Add support for var in refinements (#19982)

Closes #19809

Author: hamzaremmal

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -1943,12 +1943,27 @@ object desugar {
       case AndType(tp1, tp2) => stripToCore(tp1) ::: stripToCore(tp2)
       case _ => defn.AnyType :: Nil
     }
+
+    val refinements1 = Trees.flatten:
+      refinements.mapConserve {
+        case tree: ValDef if tree.mods.is(Mutable) =>
+          val getter =
+            cpy.DefDef(tree)(name = tree.name, paramss = Nil, tpt = tree.tpt, rhs = tree.rhs)
+              .withFlags(tree.mods.flags & (AccessFlags | Synthetic))
+          val setterParam = makeSyntheticParameter(tpt = tree.tpt)
+          val setter =
+            cpy.DefDef(tree)(name = tree.name.setterName, paramss = List(List(setterParam)), tpt = untpd.scalaUnit, rhs = EmptyTree)
+              .withFlags(tree.mods.flags & (AccessFlags | Synthetic))
+          Thicket(getter, setter)
+        case tree => tree
+      }
+
     val parentCores = stripToCore(parent.tpe)
     val untpdParent = TypedSplice(parent)
     val (classParents, self) =
       if (parentCores.length == 1 && (parent.tpe eq parentCores.head)) (untpdParent :: Nil, EmptyValDef)
       else (parentCores map TypeTree, ValDef(nme.WILDCARD, untpdParent, EmptyTree))
-    val impl = Template(emptyConstructor, classParents, Nil, self, refinements)
+    val impl = Template(emptyConstructor, classParents, Nil, self, refinements1)
     TypeDef(tpnme.REFINE_CLASS, impl).withFlags(Trait)
   }
 

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -4381,6 +4381,7 @@ object Parsers {
 
     /** RefineStatSeq    ::=  RefineStat {semi RefineStat}
      *  RefineStat       ::=  ‘val’ VarDef
+     *                     |  ‘var’ VarDef
      *                     |  ‘def’ DefDef
      *                     |  ‘type’ {nl} TypeDef
      *  (in reality we admit class defs and vars and filter them out afterwards in `checkLegal`)
@@ -4393,10 +4394,7 @@ object Parsers {
           syntaxError(msg, tree.span)
           Nil
         tree match
-          case tree: ValDef if tree.mods.is(Mutable) =>
-            fail(em"""refinement cannot be a mutable var.
-                     |You can use an explicit getter ${tree.name} and setter ${tree.name}_= instead""")
-          case tree: MemberDef if !(tree.mods.flags & ModifierFlags).isEmpty =>
+          case tree: MemberDef if !(tree.mods.flags & (ModifierFlags &~ Mutable)).isEmpty =>
             fail(em"refinement cannot be ${(tree.mods.flags & ModifierFlags).flagStrings().mkString("`", "`, `", "`")}")
           case tree: DefDef if tree.termParamss.nestedExists(!_.rhs.isEmpty) =>
             fail(em"refinement cannot have default arguments")

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

@@ -2213,7 +2213,6 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
     val refineCls = createSymbol(refineClsDef).asClass
     val TypeDef(_, impl: Template) = typed(refineClsDef): @unchecked
     val refinements1 = impl.body
-    assert(tree.refinements.hasSameLengthAs(refinements1), i"${tree.refinements}%, % > $refinements1%, %")
     val seen = mutable.Set[Symbol]()
     for (refinement <- refinements1) { // TODO: get clarity whether we want to enforce these conditions
       typr.println(s"adding refinement $refinement")

docs/_docs/internals/syntax.md

@@ -431,6 +431,7 @@ EndMarkerTag      ::=  id | ‘if’ | ‘while’ | ‘for’ | ‘match’ | 
 ### Definitions
 ```ebnf
 RefineDcl         ::=  ‘val’ ValDcl
+                    |  ‘var’ ValDcl
                     |  ‘def’ DefDcl
                     |  ‘type’ {nl} TypeDef
 ValDcl            ::=  ids ‘:’ Type

docs/_docs/reference/syntax.md

@@ -415,6 +415,7 @@ EndMarkerTag      ::=  id | ‘if’ | ‘while’ | ‘for’ | ‘match’ | 
 ```
 RefineDcl         ::=  ‘val’ ValDcl
                     |  ‘def’ DefDcl
+                    |  ‘var’ ValDcl
                     |  ‘type’ {nl} TypeDef
 ValDcl            ::=  ids ‘:’ Type
 DefDcl            ::=  DefSig ‘:’ Type

tests/neg/i13703.check

@@ -1,10 +1,5 @@
--- Error: tests/neg/i13703.scala:3:17 ----------------------------------------------------------------------------------
-3 |val f: Foo { var i: Int } = new Foo { var i: Int = 0 } // error
-  |             ^^^^^^^^^^
-  |             refinement cannot be a mutable var.
-  |             You can use an explicit getter i and setter i_= instead
--- [E007] Type Mismatch Error: tests/neg/i13703.scala:5:78 -------------------------------------------------------------
-5 |val f2: Foo { val i: Int; def i_=(x: Int): Unit } = new Foo { var i: Int = 0 } // error
+-- [E007] Type Mismatch Error: tests/neg/i13703.scala:3:78 -------------------------------------------------------------
+3 |val f2: Foo { val i: Int; def i_=(x: Int): Unit } = new Foo { var i: Int = 0 } // error
   |                                                                              ^
   |                                                                  Found:    Object with Foo {...}
   |                                                                  Required: Foo{val i: Int; def i_=(x: Int): Unit}

tests/neg/i13703.scala

@@ -1,7 +1,5 @@
 trait Foo extends reflect.Selectable
 
-val f: Foo { var i: Int } = new Foo { var i: Int = 0 } // error
-
 val f2: Foo { val i: Int; def i_=(x: Int): Unit } = new Foo { var i: Int = 0 } // error
 
 val f3: Foo { def i: Int; def i_=(x: Int): Unit } = new Foo { var i: Int = 0 } // OK

tests/neg/i19809.check

@@ -0,0 +1,10 @@
+-- [E120] Naming Error: tests/neg/i19809.scala:3:6 ---------------------------------------------------------------------
+3 |  def x_=(x: Int): Unit // error
+  |      ^
+  |      Double definition:
+  |      def x_=(x$1: Int): Unit in trait <refinement> at line 2 and
+  |      def x_=(x: Int): Unit in trait <refinement> at line 3
+  |      have the same type after erasure.
+  |
+  |      Consider adding a @targetName annotation to one of the conflicting definitions
+  |      for disambiguation.

tests/neg/i19809.scala

@@ -0,0 +1,4 @@
+type A = Any {
+  var x : Int
+  def x_=(x: Int): Unit // error
+}

tests/neg/i4496b.scala

@@ -5,7 +5,7 @@ trait Foo2 { def a: Int }
 trait Foo3 { var a: Int }
 
 object TestStructuralVar {
-  type T0 = {var a: Int} // error
+  type T0 = {var a: Int}
   object TestStructuralVar {
     type T = {val a: Int; def a_=(x: Int): Unit}
     def upcast1(v: Foo1): T = v // error