Fix variable handling in super calls

Accesses to var parameters in super calls need to refer to the constructor
parameter, not the getter.

Fixes #13630

Author: odersky

compiler/src/dotty/tools/dotc/transform/Constructors.scala

@@ -146,10 +146,16 @@ class Constructors extends MiniPhase with IdentityDenotTransformer { thisPhase =
     //  (2) If the parameter accessor reference was to an alias getter,
     //      drop the () when replacing by the parameter.
     object intoConstr extends TreeMap {
+      private var isSuperCall = false
       override def transform(tree: Tree)(using Context): Tree = tree match {
         case Ident(_) | Select(This(_), _) =>
           var sym = tree.symbol
-          if (sym.is(ParamAccessor, butNot = Mutable)) sym = sym.subst(accessors, paramSyms)
+          if sym.is(ParamAccessor) && (!sym.is(Mutable) || isSuperCall)
+            // Variables need to go through the getter since they might have been updated,
+            // except if we are in a super call, since then the virtual getter call would
+            // be illegal.
+          then
+            sym = sym.subst(accessors, paramSyms)
           if (sym.maybeOwner.isConstructor) ref(sym).withSpan(tree.span) else tree
         case Apply(fn, Nil) =>
           val fn1 = transform(fn)
@@ -161,6 +167,7 @@ class Constructors extends MiniPhase with IdentityDenotTransformer { thisPhase =
       }
 
       def apply(tree: Tree, prevOwner: Symbol)(using Context): Tree =
+        isSuperCall = isSuperConstrCall(tree)
         transform(tree).changeOwnerAfter(prevOwner, constr.symbol, thisPhase)
     }
 

tests/run/i13630.check

@@ -0,0 +1 @@
+it worked

tests/run/i13630.scala

@@ -0,0 +1,9 @@
+class ClassWithLambda(sup: () => Long)
+class ClassWithVar(var msg: String) extends ClassWithLambda(() => 1)
+
+object Test:
+  val _ = new ClassWithVar("foo")
+
+  def main(args: Array[String]): Unit = {
+    println("it worked")
+  }