FIR checker: add convention check for ++ and +=, etc

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

@@ -1115,6 +1115,11 @@ object DIAGNOSTICS_LIST : DiagnosticList("FirErrors") {
             parameter<ConeKotlinType>("leftType")
             parameter<ConeKotlinType>("rightType")
         }
+        val INC_DEC_SHOULD_NOT_RETURN_UNIT by error<KtExpression>(PositioningStrategy.OPERATOR)
+        val ASSIGNMENT_OPERATOR_SHOULD_RETURN_UNIT by error<KtExpression>(PositioningStrategy.OPERATOR) {
+            parameter<FirNamedFunctionSymbol>("functionSymbol")
+            parameter<String>("operator")
+        }
     }
 
     val TYPE_ALIAS by object : DiagnosticGroup("Type alias") {

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

@@ -577,6 +577,8 @@ object FirErrors {
     val EQUALITY_NOT_APPLICABLE by error3<KtBinaryExpression, String, ConeKotlinType, ConeKotlinType>()
     val EQUALITY_NOT_APPLICABLE_WARNING by warning3<KtBinaryExpression, String, ConeKotlinType, ConeKotlinType>()
     val INCOMPATIBLE_ENUM_COMPARISON_ERROR by error2<KtElement, ConeKotlinType, ConeKotlinType>()
+    val INC_DEC_SHOULD_NOT_RETURN_UNIT by error0<KtExpression>(SourceElementPositioningStrategies.OPERATOR)
+    val ASSIGNMENT_OPERATOR_SHOULD_RETURN_UNIT by error2<KtExpression, FirNamedFunctionSymbol, String>(SourceElementPositioningStrategies.OPERATOR)
 
     // Type alias
     val TOPLEVEL_TYPEALIASES_ONLY by error0<KtTypeAlias>()

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

@@ -50,7 +50,8 @@ object CommonExpressionCheckers : ExpressionCheckers() {
             FirConventionFunctionCallChecker,
             FirDivisionByZeroChecker,
             FirConstructorCallChecker,
-            FirSpreadOfNullableChecker
+            FirSpreadOfNullableChecker,
+            FirAssignmentOperatorCallChecker,
         )
 
     override val tryExpressionCheckers: Set<FirTryExpressionChecker>

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

@@ -16,8 +16,11 @@ import org.jetbrains.kotlin.fir.analysis.diagnostics.*
 import org.jetbrains.kotlin.fir.analysis.getChild
 import org.jetbrains.kotlin.fir.declarations.*
 import org.jetbrains.kotlin.fir.declarations.utils.*
-import org.jetbrains.kotlin.fir.expressions.*
+import org.jetbrains.kotlin.fir.expressions.FirExpression
+import org.jetbrains.kotlin.fir.expressions.FirFunctionCall
+import org.jetbrains.kotlin.fir.expressions.FirVariableAssignment
 import org.jetbrains.kotlin.fir.expressions.impl.FirEmptyExpressionBlock
+import org.jetbrains.kotlin.fir.expressions.toResolvedCallableSymbol
 import org.jetbrains.kotlin.fir.resolve.SessionHolder
 import org.jetbrains.kotlin.fir.resolve.fullyExpandedType
 import org.jetbrains.kotlin.fir.resolve.inference.isBuiltinFunctionalType
@@ -578,7 +581,11 @@ fun checkTypeMismatch(
                     rValueType = lValueType
                     lValueType = tempType
                 }
-                reporter.reportOn(source, FirErrors.RESULT_TYPE_MISMATCH, lValueType, rValueType, context)
+                if (rValueType.isUnit) {
+                    reporter.reportOn(source, FirErrors.INC_DEC_SHOULD_NOT_RETURN_UNIT, context)
+                } else {
+                    reporter.reportOn(source, FirErrors.RESULT_TYPE_MISMATCH, lValueType, rValueType, context)
+                }
             }
             else -> {
                 reporter.reportOn(source, FirErrors.ASSIGNMENT_TYPE_MISMATCH, lValueType, rValueType, context)

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

@@ -0,0 +1,40 @@
+/*
+ * 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.expression
+
+import org.jetbrains.kotlin.fir.analysis.checkers.context.CheckerContext
+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.expressions.FirFunctionCall
+import org.jetbrains.kotlin.fir.expressions.FirFunctionCallOrigin
+import org.jetbrains.kotlin.fir.expressions.FirOperationNameConventions
+import org.jetbrains.kotlin.fir.references.FirResolvedNamedReference
+import org.jetbrains.kotlin.fir.symbols.impl.FirNamedFunctionSymbol
+import org.jetbrains.kotlin.fir.types.coneType
+import org.jetbrains.kotlin.fir.types.isUnit
+
+object FirAssignmentOperatorCallChecker : FirFunctionCallChecker() {
+    override fun check(expression: FirFunctionCall, context: CheckerContext, reporter: DiagnosticReporter) {
+        val resolvedCallee = expression.calleeReference as? FirResolvedNamedReference ?: return
+        val resolvedCalleeSymbol = resolvedCallee.resolvedSymbol as? FirNamedFunctionSymbol ?: return
+        val resolvedCalleeName = resolvedCalleeSymbol.name
+        if (expression.origin != FirFunctionCallOrigin.Operator ||
+            resolvedCalleeName !in FirOperationNameConventions.ASSIGNMENT_NAMES
+        ) {
+            return
+        }
+        if (!expression.typeRef.coneType.isUnit) {
+            reporter.reportOn(
+                expression.source,
+                FirErrors.ASSIGNMENT_OPERATOR_SHOULD_RETURN_UNIT,
+                resolvedCalleeSymbol,
+                FirOperationNameConventions.ASSIGNMENT_NAMES[resolvedCalleeName]!!.operator,
+                context
+            )
+        }
+    }
+}

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

@@ -64,6 +64,7 @@ import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.ARGUMENT_PASSED_T
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.ARGUMENT_TYPE_MISMATCH
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.ARRAY_EQUALITY_OPERATOR_CAN_BE_REPLACED_WITH_EQUALS
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.ASSIGNED_VALUE_IS_NEVER_READ
+import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.ASSIGNMENT_OPERATOR_SHOULD_RETURN_UNIT
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.ASSIGNMENT_TYPE_MISMATCH
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.ASSIGN_OPERATOR_AMBIGUITY
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.BACKING_FIELD_IN_INTERFACE
@@ -219,6 +220,7 @@ import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.INCOMPATIBLE_TYPE
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.INCOMPATIBLE_TYPES_WARNING
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.INCONSISTENT_TYPE_PARAMETER_BOUNDS
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.INCONSISTENT_TYPE_PARAMETER_VALUES
+import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.INC_DEC_SHOULD_NOT_RETURN_UNIT
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.INFERENCE_ERROR
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.INFIX_MODIFIER_REQUIRED
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.INITIALIZATION_BEFORE_DECLARATION
@@ -238,8 +240,8 @@ import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.INNER_CLASS_INSID
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.INNER_CLASS_OF_GENERIC_THROWABLE_SUBCLASS
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.INSTANCE_ACCESS_BEFORE_SUPER_CALL
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.INTERFACE_WITH_SUPERCLASS
-import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.INVALID_DEFAULT_FUNCTIONAL_PARAMETER_FOR_INLINE
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.INVALID_CHARACTERS
+import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.INVALID_DEFAULT_FUNCTIONAL_PARAMETER_FOR_INLINE
 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.INVISIBLE_REFERENCE
@@ -290,9 +292,9 @@ import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.NOT_A_LOOP_LABEL
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.NOT_A_SUPERTYPE
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.NOT_NULL_ASSERTION_ON_CALLABLE_REFERENCE
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.NOT_NULL_ASSERTION_ON_LAMBDA_EXPRESSION
+import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.NOT_YET_SUPPORTED_IN_INLINE
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.NO_ACTUAL_CLASS_MEMBER_FOR_EXPECTED_CLASS
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.NO_ACTUAL_FOR_EXPECT
-import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.NOT_YET_SUPPORTED_IN_INLINE
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.NO_COMPANION_OBJECT
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.NO_ELSE_IN_WHEN
 import org.jetbrains.kotlin.fir.analysis.diagnostics.FirErrors.NO_GET_METHOD
@@ -1471,6 +1473,13 @@ class FirDefaultErrorMessages {
                 RENDER_TYPE,
                 RENDER_TYPE
             )
+            map.put(INC_DEC_SHOULD_NOT_RETURN_UNIT, "Functions inc(), dec() shouldn't return Unit to be used by operators ++, --")
+            map.put(
+                ASSIGNMENT_OPERATOR_SHOULD_RETURN_UNIT,
+                "Function ''{0}'' should return Unit to be used by corresponding operator ''{1}''",
+                SYMBOL,
+                TO_STRING
+            )
 
             // Type alias
             map.put(TOPLEVEL_TYPEALIASES_ONLY, "Nested and local type aliases are not supported")

compiler/fir/tree/src/org/jetbrains/kotlin/fir/expressions/FirOperation.kt

@@ -60,6 +60,7 @@ object FirOperationNameConventions {
             FirOperation.REM_ASSIGN to OperatorNameConventions.REM_ASSIGN
         )
     )
+    val ASSIGNMENT_NAMES = ASSIGNMENTS.map { (k, v) -> v to k }.toMap()
 
     val ASSIGNMENTS_TO_SIMPLE_OPERATOR: Map<FirOperation, Name> = EnumMap(
         mapOf(

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

@@ -35,12 +35,12 @@ class UnitIncDec() {
 
 fun testUnitIncDec() {
   var x = UnitIncDec()
-  <!RESULT_TYPE_MISMATCH!>x++<!>
+  x<!INC_DEC_SHOULD_NOT_RETURN_UNIT!>++<!>
-  <!RESULT_TYPE_MISMATCH!>++x<!>
+  <!INC_DEC_SHOULD_NOT_RETURN_UNIT!>++<!>x
-  <!RESULT_TYPE_MISMATCH!>x--<!>
+  x<!INC_DEC_SHOULD_NOT_RETURN_UNIT!>--<!>
-  <!RESULT_TYPE_MISMATCH!>--x<!>
+  <!INC_DEC_SHOULD_NOT_RETURN_UNIT!>--<!>x
-  x = <!RESULT_TYPE_MISMATCH!>x++<!>
+  x = x<!INC_DEC_SHOULD_NOT_RETURN_UNIT!>++<!>
-  x = <!RESULT_TYPE_MISMATCH!>x--<!>
+  x = x<!INC_DEC_SHOULD_NOT_RETURN_UNIT!>--<!>
-  x = <!RESULT_TYPE_MISMATCH!>++x<!>
+  x = <!INC_DEC_SHOULD_NOT_RETURN_UNIT!>++<!>x
-  x = <!RESULT_TYPE_MISMATCH!>--x<!>
+  x = <!INC_DEC_SHOULD_NOT_RETURN_UNIT!>--<!>x
 }

compiler/testData/diagnostics/tests/operatorsOverloading/kt1028.fir.kt

@@ -29,7 +29,7 @@ class Test()
     fun test()
     {
         val control = Control()
-        control.MouseMoved += { it.X } // here
+        control.MouseMoved <!ASSIGNMENT_OPERATOR_SHOULD_RETURN_UNIT!>+=<!> { it.X } // here
         control.MouseMoved.plusAssign( { it.X } ) // ok
     }
 }

compiler/testData/diagnostics/tests/regressions/kt618.fir.kt

@@ -20,9 +20,9 @@ class B() {
 
 fun main() {
     var c = B()
-    c += B()
+    c <!ASSIGNMENT_OPERATOR_SHOULD_RETURN_UNIT!>+=<!> B()
-    c *= B()
+    c <!ASSIGNMENT_OPERATOR_SHOULD_RETURN_UNIT!>*=<!> B()
-    c /= B()
+    c <!ASSIGNMENT_OPERATOR_SHOULD_RETURN_UNIT!>/=<!> B()
-    c -= B()
+    c <!ASSIGNMENT_OPERATOR_SHOULD_RETURN_UNIT!>-=<!> B()
-    c %= B()
+    c <!ASSIGNMENT_OPERATOR_SHOULD_RETURN_UNIT!>%=<!> B()
 }

compiler/testData/diagnostics/tests/smartCasts/incDecToNull.fir.kt

@@ -4,7 +4,7 @@ class IncDec {
 
 fun foo(): IncDec {
     var x = IncDec()
-    x = <!RESULT_TYPE_MISMATCH!>x++<!>
+    x = x<!INC_DEC_SHOULD_NOT_RETURN_UNIT!>++<!>
-    <!RESULT_TYPE_MISMATCH!>x++<!>
+    x<!INC_DEC_SHOULD_NOT_RETURN_UNIT!>++<!>
     return x
 }

idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDataClassConverters.kt

@@ -3027,6 +3027,20 @@ internal val KT_DIAGNOSTIC_CONVERTER = KtDiagnosticConverterBuilder.buildConvert
             token,
         )
     }
+    add(FirErrors.INC_DEC_SHOULD_NOT_RETURN_UNIT) { firDiagnostic ->
+        IncDecShouldNotReturnUnitImpl(
+            firDiagnostic as FirPsiDiagnostic,
+            token,
+        )
+    }
+    add(FirErrors.ASSIGNMENT_OPERATOR_SHOULD_RETURN_UNIT) { firDiagnostic ->
+        AssignmentOperatorShouldReturnUnitImpl(
+            firSymbolBuilder.functionLikeBuilder.buildFunctionSymbol(firDiagnostic.a.fir),
+            firDiagnostic.b,
+            firDiagnostic as FirPsiDiagnostic,
+            token,
+        )
+    }
     add(FirErrors.TOPLEVEL_TYPEALIASES_ONLY) { firDiagnostic ->
         ToplevelTypealiasesOnlyImpl(
             firDiagnostic as FirPsiDiagnostic,

idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnostics.kt

@@ -2113,6 +2113,16 @@ sealed class KtFirDiagnostic<PSI : PsiElement> : KtDiagnosticWithPsi<PSI> {
         abstract val rightType: KtType
     }
 
+    abstract class IncDecShouldNotReturnUnit : KtFirDiagnostic<KtExpression>() {
+        override val diagnosticClass get() = IncDecShouldNotReturnUnit::class
+    }
+
+    abstract class AssignmentOperatorShouldReturnUnit : KtFirDiagnostic<KtExpression>() {
+        override val diagnosticClass get() = AssignmentOperatorShouldReturnUnit::class
+        abstract val functionSymbol: KtFunctionLikeSymbol
+        abstract val operator: String
+    }
+
     abstract class ToplevelTypealiasesOnly : KtFirDiagnostic<KtTypeAlias>() {
         override val diagnosticClass get() = ToplevelTypealiasesOnly::class
     }

idea/idea-frontend-fir/src/org/jetbrains/kotlin/idea/frontend/api/fir/diagnostics/KtFirDiagnosticsImpl.kt

@@ -3401,6 +3401,22 @@ internal class IncompatibleEnumComparisonErrorImpl(
     override val firDiagnostic: FirPsiDiagnostic by weakRef(firDiagnostic)
 }
 
+internal class IncDecShouldNotReturnUnitImpl(
+    firDiagnostic: FirPsiDiagnostic,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.IncDecShouldNotReturnUnit(), KtAbstractFirDiagnostic<KtExpression> {
+    override val firDiagnostic: FirPsiDiagnostic by weakRef(firDiagnostic)
+}
+
+internal class AssignmentOperatorShouldReturnUnitImpl(
+    override val functionSymbol: KtFunctionLikeSymbol,
+    override val operator: String,
+    firDiagnostic: FirPsiDiagnostic,
+    override val token: ValidityToken,
+) : KtFirDiagnostic.AssignmentOperatorShouldReturnUnit(), KtAbstractFirDiagnostic<KtExpression> {
+    override val firDiagnostic: FirPsiDiagnostic by weakRef(firDiagnostic)
+}
+
 internal class ToplevelTypealiasesOnlyImpl(
     firDiagnostic: FirPsiDiagnostic,
     override val token: ValidityToken,