Don't instantiate hk type constructors too early

The issue scala#6385 contains more explanations.

Author: odersky

compiler/src/dotty/tools/dotc/core/ConstraintHandling.scala

@@ -520,16 +520,6 @@ trait ConstraintHandling[AbstractContext] {
     }
   }
 
-  /** Instantiate `param` to `tp` if the constraint stays satisfiable */
-  protected def tryInstantiate(param: TypeParamRef, tp: Type)(implicit actx: AbstractContext): Boolean = {
-    val saved = constraint
-    constraint =
-      if (addConstraint(param, tp, fromBelow = true) &&
-          addConstraint(param, tp, fromBelow = false)) constraint.replace(param, tp)
-      else saved
-    constraint ne saved
-  }
-
   /** Check that constraint is fully propagated. See comment in Config.checkConstraintsPropagated */
   def checkPropagated(msg: => String)(result: Boolean)(implicit actx: AbstractContext): Boolean = {
     if (Config.checkConstraintsPropagated && result && addConstraintInvocations == 0) {

compiler/src/dotty/tools/dotc/core/TypeComparer.scala

@@ -818,7 +818,7 @@ class TypeComparer(initctx: Context) extends ConstraintHandling[AbsentContext] {
           tycon1.dealiasKeepRefiningAnnots match {
             case tycon1: TypeParamRef =>
               (tycon1 == tycon2 ||
-              canConstrain(tycon1) && tryInstantiate(tycon1, tycon2)) &&
+               canConstrain(tycon1) && isSubType(tycon1, tycon2)) &&
               isSubArgs(args1, args2, tp1, tparams)
             case tycon1: TypeRef =>
               tycon2.dealiasKeepRefiningAnnots match {
@@ -892,7 +892,7 @@ class TypeComparer(initctx: Context) extends ConstraintHandling[AbsentContext] {
                     tl => tparams1.map(tparam => tl.integrate(tparams, tparam.paramInfo).bounds),
                     tl => tp1base.tycon.appliedTo(args1.take(lengthDiff) ++
                             tparams1.indices.toList.map(tl.paramRefs(_))))
-                (assumedTrue(tycon2) || tryInstantiate(tycon2, tycon1.ensureLambdaSub)) &&
+                (assumedTrue(tycon2) || isSubType(tycon1.ensureLambdaSub, tycon2)) &&
                 recur(tp1, tycon1.appliedTo(args2))
               }
             }
@@ -977,7 +977,7 @@ class TypeComparer(initctx: Context) extends ConstraintHandling[AbsentContext] {
         case param1: TypeParamRef =>
           def canInstantiate = tp2 match {
             case AppliedType(tycon2, args2) =>
-              tryInstantiate(param1, tycon2.ensureLambdaSub) && isSubArgs(args1, args2, tp1, tycon2.typeParams)
+              isSubType(param1, tycon2.ensureLambdaSub) && isSubArgs(args1, args2, tp1, tycon2.typeParams)
             case _ =>
               false
           }

compiler/test/dotc/pos-test-pickling.blacklist

@@ -1,3 +1,4 @@
+i94-nada.scala
 i1812.scala
 i1867.scala
 i3067.scala

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -164,6 +164,7 @@ class CompilationTests extends ParallelTesting {
       compileFile("tests/neg-custom-args/nopredef.scala", defaultOptions.and("-Yno-predef")),
       compileFile("tests/neg-custom-args/noimports.scala", defaultOptions.and("-Yno-imports")),
       compileFile("tests/neg-custom-args/noimports2.scala", defaultOptions.and("-Yno-imports")),
+      compileFile("tests/neg-custom-args/i1650.scala", allowDeepSubtypes),
       compileFile("tests/neg-custom-args/i3882.scala", allowDeepSubtypes),
       compileFile("tests/neg-custom-args/i4372.scala", allowDeepSubtypes),
       compileFile("tests/neg-custom-args/i1754.scala", allowDeepSubtypes),

tests/neg/i1650.scala renamed to tests/neg-custom-args/i1650.scala

@@ -0,0 +1,11 @@
+class Box[F[_]]
+
+class C[X]
+class D[X] extends C[String]
+
+object Test {
+  def f[F[_]](x: Box[F]) = ???
+  def db: Box[D] = ???
+  def cb: Box[C] = db  // error
+  f[[X] => C[X]](db)   // error
+}

tests/neg/i6385a.scala

@@ -0,0 +1,17 @@
+trait Tc1[A]
+trait Tc2[A] extends Tc1[A]
+
+class PinTypeTo[K[_]]
+object PinTypeTo {
+  implicit val pinType: PinTypeTo[Tc2] = new PinTypeTo[Tc2]
+}
+
+class X
+object X {
+  implicit def Tc2Instance[F[x] >: Tc2[x]: PinTypeTo]: F[X] = new Tc2[X] {}
+}
+
+object app extends App {
+  implicitly[Tc2[X]] // ok
+  implicitly[Tc1[X]]//(X.Tc2Instance[Tc2]) // fails
+}