FIR checker: add diagnostic VAR_OVERRIDDEN_BY_VAL

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

@@ -96,6 +96,13 @@ object FirOverrideChecker : FirRegularClassChecker() {
         return null
     }
 
+    private fun FirProperty.checkMutability(
+        overriddenSymbols: List<FirCallableSymbol<*>>,
+    ): FirMemberDeclaration? {
+        if (isVar) return null
+        return overriddenSymbols.find { (it.fir as? FirProperty)?.isVar == true }?.fir?.safeAs()
+    }
+
     private fun FirCallableMemberDeclaration<*>.checkReturnType(
         overriddenSymbols: List<FirCallableSymbol<*>>,
         typeCheckerContext: AbstractTypeCheckerContext,
@@ -178,6 +185,10 @@ object FirOverrideChecker : FirRegularClassChecker() {
             reporter.reportOverridingFinalMember(property, it)
         }
 
+        property.checkMutability(overriddenPropertySymbols)?.let {
+            reporter.reportVarOverriddenByVal(property, it)
+        }
+
         val restriction = property.checkReturnType(
             overriddenSymbols = overriddenPropertySymbols,
             typeCheckerContext = typeCheckerContext,
@@ -209,6 +220,13 @@ object FirOverrideChecker : FirRegularClassChecker() {
         }
     }
 
+    private fun DiagnosticReporter.reportVarOverriddenByVal(
+        overriding: FirMemberDeclaration,
+        overridden: FirMemberDeclaration
+    ) {
+        overriding.source?.let { report(FirErrors.VAR_OVERRIDDEN_BY_VAL.on(it, overriding, overridden)) }
+    }
+
     private fun DiagnosticReporter.reportReturnTypeMismatchOnFunction(
         overriding: FirMemberDeclaration,
         overridden: FirMemberDeclaration

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

@@ -154,6 +154,7 @@ import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.VARIABLE_INITIALI
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.VARIABLE_NEVER_READ
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.VARIANCE_ON_TYPE_PARAMETER_NOT_ALLOWED
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.VAR_ANNOTATION_PARAMETER
+import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.VAR_OVERRIDDEN_BY_VAL
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.VAR_TYPE_MISMATCH_ON_OVERRIDE
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.WRONG_INVOCATION_KIND
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.WRONG_NUMBER_OF_TYPE_ARGUMENTS
@@ -353,6 +354,13 @@ class FirDefaultErrorMessages : DefaultErrorMessages.Extension {
                 FQ_NAMES_IN_TYPES
             )
 
+            map.put(
+                VAR_OVERRIDDEN_BY_VAL,
+                "Var-property {0} cannot be overridden by val-property {1}",
+                FQ_NAMES_IN_TYPES,
+                FQ_NAMES_IN_TYPES
+            )
+
             // Redeclarations
             map.put(MANY_COMPANION_OBJECTS, "Only one companion object is allowed per class")
             map.put(CONFLICTING_OVERLOADS, "Conflicting overloads: {0}", TO_STRING) // *

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

@@ -145,6 +145,8 @@ object FirErrors {
     val PROPERTY_TYPE_MISMATCH_ON_OVERRIDE by error2<FirSourceElement, KtNamedDeclaration, FirMemberDeclaration, FirMemberDeclaration>(SourceElementPositioningStrategies.DECLARATION_RETURN_TYPE)
     val VAR_TYPE_MISMATCH_ON_OVERRIDE by error2<FirSourceElement, KtNamedDeclaration, FirMemberDeclaration, FirMemberDeclaration>(SourceElementPositioningStrategies.DECLARATION_RETURN_TYPE)
 
+    val VAR_OVERRIDDEN_BY_VAL by error2<FirSourceElement, KtNamedDeclaration, FirMemberDeclaration, FirMemberDeclaration>(SourceElementPositioningStrategies.VAL_OR_VAR_NODE)
+
     // Redeclarations
     val MANY_COMPANION_OBJECTS by error0<FirSourceElement, PsiElement>()
     val CONFLICTING_OVERLOADS by error1<FirSourceElement, PsiElement, String>(SourceElementPositioningStrategies.DECLARATION_SIGNATURE_OR_DEFAULT)

compiler/testData/diagnostics/tests/OverridingVarByVal.fir.kt

@@ -1,15 +0,0 @@
-open class Var() {
-  open var v : Int = 1
-}
-
-interface VarT {
-  var v : Int
-}
-
-class Val() : Var(), VarT {
-  override val v : Int = 1
-}
-
-class Var2() : Var() {
-  override var v : Int = 1
-}

compiler/testData/diagnostics/tests/OverridingVarByVal.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 open class Var() {
   open var v : Int = 1
 }

compiler/testData/diagnostics/tests/annotations/rendering/typeMismatchOnOverride.fir.kt

@@ -17,7 +17,7 @@ interface A {
 interface B : A {
     override val p1: <!PROPERTY_TYPE_MISMATCH_ON_OVERRIDE!>Int<!>
     @An
-    override val p2: @An String
+    override <!VAR_OVERRIDDEN_BY_VAL!>val<!> p2: @An String
     override fun test(arg: String): <!RETURN_TYPE_MISMATCH_ON_OVERRIDE!>Int<!>
 }
 

compiler/testData/diagnostics/tests/regressions/kt3810.fir.kt

@@ -1,5 +0,0 @@
-interface A {
-    var foo: String
-}
-
-class B(override val foo: String) : A

compiler/testData/diagnostics/tests/regressions/kt3810.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 interface A {
     var foo: String
 }

compiler/testData/diagnostics/tests/scopes/kt323.fir.kt

@@ -6,7 +6,7 @@ open class A {
 }
 
 class B : A() {
-    override val a = 34
+    override <!VAR_OVERRIDDEN_BY_VAL!>val<!> a = 34
 
     var b : Int
         public get() = 23

compiler/tests-spec/testData/diagnostics/linked/declarations/classifier-declaration/class-declaration/abstract-classes/p-2/neg/1.1.fir.kt

@@ -22,9 +22,9 @@ fun case1() {
 
 class BaseImplCase2(
     override var a: Any, override
-    val  b: Any, override var c: Any, override
+    <!VAR_OVERRIDDEN_BY_VAL!>val<!>  b: Any, override var c: Any, override
 
- val  d: Any = "5") : Base()
+ <!VAR_OVERRIDDEN_BY_VAL!>val<!>  d: Any = "5") : Base()
 {
     override fun foo() {}
     override internal fun boo() {}
@@ -42,14 +42,14 @@ class ImplBaseCase2() : Base() {
         set(value) {}
     override
 
-     val  b: Any
+     <!VAR_OVERRIDDEN_BY_VAL!>val<!>  b: Any
         get() = TODO()
     override var c: Any
         get() = TODO()
         set(value) {}
     override
 
-     val  d: Any
+     <!VAR_OVERRIDDEN_BY_VAL!>val<!>  d: Any
         get() = TODO()
 
     override fun foo() {}