Fix scala-js/scala-js#4929: Fix logic for moving early assignements in JS ctors.

Previously, we moved all statements in the constructors after the
super constructor call. However, it turns out that there are
statements that must be kept before, notably local `val`s
generated for default arguments to the super constructor.

We now keep statements where they are by default. We only move
statements of the form `C.this.field = ident;`, which are the
only ones that require access to `this`.

Forward port of the upstream commit
scala-js/scala-js@2e4594f

Author: sjrd

Diff for: compiler/src/dotty/tools/backend/sjs/JSCodeGen.scala

@@ -1146,42 +1146,72 @@ class JSCodeGen()(using genCtx: Context) {
 
   private def genPrimaryJSClassCtor(dd: DefDef): PrimaryJSCtor = {
     val sym = dd.symbol
-    val Block(stats, _) = dd.rhs: @unchecked
     assert(sym.isPrimaryConstructor, s"called with non-primary ctor: $sym")
 
+    var preSuperStats = List.newBuilder[js.Tree]
     var jsSuperCall: Option[js.JSSuperConstructorCall] = None
-    val jsStats = List.newBuilder[js.Tree]
+    val postSuperStats = List.newBuilder[js.Tree]
 
-    /* Move all statements after the super constructor call since JS
-     * cannot access `this` before the super constructor call.
+    /* Move param accessor initializers after the super constructor call since
+     * JS cannot access `this` before the super constructor call.
      *
      * dotc inserts statements before the super constructor call for param
      * accessor initializers (including val's and var's declared in the
-     * params). We move those after the super constructor call, and are
-     * therefore executed later than for a Scala class.
+     * params). Those statements are assignments whose rhs'es are always simple
+     * Idents (the constructor params).
+     *
+     * There can also be local `val`s before the super constructor call for
+     * default arguments to the super constructor. These must remain before.
+     *
+     * Our strategy is therefore to move only the field assignments after the
+     * super constructor call. They are therefore executed later than for a
+     * Scala class (as specified for non-native JS classes semantics).
+     * However, side effects and evaluation order of all the other
+     * computations remains unchanged.
      */
     withPerMethodBodyState(sym) {
-      stats.foreach {
-        case tree @ Apply(fun @ Select(Super(This(_), _), _), args)
-            if fun.symbol.isClassConstructor =>
-          assert(jsSuperCall.isEmpty, s"Found 2 JS Super calls at ${dd.sourcePos}")
-          implicit val pos: Position = tree.span
-          jsSuperCall = Some(js.JSSuperConstructorCall(genActualJSArgs(fun.symbol, args)))
+      def isThisField(tree: Tree): Boolean = tree match {
+        case Select(ths: This, _) => ths.symbol == currentClassSym.get
+        case tree: Ident          => desugarIdent(tree).exists(isThisField(_))
+        case _                    => false
+      }
 
-        case stat =>
-          val jsStat = genStat(stat)
-          assert(jsSuperCall.isDefined || !jsStat.isInstanceOf[js.VarDef],
-              "Trying to move a local VarDef after the super constructor call of a non-native JS class at " +
-              dd.sourcePos)
-          jsStats += jsStat
+      def rec(tree: Tree): Unit = {
+        tree match {
+          case Block(stats, expr) =>
+            stats.foreach(rec(_))
+            rec(expr)
+
+          case tree @ Apply(fun @ Select(Super(This(_), _), _), args)
+              if fun.symbol.isClassConstructor =>
+            assert(jsSuperCall.isEmpty, s"Found 2 JS Super calls at ${dd.sourcePos}")
+            implicit val pos: Position = tree.span
+            jsSuperCall = Some(js.JSSuperConstructorCall(genActualJSArgs(fun.symbol, args)))
+
+          case tree if jsSuperCall.isDefined =>
+            // Once we're past the super constructor call, everything goes after.
+            postSuperStats += genStat(tree)
+
+          case Assign(lhs, Ident(_)) if isThisField(lhs) =>
+            /* If that shape appears before the jsSuperCall, it is a param
+             * accessor initializer. We move it.
+             */
+            postSuperStats += genStat(tree)
+
+          case stat =>
+            // Other statements are left before.
+            preSuperStats += genStat(stat)
+        }
       }
+
+      rec(dd.rhs)
     }
 
     assert(jsSuperCall.isDefined,
         s"Did not find Super call in primary JS construtor at ${dd.sourcePos}")
 
     new PrimaryJSCtor(sym, genParamsAndInfo(sym, dd.paramss),
-        js.JSConstructorBody(Nil, jsSuperCall.get, jsStats.result())(dd.span))
+        js.JSConstructorBody(preSuperStats.result(), jsSuperCall.get, postSuperStats.result())(dd.span))
   }
 
   private def genSecondaryJSClassCtor(dd: DefDef): SplitSecondaryJSCtor = {