FIR checker: report KClass with incorrect type argument

compiler/fir/checkers/checkers-component-generator/src/org/jetbrains/kotlin/fir/checkers/generator/diagnostics/FirDiagnosticsList.kt

@@ -17,7 +17,6 @@ import org.jetbrains.kotlin.fir.PrivateForInline
 import org.jetbrains.kotlin.fir.declarations.FirCallableDeclaration
 import org.jetbrains.kotlin.fir.declarations.FirClass
 import org.jetbrains.kotlin.fir.declarations.FirMemberDeclaration
-import org.jetbrains.kotlin.fir.declarations.FirTypeParameter
 import org.jetbrains.kotlin.fir.expressions.FirExpression
 import org.jetbrains.kotlin.fir.symbols.AbstractFirBasedSymbol
 import org.jetbrains.kotlin.fir.symbols.impl.FirClassLikeSymbol
@@ -247,6 +246,10 @@ object DIAGNOSTICS_LIST : DiagnosticList() {
         val GENERIC_THROWABLE_SUBCLASS by error<FirSourceElement, KtTypeParameterList>()
         val INNER_CLASS_OF_GENERIC_THROWABLE_SUBCLASS by error<FirSourceElement, KtClassOrObject>(PositioningStrategy.DECLARATION_NAME)
 
+        val KCLASS_WITH_NULLABLE_TYPE_PARAMETER_IN_SIGNATURE by error<FirSourceElement, KtNamedDeclaration>(PositioningStrategy.DECLARATION_NAME) {
+            parameter<FirTypeParameterSymbol>("typeParameter")
+        }
+
         val TYPE_PARAMETER_AS_REIFIED by error<FirSourceElement, PsiElement> {
             parameter<FirTypeParameterSymbol>("typeParameter")
         }

compiler/fir/checkers/gen/org/jetbrains/kotlin/fir/analysis/diagnostics/FirErrors.kt

@@ -193,6 +193,7 @@ object FirErrors {
     val TYPE_PARAMETER_IN_CATCH_CLAUSE by error0<FirSourceElement, PsiElement>()
     val GENERIC_THROWABLE_SUBCLASS by error0<FirSourceElement, KtTypeParameterList>()
     val INNER_CLASS_OF_GENERIC_THROWABLE_SUBCLASS by error0<FirSourceElement, KtClassOrObject>(SourceElementPositioningStrategies.DECLARATION_NAME)
+    val KCLASS_WITH_NULLABLE_TYPE_PARAMETER_IN_SIGNATURE by error1<FirSourceElement, KtNamedDeclaration, FirTypeParameterSymbol>(SourceElementPositioningStrategies.DECLARATION_NAME)
     val TYPE_PARAMETER_AS_REIFIED by error1<FirSourceElement, PsiElement, FirTypeParameterSymbol>()
 
     // Reflection

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

@@ -40,6 +40,7 @@ import org.jetbrains.kotlin.psi.KtModifierList
 import org.jetbrains.kotlin.psi.KtParameter
 import org.jetbrains.kotlin.psi.KtParameter.VAL_VAR_TOKEN_SET
 import org.jetbrains.kotlin.psi.psiUtil.visibilityModifierType
+import org.jetbrains.kotlin.types.AbstractTypeChecker
 import org.jetbrains.kotlin.utils.addToStdlib.safeAs
 
 /**
@@ -319,6 +320,10 @@ fun Modality.toToken(): KtModifierKeywordToken = when (this) {
 val FirFunctionCall.isIterator
     get() = this.calleeReference.name.asString() == "<iterator>"
 
+fun ConeKotlinType.isSubtypeOfAny(session: FirSession): Boolean {
+    return AbstractTypeChecker.isSubtypeOf(session.typeContext, this.fullyExpandedType(session), session.builtinTypes.anyType.type)
+}
+
 internal fun throwableClassLikeType(session: FirSession) = session.builtinTypes.throwableType.type
 
 fun ConeKotlinType.isSubtypeOfThrowable(session: FirSession) =

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/declaration/FirKClassWithIncorrectTypeArgumentChecker.kt

@@ -0,0 +1,71 @@
+/*
+ * 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.declaration
+
+import org.jetbrains.kotlin.fir.FirFakeSourceElementKind
+import org.jetbrains.kotlin.fir.FirSession
+import org.jetbrains.kotlin.fir.analysis.checkers.context.CheckerContext
+import org.jetbrains.kotlin.fir.analysis.checkers.isSubtypeOfAny
+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.declarations.*
+import org.jetbrains.kotlin.fir.resolve.diagnostics.ConeTypeParameterInQualifiedAccess
+import org.jetbrains.kotlin.fir.resolve.inference.isKClassType
+import org.jetbrains.kotlin.fir.scopes.impl.toConeType
+import org.jetbrains.kotlin.fir.types.*
+
+// FE1.0 [KClassWithIncorrectTypeArgumentChecker]
+object FirKClassWithIncorrectTypeArgumentChecker : FirFileChecker() {
+    override fun check(declaration: FirFile, context: CheckerContext, reporter: DiagnosticReporter) {
+        for (topLevelDeclaration in declaration.declarations) {
+            if (topLevelDeclaration is FirCallableMemberDeclaration<*>) {
+                checkTopLevelDeclaration(topLevelDeclaration, context, reporter)
+            }
+        }
+    }
+
+    // When a type parameter is used as a type argument for KClass, it shouldn't be nullable.
+    //  bad:  fun <T> test1() = T::class
+    //  okay: fun <T: Any> test2() = T::class
+    private fun checkTopLevelDeclaration(
+        declaration: FirCallableMemberDeclaration<*>,
+        context: CheckerContext,
+        reporter: DiagnosticReporter
+    ) {
+        // prevent duplicate reporting
+        if (declaration is FirPropertyAccessor) return
+
+        val source = declaration.source ?: return
+        if (source.kind is FirFakeSourceElementKind) return
+
+        val returnType = declaration.returnTypeRef.coneType
+        if (!returnType.isKClassWithBadArgument(context.session)) return
+
+        val typeArgument = (returnType.typeArguments[0] as ConeKotlinTypeProjection).type
+        typeArgument.typeParameterFromError?.let {
+            reporter.reportOn(source, FirErrors.KCLASS_WITH_NULLABLE_TYPE_PARAMETER_IN_SIGNATURE, it.symbol, context)
+        }
+    }
+
+    private fun ConeKotlinType.isKClassWithBadArgument(session: FirSession): Boolean {
+        if (!this.isKClassType()) return false
+        val argumentType = typeArguments.toList().singleOrNull()?.let {
+            when (it) {
+                is ConeStarProjection -> null
+                is ConeKotlinTypeProjection -> it.type
+            }
+        } ?: return false
+        argumentType.typeParameterFromError?.let { typeParameter ->
+            return !typeParameter.toConeType().isSubtypeOfAny(session)
+        }
+        return argumentType is ConeKotlinErrorType || !argumentType.isSubtypeOfAny(session)
+    }
+
+    private val ConeKotlinType.typeParameterFromError: FirTypeParameter?
+        get() = ((this as? ConeKotlinErrorType)?.diagnostic as? ConeTypeParameterInQualifiedAccess)?.symbol?.fir
+
+}

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

@@ -111,6 +111,7 @@ import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.INSTANCE_ACCESS_B
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.INTERFACE_WITH_SUPERCLASS
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.INVALID_IF_AS_EXPRESSION
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.INVALID_TYPE_OF_ANNOTATION_MEMBER
+import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.KCLASS_WITH_NULLABLE_TYPE_PARAMETER_IN_SIGNATURE
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.LEAKED_IN_PLACE_LAMBDA
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.LOCAL_ANNOTATION_CLASS_ERROR
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.LOCAL_INTERFACE_NOT_ALLOWED
@@ -395,6 +396,13 @@ class FirDefaultErrorMessages : DefaultErrorMessages.Extension {
             map.put(REIFIED_TYPE_IN_CATCH_CLAUSE, "Reified type is forbidden for catch parameter")
             map.put(TYPE_PARAMETER_IN_CATCH_CLAUSE, "Type parameter is forbidden for catch parameter")
 
+            map.put(
+                KCLASS_WITH_NULLABLE_TYPE_PARAMETER_IN_SIGNATURE,
+                "Declaration has an inconsistent return type. " +
+                        "Please add upper bound Any for type parameter ''{0}'' or specify return type explicitly",
+                SYMBOL
+            )
+
             map.put(TYPE_PARAMETER_AS_REIFIED, "Cannot use ''{0}'' as reified type parameter. Use a class instead", SYMBOL)
 
             // Reflection

compiler/fir/entrypoint/src/org/jetbrains/kotlin/fir/checkers/CommonDeclarationCheckers.kt

@@ -70,6 +70,7 @@ object CommonDeclarationCheckers : DeclarationCheckers() {
     )
 
     override val fileCheckers: Set<FirFileChecker> = setOf(
+        FirKClassWithIncorrectTypeArgumentChecker,
         FirTopLevelFunctionsChecker,
         FirTopLevelPropertiesChecker,
     )