[FIR] Introduce ConeStubType to have subtyping for non-fixed variables

Currently, it's needed after changes in d7b47108f70a107818a04c8b8db33bfbf7c7e590

 Later it'll be also used for builder-inference

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

@@ -8,6 +8,7 @@ package org.jetbrains.kotlin.fir.types
 import org.jetbrains.kotlin.fir.symbols.ConeClassLikeLookupTag
 import org.jetbrains.kotlin.fir.symbols.ConeClassifierLookupTag
 import org.jetbrains.kotlin.name.ClassId
+import org.jetbrains.kotlin.name.Name
 import org.jetbrains.kotlin.types.model.*
 
 sealed class ConeKotlinTypeProjection : TypeArgumentMarker {
@@ -193,3 +194,17 @@ class ConeIntersectionType(
 fun ConeIntersectionType.mapTypes(func: (ConeKotlinType) -> ConeKotlinType): ConeIntersectionType {
     return ConeIntersectionType(intersectedTypes.map(func))
 }
+
+class ConeStubType(val variable: ConeTypeVariable, override val nullability: ConeNullability) : StubTypeMarker, ConeKotlinType() {
+    override val typeArguments: Array<out ConeKotlinTypeProjection>
+        get() = emptyArray()
+}
+
+open class ConeTypeVariable(name: String) : TypeVariableMarker {
+    val typeConstructor = ConeTypeVariableTypeConstructor(name)
+    val defaultType = ConeTypeVariableType(ConeNullability.NOT_NULL, typeConstructor)
+}
+
+class ConeTypeVariableTypeConstructor(val debugName: String) : ConeClassifierLookupTag(), TypeVariableTypeConstructorMarker {
+    override val name: Name get() = Name.identifier(debugName)
+}

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

@@ -46,5 +46,6 @@ fun ConeKotlinType.render(): String {
                 postfix = ")"
             )
         }
+        is ConeStubType -> "stub type: $variable"
     } + nullabilitySuffix
 }

compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/ConversionUtils.kt

@@ -70,6 +70,7 @@ fun ConeKotlinType.toIrType(session: FirSession, declarationStorage: Fir2IrDecla
             // TODO: add intersectionTypeApproximation
             intersectedTypes.first().toIrType(session, declarationStorage)
         }
+        is ConeStubType -> createErrorType()
     }
 }
 

compiler/fir/java/src/org/jetbrains/kotlin/fir/java/FirJavaElementFinder.kt

@@ -222,7 +222,8 @@ private const val ERROR_TYPE_STUB = CommonClassNames.JAVA_LANG_OBJECT
 private fun ConeKotlinType.mapToCanonicalString(session: FirSession): String {
     return when (this) {
         is ConeClassLikeType -> mapToCanonicalString(session)
-        is ConeTypeVariableType, is ConeFlexibleType, is ConeCapturedType, is ConeDefinitelyNotNullType, is ConeIntersectionType ->
+        is ConeTypeVariableType, is ConeFlexibleType, is ConeCapturedType,
+        is ConeDefinitelyNotNullType, is ConeIntersectionType, is ConeStubType ->
             error("Unexpected type: $this [${this::class}]")
         is ConeLookupTagBasedType -> lookupTag.name.asString()
     }
@@ -250,7 +251,7 @@ private fun ConeClassType.mapToCanonicalString(session: FirSession): String {
         } + "[]"
     }
 
-    val context = ConeTypeCheckerContext(false, session)
+    val context = ConeTypeCheckerContext(isErrorTypeEqualsToAnything = false, isStubTypeEqualsToAnything = true, session = session)
 
     with(context) {
         val typeConstructor = typeConstructor()

compiler/fir/java/src/org/jetbrains/kotlin/fir/java/FirJavaElementFinder.kt.183

@@ -223,7 +223,8 @@ private const val ERROR_TYPE_STUB = CommonClassNames.JAVA_LANG_OBJECT
 private fun ConeKotlinType.mapToCanonicalString(session: FirSession): String {
     return when (this) {
         is ConeClassLikeType -> mapToCanonicalString(session)
-        is ConeTypeVariableType, is ConeFlexibleType, is ConeCapturedType, is ConeDefinitelyNotNullType, is ConeIntersectionType ->
+        is ConeTypeVariableType, is ConeFlexibleType, is ConeCapturedType,
+        is ConeDefinitelyNotNullType, is ConeIntersectionType, is ConeStubType ->
             error("Unexpected type: $this [${this::class}]")
         is ConeLookupTagBasedType -> lookupTag.name.asString()
     }
@@ -251,7 +252,7 @@ private fun ConeClassType.mapToCanonicalString(session: FirSession): String {
         } + "[]"
     }
 
-    val context = ConeTypeCheckerContext(false, session)
+    val context = ConeTypeCheckerContext(isErrorTypeEqualsToAnything = false, isStubTypeEqualsToAnything = true, session = session)
 
     with(context) {
         val typeConstructor = typeConstructor()

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/ResolveUtils.kt

@@ -178,6 +178,7 @@ fun <T : ConeKotlinType> T.withNullability(nullability: ConeNullability): T {
             ConeNullability.UNKNOWN -> this // TODO: is that correct?
             ConeNullability.NOT_NULL -> this
         } as T
+        is ConeStubType -> ConeStubType(variable, nullability) as T
         else -> error("sealed: ${this::class}")
     }
 }

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/SamResolution.kt

@@ -51,7 +51,7 @@ class FirSamResolverImpl(
                 getFunctionTypeForPossibleSamType(type.lowerBound) ?: return null,
                 getFunctionTypeForPossibleSamType(type.upperBound) ?: return null
             )
-            is ConeClassErrorType -> null
+            is ConeClassErrorType, is ConeStubType -> null
             // TODO: support those types as well
             is ConeAbbreviatedType, is ConeTypeParameterType, is ConeTypeVariableType,
             is ConeCapturedType, is ConeDefinitelyNotNullType, is ConeIntersectionType -> null

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/calls/ConeInferenceContext.kt

@@ -213,7 +213,8 @@ interface ConeInferenceContext : TypeSystemInferenceExtensionContext,
     }
 
     override fun createStubType(typeVariable: TypeVariableMarker): StubTypeMarker {
-        TODO("not implemented")
+        require(typeVariable is ConeTypeVariable) { "$typeVariable should subtype of ${ConeTypeVariable::class.qualifiedName}" }
+        return ConeStubType(typeVariable, ConeNullability.create(typeVariable.defaultType().isMarkedNullable()))
     }
 
     override fun KotlinTypeMarker.removeAnnotations(): KotlinTypeMarker {

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/calls/InferenceUtil.kt

@@ -41,18 +41,9 @@ fun ConeTypeContext.hasNullableSuperType(type: ConeKotlinType): Boolean {
     return false
 }
 
-class ConeTypeVariableTypeConstructor(val debugName: String) : ConeClassifierLookupTag(), TypeVariableTypeConstructorMarker {
-    override val name: Name get() = Name.identifier(debugName)
-}
-
 class TypeParameterBasedTypeVariable(val typeParameterSymbol: FirTypeParameterSymbol) :
     ConeTypeVariable(typeParameterSymbol.name.identifier)
 
-open class ConeTypeVariable(name: String) : TypeVariableMarker {
-    val typeConstructor = ConeTypeVariableTypeConstructor(name)
-    val defaultType = ConeTypeVariableType(ConeNullability.NOT_NULL, typeConstructor)
-}
-
 class InferenceComponents(
     val ctx: TypeSystemInferenceExtensionContextDelegate,
     val session: FirSession,

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/substitution/ConeSubstitutor.kt

@@ -75,6 +75,7 @@ abstract class AbstractConeSubstitutor : ConeSubstitutor() {
             is ConeCapturedType -> return null
             is ConeDefinitelyNotNullType -> this.substituteOriginal()
             is ConeIntersectionType -> this.substituteIntersectedTypes()
+            is ConeStubType -> return null
         }
     }
 

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/types/ConeTypeContext.kt

@@ -12,7 +12,6 @@ import org.jetbrains.kotlin.fir.FirSession
 import org.jetbrains.kotlin.fir.declarations.*
 import org.jetbrains.kotlin.fir.resolve.*
 import org.jetbrains.kotlin.fir.resolve.calls.ConeInferenceContext
-import org.jetbrains.kotlin.fir.resolve.calls.ConeTypeVariableTypeConstructor
 import org.jetbrains.kotlin.fir.resolve.calls.hasNullableSuperType
 import org.jetbrains.kotlin.fir.resolve.substitution.ConeSubstitutor
 import org.jetbrains.kotlin.fir.resolve.substitution.substitutorByMap
@@ -64,6 +63,7 @@ interface ConeTypeContext : TypeSystemContext, TypeSystemOptimizationContext, Ty
             is ConeDefinitelyNotNullType -> this
             is ConeIntersectionType -> this
             is ConeFlexibleType -> null
+            is ConeStubType -> this
             else -> error("Unknown simpleType: $this")
         }
     }
@@ -131,6 +131,7 @@ interface ConeTypeContext : TypeSystemContext, TypeSystemOptimizationContext, Ty
                 ?: ErrorTypeConstructor("Failed to expand alias: ${this}")
             is ConeLookupTagBasedType -> this.lookupTag.toSymbol(session) ?: ErrorTypeConstructor("Unresolved: ${this.lookupTag}")
             is ConeIntersectionType -> this
+            is ConeStubType -> variable.typeConstructor
             else -> error("?: ${this}")
         }
 
@@ -352,6 +353,7 @@ interface ConeTypeContext : TypeSystemContext, TypeSystemOptimizationContext, Ty
         if (this is ConeCapturedType) return true
         if (this is ConeTypeVariableType) return false
         if (this is ConeIntersectionType) return false
+        if (this is ConeStubType) return true
         require(this is ConeLookupTagBasedType)
         val typeConstructor = this.typeConstructor()
         return typeConstructor is FirClassSymbol<*> ||
@@ -367,7 +369,7 @@ interface ConeTypeContext : TypeSystemContext, TypeSystemOptimizationContext, Ty
     }
 
     override fun SimpleTypeMarker.isStubType(): Boolean {
-        return false // TODO
+        return this is StubTypeMarker
     }
 
     override fun intersectTypes(types: List<SimpleTypeMarker>): SimpleTypeMarker {