Fix alignment of trait parameters and arguments

As part of the fix, we now also detect missing trait constructors.

Fixes #11214
Fixes #11344

Author: odersky

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

@@ -258,7 +258,7 @@ class Mixin extends MiniPhase with SymTransformer { thisPhase =>
            && !wasOneOf(getter, Deferred)
            && !getter.isConstExprFinalVal
       yield
-        if (isCurrent(getter) || getter.name.is(ExpandedName)) {
+        if (isInImplementingClass(getter) || getter.name.is(ExpandedName)) {
           val rhs =
             if (wasOneOf(getter, ParamAccessor))
               nextArgument()
@@ -271,6 +271,9 @@ class Mixin extends MiniPhase with SymTransformer { thisPhase =>
           // transformFollowing call is needed to make memoize & lazy vals run
           transformFollowing(DefDef(mkForwarderSym(getter.asTerm), rhs))
         }
+        else if wasOneOf(getter, ParamAccessor) then
+          // mixin parameter field is defined by an override; evaluate the argument and throw it away
+          nextArgument()
         else EmptyTree
     }
 

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

@@ -42,7 +42,7 @@ class MixinOps(cls: ClassSymbol, thisPhase: DenotTransformer)(using Context) {
   /** Is `sym` a member of implementing class `cls`?
    *  The test is performed at phase `thisPhase`.
    */
-  def isCurrent(sym: Symbol): Boolean =
+  def isInImplementingClass(sym: Symbol): Boolean =
     atPhase(thisPhase) {
       cls.info.nonPrivateMember(sym.name).hasAltWith(_.symbol == sym)
     }
@@ -71,7 +71,7 @@ class MixinOps(cls: ClassSymbol, thisPhase: DenotTransformer)(using Context) {
     meth.is(Method, butNot = PrivateOrAccessorOrDeferred) &&
     (ctx.settings.mixinForwarderChoices.isTruthy || meth.owner.is(Scala2x) || needsDisambiguation || hasNonInterfaceDefinition ||
      generateJUnitForwarder || generateSerializationForwarder) &&
-    isCurrent(meth)
+    isInImplementingClass(meth)
   }
 
   final val PrivateOrAccessor: FlagSet = Private | Accessor

tests/pos/i11214.scala renamed to tests/neg/i11214.scala

@@ -1,5 +1,5 @@
 trait Pet(val name: String)
 trait FeatheredPet extends Pet
 
-class Bird(override val name: String) extends FeatheredPet:
+class Bird(override val name: String) extends FeatheredPet: // error
    override def toString = s"bird name: $name"

tests/pos/i11344.scala

@@ -0,0 +1,8 @@
+trait Pet(val name: String, rest: Int):
+   def f(suffix: String) = s"$name$suffix$rest"
+
+class Birdie(override val name: String) extends Pet("huh", 1)
+
+@main def Test =
+  assert(Birdie("Polly").f("more") == "Pollymore1")
+