequence>(val x: AT) {
 
     inner class C() {
         fun case2a(x: CharSequence) {
-            if (x is AT) {
+            if (x is <!CANNOT_CHECK_FOR_ERASED!>AT<!>) {
                 ""
             }
         }
 
         fun case2b(x: CharSequence) {
             when (x) {
-                is AT -> ""
+                is <!CANNOT_CHECK_FOR_ERASED!>AT<!> -> ""
             }
         }
     }