HK reduction: Remove special-case for typerefs

src/dotty/tools/dotc/config/Config.scala

@@ -98,9 +98,8 @@ object Config {
   final val splitProjections = false
 
   /** If this flag is on, always rewrite an application `S[Ts]` where `S` is an alias for
-   *  `[Xs] -> U` to `[Xs := Ts]U`. If this flag is off, the rewriting is only done if `S` is a
-   *  reference to an instantiated parameter. Turning this flag on was observed to
-   *  give a ~6% speedup on the JUnit test suite.
+   *  `[Xs] -> U` to `[Xs := Ts]U`.
+   *  Turning this flag on was observed to give a ~6% speedup on the JUnit test suite.
    */
   final val simplifyApplications = true
 

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

@@ -470,18 +470,13 @@ class TypeApplications(val self: Type) extends AnyVal {
       case dealiased: TypeLambda =>
         def tryReduce =
           if (!args.exists(_.isInstanceOf[TypeBounds])) {
-            val followAlias = stripped match {
-              case stripped: TypeRef =>
-                stripped.symbol.is(BaseTypeArg)
-              case _ =>
-                Config.simplifyApplications && {
-                  dealiased.resType match {
-                    case AppliedType(tyconBody, _) =>
-                      variancesConform(typParams, tyconBody.typeParams)
-                        // Reducing is safe for type inference, as kind of type constructor does not change
-                    case _ => false
-                  }
-                }
+            val followAlias = Config.simplifyApplications && {
+              dealiased.resType match {
+                case AppliedType(tyconBody, _) =>
+                  variancesConform(typParams, tyconBody.typeParams)
+                    // Reducing is safe for type inference, as kind of type constructor does not change
+                case _ => false
+              }
             }
             if ((dealiased eq stripped) || followAlias) dealiased.instantiate(args)
             else HKApply(self, args)

tests/pos/i1181b.scala

@@ -0,0 +1,11 @@
+class Foo[A]
+
+object Test {
+  def foo[M[_,_]](x: M[Int,Int]) = x
+
+  type Alias[X,Y] = Foo[X]
+  val x: Alias[Int,Int] = new Foo[Int]
+
+  foo[Alias](x) // ok
+  foo(x)
+}

tests/pos/i1181c.scala

@@ -0,0 +1,11 @@
+class Foo[A]
+
+trait Bar[DD[_,_]] {
+  val x: DD[Int, Int]
+}
+
+trait Baz extends Bar[[X,Y] -> Foo[X]] {
+  def foo[M[_,_]](x: M[Int, Int]) = x
+
+  foo(x)
+}