Fix order in which errors are reported for assignment to val (#16660)

Fixes #16655

Author: odersky

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

@@ -1000,8 +1000,8 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
         def lhs1 = adapt(lhsCore, AssignProto, locked)
 
         def reassignmentToVal =
-          errorTree(cpy.Assign(tree)(lhsCore, typed(tree.rhs, lhs1.tpe.widen)),
-            ReassignmentToVal(lhsCore.symbol.name))
+          report.error(ReassignmentToVal(lhsCore.symbol.name), tree.srcPos)
+          cpy.Assign(tree)(lhsCore, typed(tree.rhs, lhs1.tpe.widen)).withType(defn.UnitType)
 
         def canAssign(sym: Symbol) =
           sym.is(Mutable, butNot = Accessor) ||

tests/neg/i16655.check

@@ -0,0 +1,6 @@
+-- [E052] Type Error: tests/neg/i16655.scala:3:4 -----------------------------------------------------------------------
+3 |  x = 5 // error
+  |  ^^^^^
+  |  Reassignment to val x
+  |
+  | longer explanation available when compiling with `-explain`

tests/neg/i16655.scala

@@ -0,0 +1,3 @@
+object Test:
+  val x = "MyString"
+  x = 5 // error