Allow to call += on a val

compiler/testData/codegen/box/operatorConventions/assignmentOperations.kt

@@ -0,0 +1,36 @@
+class A() {
+  var x = 0
+}
+
+fun A.plusAssign(y: Int) { x += y }
+fun A.minusAssign(y: Int) { x -= y }
+fun A.timesAssign(y: Int) { x *= y }
+fun A.divAssign(y: Int) { x /= y }
+fun A.modAssign(y: Int) { x %= y }
+
+fun box(): String {
+  val original = A()
+  val a = original
+
+  a += 1
+  if (!(a identityEquals original)) return "Fail 1: $a !== $original"
+  if (a.x != 1) return "Fail 2: ${a.x} != 1"
+
+  a -= 2
+  if (!(a identityEquals original)) return "Fail 3: $a !== $original"
+  if (a.x != -1) return "Fail 4: ${a.x} != -1"
+
+  a *= -10
+  if (!(a identityEquals original)) return "Fail 5: $a !== $original"
+  if (a.x != 10) return "Fail 6: ${a.x} != 10"
+
+  a /= 3
+  if (!(a identityEquals original)) return "Fail 7: $a !== $original"
+  if (a.x != 3) return "Fail 8: ${a.x} != 3"
+
+  a %= 2
+  if (!(a identityEquals original)) return "Fail 9: $a !== $original"
+  if (a.x != 1) return "Fail 10: ${a.x} != 1"
+
+  return "OK"
+}

compiler/testData/diagnostics/tests/operatorsOverloading/AssignmentOperations.kt

@@ -0,0 +1,40 @@
+class A {
+  fun plusAssign(<!UNUSED_PARAMETER!>x<!>: Int) {}
+  fun minusAssign(<!UNUSED_PARAMETER!>x<!>: Int) {}
+  fun timesAssign(<!UNUSED_PARAMETER!>x<!>: Int) {}
+  fun divAssign(<!UNUSED_PARAMETER!>x<!>: Int) {}
+  fun modAssign(<!UNUSED_PARAMETER!>x<!>: Int) {}
+}
+
+fun testVal() {
+  val a = A()
+  a += 1
+  a -= 1
+  a *= 1
+  a /= 1
+  a %= 1
+}
+
+fun testExpr() {
+  A() += 1
+  A() -= 1
+  A() *= 1
+  A() /= 1
+  A() %= 1
+}
+
+class B {
+  fun plus(<!UNUSED_PARAMETER!>x<!>: Int): B = B()
+  fun minus(<!UNUSED_PARAMETER!>x<!>: Int): B = B()
+  fun times(<!UNUSED_PARAMETER!>x<!>: Int): B = B()
+  fun div(<!UNUSED_PARAMETER!>x<!>: Int): B = B()
+  fun mod(<!UNUSED_PARAMETER!>x<!>: Int): B = B()
+}
+
+fun testWrong() {
+  <!VARIABLE_EXPECTED!>B()<!> += 1
+  <!VARIABLE_EXPECTED!>B()<!> -= 1
+  <!VARIABLE_EXPECTED!>B()<!> *= 1
+  <!VARIABLE_EXPECTED!>B()<!> /= 1
+  <!VARIABLE_EXPECTED!>B()<!> %= 1
+}