Merge pull request #2896 from dotty-staging/fix#-2300

Fix #2300: Try to heal subtype failures by instantiating type variables

Author: DarkDimius

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

@@ -2191,7 +2191,10 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
         case SearchSuccess(inferred, _, _, _) =>
           adapt(inferred, pt)(ctx.retractMode(Mode.ImplicitsEnabled))
         case failure: SearchFailure =>
+          val prevConstraint = ctx.typerState.constraint
           if (pt.isInstanceOf[ProtoType] && !failure.isInstanceOf[AmbiguousImplicits]) tree
+          else if (isFullyDefined(wtp, force = ForceDegree.all) &&
+                   ctx.typerState.constraint.ne(prevConstraint)) adapt(tree, pt, original)
           else err.typeMismatch(tree, pt, failure)
       }
     }

tests/neg/i1650.scala

@@ -1,5 +1,5 @@
 object Test {
-  test4(test4$default$1) // error
+  test4(test4$default$1)
   def test4[T[P]](x: T[T[List[T[X forSome { type X }]]]]) = ??? // error // error
   def test4$default$1[T[P]]: T[Int] = ???
 }

tests/pos/i2300.scala

@@ -0,0 +1,26 @@
+object hlists {
+  sealed abstract class HList {
+    type Merge[L <: HList] <: HList
+
+    def merge[L <: HList](l: L): Merge[L]
+  }
+  final case class HCons[H, T <: HList](head : H, tail : T) extends HList {
+    type Merge[L <: HList] = HCons[H, tail.Merge[L]]
+
+    def merge[L <: HList](l: L): Merge[L] = HCons(head, tail.merge(l))
+  }
+  sealed trait HNil extends HList {
+    type Merge[L <: HList] = L
+
+    def merge[L <: HList](l: L): Merge[L] = l
+  }
+  final val HNil: HNil = { case object HNil extends HNil; HNil }
+}
+
+object Test {
+  import hlists._
+
+  val merged: HCons[Int,HCons[String,HNil]] = {
+    HCons(42, HNil) merge HCons("foo", HNil)
+  }
+}

tests/repl/errmsgs.check

@@ -31,10 +31,9 @@ scala> val a: Inv[String] = new Inv(new Inv(1))
 -- [E007] Type Mismatch Error: <console>:5:33 ----------------------------------
 5 |val a: Inv[String] = new Inv(new Inv(1))
   |                                 ^^^^^
-  |                  found:    Inv[T]
-  |                  required: String
-  |                  
-  |                  where:    T is a type variable with constraint >: Int(1)
+  |                                 found:    Inv[Int]
+  |                                 required: String
+  |                                 
 scala> val b: Inv[String] = new Inv(1)
 -- [E007] Type Mismatch Error: <console>:5:29 ----------------------------------
 5 |val b: Inv[String] = new Inv(1)