Alternative fix for #1857: Consider tvars in bounds of uninstantiated tvars

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

@@ -79,8 +79,9 @@ object Inferencing {
       case _: WildcardType | _: ProtoType =>
         false
       case tvar: TypeVar
-      if !tvar.isInstantiated && ctx.typerState.constraint.contains(tvar) =>
-        force.appliesTo(tvar) && {
+        if !tvar.isInstantiated && ctx.typerState.constraint.contains(tvar) =>
+        val forceApplies = force.appliesTo(tvar)
+        if (forceApplies) {
           val direction = instDirection(tvar.origin)
           if (direction != 0) {
             //if (direction > 0) println(s"inst $tvar dir = up")
@@ -95,8 +96,13 @@ object Inferencing {
             if (minimize) instantiate(tvar, fromBelow = true)
             else toMaximize = true
           }
-          foldOver(x, tvar)
         }
+        foldOver(x && forceApplies,
+          if (tvar.isInstantiated) tvar
+          else ctx.typerState.constraint.entry(tvar.origin)
+        )
+      case tp: PolyParam =>
+        apply(x, ctx.typerState.constraint.typeVarOfParam(tp))
       case tp =>
         foldOver(x, tp)
     }

tests/pos/i1857a.scala

@@ -0,0 +1,39 @@
+class Wrapper[Host]
+
+
+class SubWrapper extends Wrapper[Foo]
+
+class Foo
+object Foo {
+  implicit val fooWrapper: SubWrapper = new SubWrapper
+}
+
+class Inv[A, B]
+
+object RunMe {
+  def main(args: Array[String]): Unit = {
+    def wrap1a[A, W <: Wrapper[Foo]](host: A)(implicit w: W): W = ???
+    def wrap1b[A, W <: Wrapper[Foo]](host: A)(implicit w: W): A = ???
+    def wrap1c[A, W <: Wrapper[Foo]](host: A)(implicit w: W): Inv[W, A] = ???
+    def wrap1d[A, W <: Wrapper[Foo]](host: A)(implicit w: W): Nothing = ???
+
+    def wrap2a[A, W <: Wrapper[A]](host: A)(implicit w: W): W = ???
+    def wrap2b[A, W <: Wrapper[A]](host: A)(implicit w: W): A = ???
+    def wrap2c[A, W <: Wrapper[A]](host: A)(implicit w: W): Inv[W, A] = ???
+    def wrap2d[A, W <: Wrapper[A]](host: A)(implicit w: W): Nothing = ???
+
+    val f: Foo = new Foo
+
+    // work with master
+    wrap1a(f)
+    wrap1b(f)
+    wrap1c(f)
+    wrap1d(f)
+
+    // do not work with master
+    wrap2a(f)
+    wrap2b(f)
+    wrap2c(f)
+    wrap2d(f)
+  }
+}

tests/pos/tparam_inf.scala

@@ -23,6 +23,14 @@ object Test {
     foo2(10)
   }
 
+  def test1b: Unit = {
+    implicit val ii: Int = 42
+    implicit val ss: String = "foo"
+
+    foo1(10)
+    foo2(10)
+  }
+
   def hf[T]: HasFoo[T] = ???
   def test2: Unit = {
     implicit val ii: Int = 42