[FIR] Add exhaustive checker for when expressions

Also remove generating default else branch introduced in 38fa0122 and 54eb4cf9

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

@@ -41,15 +41,15 @@ abstract class FirSession(val sessionProvider: FirSessionProvider?) {
 }
 
 class BuiltinTypes {
-    val unitType: FirTypeRef = FirImplicitUnitTypeRef(null)
-    val anyType: FirTypeRef = FirImplicitAnyTypeRef(null)
-    val nullableAnyType: FirTypeRef = FirImplicitNullableAnyTypeRef(null)
-    val enumType: FirTypeRef = FirImplicitEnumTypeRef(null)
-    val annotationType: FirTypeRef = FirImplicitAnnotationTypeRef(null)
-    val booleanType: FirTypeRef = FirImplicitBooleanTypeRef(null)
-    val nothingType: FirTypeRef = FirImplicitNothingTypeRef(null)
-    val nullableNothingType: FirTypeRef = FirImplicitNullableNothingTypeRef(null)
-    val stringType: FirTypeRef = FirImplicitStringTypeRef(null)
+    val unitType: FirImplicitBuiltinTypeRef = FirImplicitUnitTypeRef(null)
+    val anyType: FirImplicitBuiltinTypeRef = FirImplicitAnyTypeRef(null)
+    val nullableAnyType: FirImplicitBuiltinTypeRef = FirImplicitNullableAnyTypeRef(null)
+    val enumType: FirImplicitBuiltinTypeRef = FirImplicitEnumTypeRef(null)
+    val annotationType: FirImplicitBuiltinTypeRef = FirImplicitAnnotationTypeRef(null)
+    val booleanType: FirImplicitBuiltinTypeRef = FirImplicitBooleanTypeRef(null)
+    val nothingType: FirImplicitBuiltinTypeRef = FirImplicitNothingTypeRef(null)
+    val nullableNothingType: FirImplicitBuiltinTypeRef = FirImplicitNullableNothingTypeRef(null)
+    val stringType: FirImplicitBuiltinTypeRef = FirImplicitStringTypeRef(null)
 }
 
 interface FirSessionProvider {

compiler/fir/tree/src/org/jetbrains/kotlin/fir/declarations/FirDeclarationUtil.kt

@@ -5,6 +5,7 @@
 
 package org.jetbrains.kotlin.fir.declarations
 
+import org.jetbrains.kotlin.descriptors.ClassKind
 import org.jetbrains.kotlin.fir.declarations.impl.FirEnumEntryImpl
 import org.jetbrains.kotlin.fir.declarations.impl.FirModifiableRegularClass
 import org.jetbrains.kotlin.fir.declarations.impl.FirTypeParameterImpl
@@ -59,4 +60,9 @@ val FirTypeAlias.expandedConeType: ConeClassLikeType? get() = expandedTypeRef.co
 
 val FirRegularClass.classId get() = symbol.classId
 
-val FirClass.superConeTypes get() = superTypeRefs.mapNotNull { it.coneTypeSafe<ConeClassLikeType>() }
+val FirClass.superConeTypes get() = superTypeRefs.mapNotNull { it.coneTypeSafe<ConeClassLikeType>() }
+
+fun FirRegularClass.collectEnumEntries(): Collection<FirEnumEntry> {
+    assert(classKind == ClassKind.ENUM_CLASS)
+    return declarations.filterIsInstance<FirEnumEntry>()
+}

compiler/fir/tree/src/org/jetbrains/kotlin/fir/expressions/FirWhenExpression.kt

@@ -24,9 +24,12 @@ interface FirWhenExpression : FirExpression, FirResolvable {
     val subject: FirExpression?
     val subjectVariable: FirVariable<*>?
     val branches: List<FirWhenBranch>
+    val isExhaustive: Boolean
 
     override fun <R, D> accept(visitor: FirVisitor<R, D>, data: D): R = visitor.visitWhenExpression(this, data)
 
+    fun replaceIsExhaustive(newIsExhaustive: Boolean)
+
     override fun <D> transformCalleeReference(transformer: FirTransformer<D>, data: D): FirWhenExpression
 
     fun <D> transformSubject(transformer: FirTransformer<D>, data: D): FirWhenExpression

compiler/fir/tree/src/org/jetbrains/kotlin/fir/expressions/impl/FirWhenExpressionImpl.kt

@@ -32,6 +32,7 @@ class FirWhenExpressionImpl(
     override val annotations: MutableList<FirAnnotationCall> = mutableListOf()
     override var calleeReference: FirReference = FirStubReference()
     override val branches: MutableList<FirWhenBranch> = mutableListOf()
+    override var isExhaustive: Boolean = false
 
     override fun <R, D> acceptChildren(visitor: FirVisitor<R, D>, data: D) {
         typeRef.accept(visitor, data)
@@ -81,4 +82,8 @@ class FirWhenExpressionImpl(
     override fun replaceTypeRef(newTypeRef: FirTypeRef) {
         typeRef = newTypeRef
     }
+
+    override fun replaceIsExhaustive(newIsExhaustive: Boolean) {
+        isExhaustive = newIsExhaustive
+    }
 }

compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/ImplementationConfigurator.kt

@@ -425,6 +425,7 @@ object ImplementationConfigurator : AbstractFirTreeImplementationConfigurator()
 
         impl(whenExpression) {
             default("calleeReference", "FirStubReference()")
+            defaultFalse("isExhaustive")
             useTypes(stubReferenceType)
         }
 

compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/NodeConfigurator.kt

@@ -500,6 +500,7 @@ object NodeConfigurator : AbstractFieldConfigurator() {
             +field("subject", expression, nullable = true).withTransform()
             +field("subjectVariable", variable.withArgs("F" to "*"), nullable = true)
             +fieldList("branches", whenBranch).withTransform()
+            +booleanField("isExhaustive", withReplace = true)
             needTransformOtherChildren()
         }