Fix $1753 Better comparison of path types

In this case, a path went through a type parameter which was
aliased to a singleton type. Need to dealias to get to the
special case handling two paths.

1 files changed, 4 insertions, 2 deletions

diff --git a/compiler/src/dotty/tools/dotc/core/TypeComparer.scala b/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
index f78820fff..fd954c688 100644
--- a/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
+++ b/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
@@ -541,9 +541,11 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
       /** if `tp2 == p.type` and `p: q.type` then try `tp1 <:< q.type` as a last effort.*/
       def comparePaths = tp2 match {
         case tp2: TermRef =>
-          tp2.info.widenExpr match {
+          tp2.info.widenExpr.dealias match {
             case tp2i: SingletonType =>
-              isSubType(tp1, tp2i) // see z1720.scala for a case where this can arise even in typer.
+              isSubType(tp1, tp2i)
+                // see z1720.scala for a case where this can arise even in typer.
+                // Also, i1753.scala, to show why the dealias above is necessary.
             case _ => false
           }
         case _ =>