FIR: do not leak ConeTypeVariableType via diagnostics

ConeTypeVariableType is an internal part of type inference
and should not be leaked outside it

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/coneDiagnosticToFirDiagnostic.kt

@@ -190,12 +190,15 @@ private fun mapInapplicableCandidateError(
                 rootCause.argument.source,
                 rootCause.forbiddenNamedArgumentsTarget
             )
-            is ArgumentTypeMismatch -> FirErrors.ARGUMENT_TYPE_MISMATCH.createOn(
-                rootCause.argument.source ?: source,
-                rootCause.expectedType,
-                rootCause.actualType,
-                rootCause.isMismatchDueToNullability
-            )
+            is ArgumentTypeMismatch -> {
+                val typeContext = diagnostic.candidate.callInfo.session.typeContext
+                FirErrors.ARGUMENT_TYPE_MISMATCH.createOn(
+                    rootCause.argument.source ?: source,
+                    rootCause.expectedType.removeTypeVariableTypes(typeContext),
+                    rootCause.argument.typeRef.coneType.removeTypeVariableTypes(typeContext),
+                    rootCause.isMismatchDueToNullability
+                )
+            }
             is NullForNotNullType -> FirErrors.NULL_FOR_NONNULL_TYPE.createOn(
                 rootCause.argument.source ?: source
             )
@@ -299,8 +302,8 @@ private fun ConstraintSystemError.toDiagnostic(
             argument?.let {
                 return FirErrors.ARGUMENT_TYPE_MISMATCH.createOn(
                     it.source ?: source,
-                    lowerConeType,
-                    upperConeType,
+                    lowerConeType.removeTypeVariableTypes(typeContext),
+                    upperConeType.removeTypeVariableTypes(typeContext),
                     typeMismatchDueToNullability
                 )
             }
@@ -319,8 +322,8 @@ private fun ConstraintSystemError.toDiagnostic(
 
                     FirErrors.TYPE_MISMATCH.createOn(
                         qualifiedAccessSource ?: source,
-                        upperConeType,
-                        inferredType,
+                        upperConeType.removeTypeVariableTypes(typeContext),
+                        inferredType.removeTypeVariableTypes(typeContext),
                         typeMismatchDueToNullability
                     )
                 }

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/removeTypeVariableTypes.kt

@@ -0,0 +1,33 @@
+/*
+ * 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.diagnostics
+
+import org.jetbrains.kotlin.fir.resolve.diagnostics.ConeUnknownLambdaParameterTypeDiagnostic
+import org.jetbrains.kotlin.fir.resolve.substitution.AbstractConeSubstitutor
+import org.jetbrains.kotlin.fir.symbols.ConeTypeParameterLookupTag
+import org.jetbrains.kotlin.fir.types.*
+import org.jetbrains.kotlin.fir.types.impl.ConeTypeParameterTypeImpl
+
+internal fun ConeKotlinType.removeTypeVariableTypes(typeContext: ConeTypeContext): ConeKotlinType {
+    val substitutor = TypeVariableTypeRemovingSubstitutor(typeContext)
+    return substitutor.substituteOrSelf(this)
+}
+
+private class TypeVariableTypeRemovingSubstitutor(typeContext: ConeTypeContext) : AbstractConeSubstitutor(typeContext) {
+    override fun substituteType(type: ConeKotlinType): ConeKotlinType? = when (type) {
+        is ConeTypeVariableType -> convertTypeVariableType(type)
+        else -> null
+    }
+
+    private fun convertTypeVariableType(type: ConeTypeVariableType): ConeKotlinType {
+        val originalTypeParameter = type.lookupTag.originalTypeParameter
+        if (originalTypeParameter != null) {
+            check(originalTypeParameter is ConeTypeParameterLookupTag)
+            return ConeTypeParameterTypeImpl(originalTypeParameter, type.isNullable, type.attributes)
+        }
+        return ConeClassErrorType(ConeUnknownLambdaParameterTypeDiagnostic())
+    }
+}

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/diagnostics/FirDiagnostics.kt

@@ -216,6 +216,10 @@ class ConePropertyAsOperator(val symbol: FirPropertySymbol) : ConeDiagnostic {
     override val reason: String get() = "Cannot use a property as an operator"
 }
 
+class ConeUnknownLambdaParameterTypeDiagnostic : ConeDiagnostic {
+    override val reason: String get() = "Unknown return lambda parameter type"
+}
+
 private fun describeSymbol(symbol: FirBasedSymbol<*>): String {
     return when (symbol) {
         is FirClassLikeSymbol<*> -> symbol.classId.asString()

compiler/testData/cli/jvm/instanceAccessBeforeSuperCall.out

@@ -13,10 +13,10 @@ compiler/testData/cli/jvm/instanceAccessBeforeSuperCall.kt:2:74: error: unresolv
 compiler/testData/cli/jvm/instanceAccessBeforeSuperCall.kt:7:20: error: 'this' is not defined in this context
 class B(other: B = this)
                    ^
-compiler/testData/cli/jvm/instanceAccessBeforeSuperCall.kt:10:32: error: argument type mismatch: actual type is kotlin/Int but kotlin/Function0<TypeVariable(_L)> was expected
+compiler/testData/cli/jvm/instanceAccessBeforeSuperCall.kt:10:32: error: argument type mismatch: actual type is kotlin/Function0<ERROR CLASS: Cannot access ''<this>'' before superclass constructor has been called> but kotlin/Int was expected
     constructor(x: Int) : this({
                                ^
-compiler/testData/cli/jvm/instanceAccessBeforeSuperCall.kt:12:9: error: argument type mismatch: actual type is ERROR CLASS: Cannot access ''<this>'' before superclass constructor has been called but TypeVariable(_L) was expected
+compiler/testData/cli/jvm/instanceAccessBeforeSuperCall.kt:12:9: error: argument type mismatch: actual type is ERROR CLASS: Cannot access ''<this>'' before superclass constructor has been called but ERROR CLASS: Unknown return lambda parameter type was expected
         this
         ^
 compiler/testData/cli/jvm/instanceAccessBeforeSuperCall.kt:12:9: error: cannot access '<this>' before superclass constructor has been called