Fix higher-order unification incorrectly substituting tparams

When creating a fresh type lambda for the purpose of higher-order type
inference, we incorrectly substituted references to type parameters
before this commit. We want to construct:

    bodyArgs := otherArgs.take(d), T_0, ..., T_k-1
    [T_0, ..., T_k-1] =>> otherTycon[bodyArgs]

For this type to be valid, we need the bounds of `T_i` to be the bounds of
the (d+i) type parameter of `otherTycon` after substituting references to
each type parameter of `otherTycon` by the corresponding argument in `bodyArgs`.

The previous implementation incorrectly substituted only the last `k` type
parameters, this was not enough for correctness. It could also lead to a crash
because it called `integrate` which implicitly assumes it is passed a full list
of type parameters (this is now documented).

Fixes #15983.

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

@@ -204,6 +204,12 @@ class TypeApplications(val self: Type) extends AnyVal {
     }
   }
 
+  /** Substitute in `self` the type parameters of `tycon` by some other types. */
+  final def substTypeParams(tycon: Type, to: List[Type])(using Context): Type =
+    (tycon.typeParams: @unchecked) match
+      case LambdaParam(lam, _) :: _ => self.substParams(lam, to)
+      case params: List[Symbol @unchecked] => self.subst(params, to)
+
   /** If `self` is a higher-kinded type, its type parameters, otherwise Nil */
   final def hkTypeParams(using Context): List[TypeParamInfo] =
     if (isLambdaSub) typeParams else Nil

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

@@ -1066,12 +1066,16 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
      *
      *  - k := args.length
      *  - d := otherArgs.length - k
+     *  - T_0, ..., T_k-1 fresh type parameters
+     *  - bodyArgs := otherArgs.take(d), T_0, ..., T_k-1
      *
-     *  `adaptedTycon` will be:
+     *  Then,
      *
-     *    [T_0, ..., T_k-1] =>> otherTycon[otherArgs(0), ..., otherArgs(d-1), T_0, ..., T_k-1]
+     *    adaptedTycon := [T_0, ..., T_k-1] =>> otherTycon[bodyArgs]
      *
-     *  where `T_n` has the same bounds as `otherTycon.typeParams(d+n)`
+     *  where the bounds of `T_i` are set based on the bounds of `otherTycon.typeParams(d+i)`
+     *  after substituting type parameter references by the corresponding argument
+     *  in `bodyArgs` (see `adaptedBounds` in the implementation).
      *
      *  Historical note: this strategy is known in Scala as "partial unification"
      *  (even though the type constructor variable isn't actually unified but only
@@ -1096,11 +1100,18 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
         variancesConform(remainingTparams, tparams) && {
           val adaptedTycon =
             if d > 0 then
+              val initialArgs = otherArgs.take(d)
+              /** The arguments passed to `otherTycon` in the body of `tl` */
+              def bodyArgs(tl: HKTypeLambda) = initialArgs ++ tl.paramRefs
+              /** The bounds of the type parameters of `tl` */
+              def adaptedBounds(tl: HKTypeLambda) =
+                val bodyArgsComputed = bodyArgs(tl)
+                remainingTparams.map(_.paramInfo)
+                  .mapconserve(_.substTypeParams(otherTycon, bodyArgsComputed).bounds)
+
               HKTypeLambda(remainingTparams.map(_.paramName))(
-                tl => remainingTparams.map(remainingTparam =>
-                  tl.integrate(remainingTparams, remainingTparam.paramInfo).bounds),
-                tl => otherTycon.appliedTo(
-                  otherArgs.take(d) ++ tl.paramRefs))
+                adaptedBounds,
+                tl => otherTycon.appliedTo(bodyArgs(tl)))
             else
               otherTycon
           (assumedTrue(tycon) || directionalIsSubType(tycon, adaptedTycon)) &&

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

@@ -3600,10 +3600,14 @@ object Types {
 
     /** The type `[tparams := paramRefs] tp`, where `tparams` can be
      *  either a list of type parameter symbols or a list of lambda parameters
+     *
+     *  @pre If `tparams` is a list of lambda parameters, then it must be the
+     *       full, in-order list of type parameters of some type constructor, as
+     *       can be obtained using `TypeApplications#typeParams`.
      */
     def integrate(tparams: List[ParamInfo], tp: Type)(using Context): Type =
       (tparams: @unchecked) match {
-        case LambdaParam(lam, _) :: _ => tp.subst(lam, this)
+        case LambdaParam(lam, _) :: _ => tp.subst(lam, this) // This is where the precondition is necessary.
         case params: List[Symbol @unchecked] => tp.subst(params, paramRefs)
       }
 

tests/pos/i15983a.scala

@@ -0,0 +1,11 @@
+class OtherC[A, B, C <: B]
+
+trait crash {
+  type OtherT[A, B, C <: B]
+
+  def indexK[F[_]]: F[Any] = ???
+
+  def res: OtherT[Any, Any, Any] = indexK
+
+  def res2: OtherC[Any, Any, Any] = indexK
+}

tests/pos/i15983b.scala

@@ -0,0 +1,11 @@
+class OtherC[A, B, C <: B, D <: C]
+
+trait crash {
+  type OtherT[A, B, C <: B, D <: C]
+
+  def indexK[F[X, Y <: X]]: F[Any, Any] = ???
+
+  def res: OtherT[Any, Any, Any, Any] = indexK
+
+  def res2: OtherC[Any, Any, Any, Any] = indexK
+}