Forbid user from overriding vars

The current check allowed the user to override vars, we don't want that

Only the compiler should be allowed to do this, and for inline traits only

Author: timotheeandres

compiler/src/dotty/tools/dotc/typer/RefChecks.scala

@@ -341,6 +341,7 @@ object RefChecks {
      */
     def checkOverride(checkSubType: (Type, Type) => Context ?=> Boolean, member: Symbol, other: Symbol): Unit =
       def overridesInlineTraitMember = other.owner.ownersIterator.exists(_.isInlineTrait) && member.is(Synthetic)
+      def isInlinedFromInlineTrait = other.owner.isAllOf(InlineTrait) && member.is(Synthetic)
 
       def memberTp(self: Type) =
         if (member.isClass) TypeAlias(member.typeRef.EtaExpand(member.typeParams))
@@ -505,7 +506,7 @@ object RefChecks {
           overrideError("needs `override` modifier")
       else if (other.is(AbsOverride) && other.isIncompleteIn(clazz) && !member.is(AbsOverride))
         overrideError("needs `abstract override` modifiers")
-      else if member.is(Override) && other.is(Mutable) && !other.owner.isAllOf(InlineTrait) then
+      else if member.is(Override) && other.is(Mutable) && !isInlinedFromInlineTrait then
         overrideError("cannot override a mutable variable")
       else if (member.isAnyOverride &&
         !(member.owner.thisType.baseClasses exists (_ isSubClass other.owner)) &&

tests/neg/inline-trait-body-override-var.scala

@@ -0,0 +1,5 @@
+inline trait A:
+  var x: Int = 1
+
+class B extends A:
+  override var x = 2 // error

tests/run/inline-trait-body-override-var.check

@@ -0,0 +1,4 @@
+1
+2
+3
+4

tests/run/inline-trait-body-override-var.scala

@@ -6,3 +6,10 @@ class B extends A:
     val old = x
     x += 1
     old
+
+@main def Test =
+  val b = B()
+  println(b.f)
+  println(b.f)
+  println(b.f)
+  println(b.f)