Use other tree for actual symbol of Assign

Author: som-snytt

Diff for: compiler/src/dotty/tools/dotc/ast/Trees.scala

@@ -1821,16 +1821,15 @@ object Trees {
       }
     }
 
-    def rename(tree: NameTree, newName: Name)(using Context): tree.ThisTree[T] = {
-      tree match {
+    def rename(tree: NameTree, newName: Name)(using Context): tree.ThisTree[T] =
+      tree.match
         case tree: Ident => cpy.Ident(tree)(newName)
         case tree: Select => cpy.Select(tree)(tree.qualifier, newName)
         case tree: Bind => cpy.Bind(tree)(newName, tree.body)
         case tree: ValDef => cpy.ValDef(tree)(name = newName.asTermName)
         case tree: DefDef => cpy.DefDef(tree)(name = newName.asTermName)
         case tree: TypeDef => cpy.TypeDef(tree)(name = newName.asTypeName)
-      }
-    }.asInstanceOf[tree.ThisTree[T]]
+      .asInstanceOf[tree.ThisTree[T]]
 
     object TypeDefs:
       def unapply(xs: List[Tree]): Option[List[TypeDef]] = xs match

Diff for: compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -1528,14 +1528,16 @@ class AmbiguousExtensionMethod(tree: untpd.Tree, expansion1: tpd.Tree, expansion
 
 class ReassignmentToVal(name: Name)(using Context)
   extends TypeMsg(ReassignmentToValID) {
-  def msg(using Context) = i"""Reassignment to val $name"""
+  def msg(using Context) = i"""Reassignment to value $name"""
   def explain(using Context) =
-    i"""|You can not assign a new value to $name as values can't be changed.
-        |Keep in mind that every statement has a value, so you may e.g. use
-        |  ${hl("val")} $name ${hl("= if (condition) 2 else 5")}
-        |In case you need a reassignable name, you can declare it as
-        |variable
+    i"""|Members defined using `val` or `def` can't be assigned to.
+        |If you need to change the value of $name, use `var` instead:
         |  ${hl("var")} $name ${hl("=")} ...
+        |However, it's more common to initialize a variable just once
+        |with a complex expression or even a block with many statements:
+        |  ${hl("val")} $name ${hl("= if (condition) 1 else -1")}
+        |Also, assignment syntax can be used if there is a corresponding setter:
+        |  ${hl("def")} ${name}${hl("_=(x: Int): Unit = _v = x")}
         |"""
 }
 

Diff for: compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -1373,9 +1373,11 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
 
   def typedAssign(tree: untpd.Assign, pt: Type)(using Context): Tree =
     tree.lhs match {
-      case lhs @ Apply(fn, args) =>
-        typed(untpd.Apply(untpd.Select(fn, nme.update), args :+ tree.rhs), pt)
-      case untpd.TypedSplice(Apply(MaybePoly(Select(fn, app), targs), args)) if app == nme.apply =>
+      case Apply(fn, args) =>
+        val appliedUpdate =
+          untpd.Apply(untpd.Select(fn, nme.update), args :+ tree.rhs)
+        typed(appliedUpdate, pt)
+      case untpd.TypedSplice(Apply(MaybePoly(Select(fn, nme.apply), targs), args)) =>
         val rawUpdate: untpd.Tree = untpd.Select(untpd.TypedSplice(fn), nme.update)
         val wrappedUpdate =
           if (targs.isEmpty) rawUpdate
@@ -1389,7 +1391,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
         def lhs1 = adapt(lhsCore, LhsProto, locked)
 
         def reassignmentToVal =
-          report.error(ReassignmentToVal(lhsCore.symbol.name), tree.srcPos)
+          report.error(ReassignmentToVal(lhs1.symbol.name), tree.srcPos)
           cpy.Assign(tree)(lhsCore, typed(tree.rhs, lhs1.tpe.widen)).withType(defn.UnitType)
 
         def canAssign(sym: Symbol) =
@@ -1478,8 +1480,9 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
               typedDynamicAssign(tree, pt)
             case tpe =>
               reassignmentToVal
-        }
+          }
     }
+  end typedAssign
 
   def typedBlockStats(stats: List[untpd.Tree])(using Context): (List[tpd.Tree], Context) =
     index(stats)

Diff for: tests/neg/i22671.check

@@ -0,0 +1,49 @@
+-- [E007] Type Mismatch Error: tests/neg/i22671.scala:37:22 ------------------------------------------------------------
+37 |    names_times(fields(0)) += fields(1).toLong // error
+   |                ^^^^^^^^^
+   |                Found:    Char
+   |                Required: String
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E008] Not Found Error: tests/neg/i22671.scala:41:6 -----------------------------------------------------------------
+41 |  x() += "42" // error
+   |  ^^^^^^
+   |  value += is not a member of Int - did you mean Int.!=? or perhaps Int.<=?
+-- [E052] Type Error: tests/neg/i22671.scala:8:6 -----------------------------------------------------------------------
+8 |  X.x = 27 // error
+  |  ^^^^^^^^
+  |  Reassignment to value x
+  |
+  | longer explanation available when compiling with `-explain`
+-- [E052] Type Error: tests/neg/i22671.scala:12:4 ----------------------------------------------------------------------
+12 |  x = 27 // error
+   |  ^^^^^^
+   |  Reassignment to value x
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E052] Type Error: tests/neg/i22671.scala:16:4 ----------------------------------------------------------------------
+16 |  y = 27 // error
+   |  ^^^^^^
+   |  Reassignment to value x
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E052] Type Error: tests/neg/i22671.scala:20:4 ----------------------------------------------------------------------
+20 |  y = 27 // error
+   |  ^^^^^^
+   |  Reassignment to value z
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E052] Type Error: tests/neg/i22671.scala:24:4 ----------------------------------------------------------------------
+24 |  x = 27 // error
+   |  ^^^^^^
+   |  Reassignment to value x
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E008] Not Found Error: tests/neg/i22671.scala:27:6 -----------------------------------------------------------------
+27 |  X.x += 27 // error
+   |  ^^^^^^
+   |  value += is not a member of Int - did you mean Int.!=? or perhaps Int.<=?
+-- [E008] Not Found Error: tests/neg/i22671.scala:28:4 -----------------------------------------------------------------
+28 |  1 += 1 // error
+   |  ^^^^
+   |  value += is not a member of Int - did you mean (1 : Int).!=? or perhaps (1 : Int).<=?

Diff for: tests/neg/i22671.scala

@@ -0,0 +1,41 @@
+
+object X:
+  def x: Int = 42
+  def x(y: Int): Int = x + y
+  val z = 26
+
+def f =
+  X.x = 27 // error
+
+def g =
+  import X.x
+  x = 27 // error
+
+def h =
+  import X.x as y
+  y = 27 // error
+
+def i =
+  import X.z as y
+  y = 27 // error
+
+def j =
+  val x = 42
+  x = 27 // error
+
+def k =
+  X.x += 27 // error
+  1 += 1 // error
+
+
+object t8763:
+  import collection.mutable
+  def bar(): Unit =
+    val names_times = mutable.Map.empty[String, mutable.Set[Long]]
+    val line = ""
+    val Array(fields) = line.split("\t")
+    names_times(fields(0)) += fields(1).toLong // error
+
+object t9834:
+  object x { def apply() = 42 ; def update(i: Int) = () }
+  x() += "42" // error