fix KT-7042 Providing return type for property setter is not reported as error

#KT-7042 Fixed

compiler/frontend/src/org/jetbrains/kotlin/diagnostics/Errors.java

@@ -340,6 +340,7 @@ public interface Errors {
     DiagnosticFactory0<PsiElement> PRIVATE_SETTER_ON_NON_PRIVATE_LATE_INIT_VAR = DiagnosticFactory0.create(ERROR);
     DiagnosticFactory0<PsiElement> ACCESSOR_VISIBILITY_FOR_ABSTRACT_PROPERTY = DiagnosticFactory0.create(ERROR);
     DiagnosticFactory2<KtTypeReference, KotlinType, KotlinType> WRONG_GETTER_RETURN_TYPE = DiagnosticFactory2.create(ERROR);
+    DiagnosticFactory0<KtTypeReference> WRONG_SETTER_RETURN_TYPE = DiagnosticFactory0.create(ERROR);
 
     DiagnosticFactory0<KtModifierListOwner>
             ABSTRACT_PROPERTY_IN_PRIMARY_CONSTRUCTOR_PARAMETERS = DiagnosticFactory0.create(ERROR, ABSTRACT_MODIFIER);

compiler/frontend/src/org/jetbrains/kotlin/diagnostics/rendering/DefaultErrorMessages.java

@@ -323,6 +323,7 @@ public class DefaultErrorMessages {
         MAP.put(USELESS_NULLABLE_CHECK, "Non-null type is checked for instance of nullable type");
         MAP.put(WRONG_SETTER_PARAMETER_TYPE, "Setter parameter type must be equal to the type of the property, i.e. ''{0}''", RENDER_TYPE, RENDER_TYPE);
         MAP.put(WRONG_GETTER_RETURN_TYPE, "Getter return type must be equal to the type of the property, i.e. ''{0}''", RENDER_TYPE, RENDER_TYPE);
+        MAP.put(WRONG_SETTER_RETURN_TYPE, "Setter return type must be Unit");
         MAP.put(NO_COMPANION_OBJECT, "Please specify constructor invocation; classifier ''{0}'' does not have a companion object", NAME);
         MAP.put(TYPE_PARAMETER_IS_NOT_AN_EXPRESSION, "Type parameter ''{0}'' is not an expression", NAME);
         MAP.put(TYPE_PARAMETER_ON_LHS_OF_DOT, "Type parameter ''{0}'' cannot have or inherit a companion object, so it cannot be on the left hand side of dot", NAME);

compiler/frontend/src/org/jetbrains/kotlin/resolve/DescriptorResolver.java

@@ -1010,6 +1010,14 @@ public class DescriptorResolver {
                                                                 setter.hasBody(), false, setter.hasModifier(EXTERNAL_KEYWORD),
                                                                 CallableMemberDescriptor.Kind.DECLARATION, null, KotlinSourceElementKt
                                                                         .toSourceElement(setter));
+            KtTypeReference returnTypeReference = setter.getReturnTypeReference();
+            if (returnTypeReference != null) {
+                KotlinType returnType = typeResolver.resolveType(scope, returnTypeReference, trace, true);
+                if (!KotlinBuiltIns.isUnit(returnType)) {
+                    trace.report(WRONG_SETTER_RETURN_TYPE.on(returnTypeReference));
+                }
+            }
+
             if (parameter != null) {
 
                 // This check is redundant: the parser does not allow a default value, but we'll keep it just in case

compiler/testData/diagnostics/tests/backingField/SetterWithExplicitType.kt

@@ -0,0 +1,26 @@
+// KT-7042 Providing return type for property setter is not reported as error
+
+var x: Int = 1
+
+// No backing field!
+var y: Int
+    get() = x
+    set(value): <!WRONG_SETTER_RETURN_TYPE!>Any<!> {
+        x = value
+    }
+
+var z: Int
+    get() = x
+    set(value): Unit {
+        x = value
+    }
+
+var u: String = ""
+    set(value): Unit {
+        field = value
+    }
+
+var v: String = ""
+    set(value): <!WRONG_SETTER_RETURN_TYPE!>String<!> {
+        field = value
+    }

compiler/testData/diagnostics/tests/backingField/SetterWithExplicitType.txt

@@ -0,0 +1,7 @@
+package
+
+public var u: kotlin.String
+public var v: kotlin.String
+public var x: kotlin.Int
+public var y: kotlin.Int
+public var z: kotlin.Int

compiler/tests/org/jetbrains/kotlin/checkers/DiagnosticsTestGenerated.java

@@ -1553,6 +1553,12 @@ public class DiagnosticsTestGenerated extends AbstractDiagnosticsTest {
                 String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/backingField/kt782packageLevel.kt");
                 doTest(fileName);
             }
+
+            @TestMetadata("SetterWithExplicitType.kt")
+            public void testSetterWithExplicitType() throws Exception {
+                String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/backingField/SetterWithExplicitType.kt");
+                doTest(fileName);
+            }
         }
 
         @TestMetadata("compiler/testData/diagnostics/tests/callableReference")