[FIR] Report VARIABLE_EXPECTED on assign to this

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/checkers/expression/FirReassignmentAndInvisibleSetterChecker.kt

@@ -18,6 +18,7 @@ import org.jetbrains.kotlin.fir.expressions.FirSmartCastExpression
 import org.jetbrains.kotlin.fir.expressions.FirVariableAssignment
 import org.jetbrains.kotlin.fir.originalForSubstitutionOverride
 import org.jetbrains.kotlin.fir.references.FirBackingFieldReference
+import org.jetbrains.kotlin.fir.references.FirThisReference
 import org.jetbrains.kotlin.fir.references.toResolvedCallableSymbol
 import org.jetbrains.kotlin.fir.references.toResolvedValueParameterSymbol
 import org.jetbrains.kotlin.fir.resolve.calls.ExpressionReceiverValue
@@ -30,6 +31,7 @@ object FirReassignmentAndInvisibleSetterChecker : FirVariableAssignmentChecker()
         checkInvisibleSetter(expression, context, reporter)
         checkValReassignmentViaBackingField(expression, context, reporter)
         checkValReassignmentOnValueParameter(expression, context, reporter)
+        checkAssignmentToThis(expression, context, reporter)
     }
 
     private fun checkInvisibleSetter(
@@ -102,4 +104,14 @@ object FirReassignmentAndInvisibleSetterChecker : FirVariableAssignmentChecker()
         val valueParameter = expression.lValue.toResolvedValueParameterSymbol() ?: return
         reporter.reportOn(expression.lValue.source, FirErrors.VAL_REASSIGNMENT, valueParameter, context)
     }
+
+    private fun checkAssignmentToThis(
+        expression: FirVariableAssignment,
+        context: CheckerContext,
+        reporter: DiagnosticReporter
+    ) {
+        if (expression.lValue is FirThisReference) {
+            reporter.reportOn(expression.lValue.source, FirErrors.VARIABLE_EXPECTED, context)
+        }
+    }
 }

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

@@ -22,7 +22,7 @@ class C() : B() {
     }
 
     fun bar(c: C) {
-        this = c  //should be an error
+        <!VARIABLE_EXPECTED!>this<!> = c  //should be an error
     }
 }
 
@@ -31,7 +31,7 @@ fun getInt() = 0
 class D() {
     inner class B() {
         fun foo() {
-            this@D = D()
+            <!VARIABLE_EXPECTED!>this@D<!> = D()
         }
     }
 }
@@ -93,7 +93,7 @@ class Test() {
         ++<!VARIABLE_EXPECTED!>2<!>
         --(r@ <!VARIABLE_EXPECTED!>2<!>)
 
-        this<!UNRESOLVED_REFERENCE!>++<!>
+        <!VARIABLE_EXPECTED!>this<!><!UNRESOLVED_REFERENCE!>++<!>
 
         var s : String = "r"
         s += "ss"