Fix #5419: explicit result types for _N in case classes

In general this is necessary to avoid widening, but it's especially needed when using an implicit function type as a case class parameter type.

Author: smarter

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -431,9 +431,9 @@ object desugar {
  lazy val creatorExpr = New(classTypeRef, constrVparamss nestedMap refOfDef)
 
  // Methods to add to a case class C[..](p1: T1, ..., pN: Tn)(moreParams)
- // def _1 = this.p1
+ // def _1: T1 = this.p1
  // ...
- // def _N = this.pN
+ // def _N: TN = this.pN
  // def copy(p1: T1 = p1: @uncheckedVariance, ...,
  // pN: TN = pN: @uncheckedVariance)(moreParams) =
  // new C[...](p1, ..., pN)(moreParams)
@@ -442,12 +442,13 @@ object desugar {
  // neg/t1843-variances.scala for a test case. The test would give
  // two errors without @uncheckedVariance, one of them spurious.
  val caseClassMeths = {
- def syntheticProperty(name: TermName, rhs: Tree) =
- DefDef(name, Nil, Nil, TypeTree(), rhs).withMods(synthetic)
+ def syntheticProperty(name: TermName, tpt: Tree, rhs: Tree) =
+ DefDef(name, Nil, Nil, tpt, rhs).withMods(synthetic)
  def productElemMeths = {
- val caseParams = constrVparamss.head.toArray
+ val caseParams = derivedVparamss.head.toArray
  for (i <- 0 until arity if nme.selectorName(i) `ne` caseParams(i).name)
- yield syntheticProperty(nme.selectorName(i), Select(This(EmptyTypeIdent), caseParams(i).name))
+ yield syntheticProperty(nme.selectorName(i), caseParams(i).tpt,
+ Select(This(EmptyTypeIdent), caseParams(i).name))
  }
  def enumTagMeths = if (isEnumCase) enumTagMeth(CaseKind.Class)._1 :: Nil else Nil
  def copyMeths = {

tests/neg/t1843-variances.scala

@@ -6,7 +6,7 @@
 
 object Crash {
  trait UpdateType[A]
- case class StateUpdate[+A](updateType : UpdateType[A], value : A) // error // error
+ case class StateUpdate[+A](updateType : UpdateType[A], value : A) // error
  case object IntegerUpdateType extends UpdateType[Integer]
 
  //However this method will cause a crash

tests/pos/case-getters.scala

@@ -0,0 +1,8 @@
+case class Foo(x: 1, y: implicit Int => Int)
+object Test {
+ val f = Foo(1, implicit (i: Int) => i)
+ val fx1: 1 = f.x
+ val fx2: 1 = f._1
+ val fy1: Int = f.y(1)
+ val fy2: Int = f._2(1)
+}