Allows for NoType in AplliedType argument lists, as well as argument lists that are shorter than type parameter lists.

3 files changed, 26 insertions, 8 deletions

diff --git a/src/dotty/tools/dotc/core/Substituters.scala b/src/dotty/tools/dotc/core/Substituters.scala
index 8255a6750..471718f63 100644
--- a/src/dotty/tools/dotc/core/Substituters.scala
+++ b/src/dotty/tools/dotc/core/Substituters.scala
@@ -54,7 +54,8 @@ trait Substituters { this: Context =>
       case tp: NamedType =>
         val sym = tp.symbol
         if (tp.prefix eq NoPrefix) {
-          if (sym eq from) return to
+          if (sym eq from)
+            return if (to.exists) to else tp
         }
         if (sym.isStatic) tp
         else tp.derivedNamedType(subst1(tp.prefix, from, to, map), tp.name)
@@ -79,8 +80,10 @@ trait Substituters { this: Context =>
       case tp: NamedType =>
         val sym = tp.symbol
         if (tp.prefix eq NoPrefix) {
-          if (sym eq from1) return to1
-          if (sym eq from2) return to2
+          if (sym eq from1)
+            return if (to1.exists) to1 else tp
+          if (sym eq from2)
+            return if (to2.exists) to2 else tp
         }
         if (sym.isStatic) tp
         else tp.derivedNamedType(subst2(tp.prefix, from1, to1, from2, to2, map), tp.name)
@@ -108,7 +111,8 @@ trait Substituters { this: Context =>
           var fs = from
           var ts = to
           while (fs.nonEmpty) {
-            if (fs.head eq sym) return ts.head
+            if (fs.head eq sym)
+              return if (ts.head.exists) ts.head else tp
             fs = fs.tail
             ts = ts.tail
           }
diff --git a/src/dotty/tools/dotc/core/TypeOps.scala b/src/dotty/tools/dotc/core/TypeOps.scala
index a0cc00b4b..8b4952489 100644
--- a/src/dotty/tools/dotc/core/TypeOps.scala
+++ b/src/dotty/tools/dotc/core/TypeOps.scala
@@ -48,9 +48,11 @@ trait TypeOps { this: Context =>
         }
 
         def instParam(ps: List[Symbol], as: List[Type]): Type =
-          if (ps.isEmpty || as.isEmpty) throwError
-          else if (tparam eq ps.head) as.head
-          else throwError
+          if (as.isEmpty) tp
+          else if (ps.isEmpty) throwError
+          else if (tparam eq ps.head)
+            if (as.head.exists) as.head else tp
+          else instParam(ps.tail, as.tail)
 
         if (tparamOwner == clazz && prefixMatches) instParamFrom(basePre)
         else toInstance(basePre.normalizedPrefix, clazz.owner, tparam)
diff --git a/src/dotty/tools/dotc/core/Types.scala b/src/dotty/tools/dotc/core/Types.scala
index 460265e3d..dc8c7c9f1 100644
--- a/src/dotty/tools/dotc/core/Types.scala
+++ b/src/dotty/tools/dotc/core/Types.scala
@@ -409,7 +409,15 @@ object Types {
      */
     final def typeParams(implicit ctx: Context): List[TypeSymbol] = this match {
       case tp: AppliedType =>
-        tp.tycon.typeParams drop tp.targs.length
+        def loop(tparams: List[TypeSymbol], targs: List[Type]): List[TypeSymbol] = tparams match {
+          case tparam :: tparams1 =>
+            if (targs.isEmpty) tparams
+            else if (targs.head eq NoType) loop(tparams1, targs.tail)
+            else tparam :: loop(tparams1, targs.tail)
+          case _ =>
+            Nil
+        }
+        loop(tp.tycon.typeParams, tp.targs)
       case tp: TypeProxy =>
         tp.underlying.typeParams
       case tp: ClassInfo =>
@@ -701,6 +709,10 @@ object Types {
 
   // --- AppliedType -----------------------------------------------------------------
 
+  /** An applied type of the form tycon[..targs].
+   *  ..targs must match the length to tycon.typeParams.
+   *  Missing type arguments are represented by NoType.
+   */
   abstract case class AppliedType(tycon: Type, targs: List[Type]) extends CachedProxyType {
 
     override def underlying(implicit ctx: Context) = tycon