4 files changed, 29 insertions, 5 deletions
diff --git a/compiler/src/dotty/tools/dotc/typer/Inferencing.scala b/compiler/src/dotty/tools/dotc/typer/Inferencing.scala
index 1cb86dd72..86649d78e 100644
--- a/compiler/src/dotty/tools/dotc/typer/Inferencing.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Inferencing.scala
@@ -119,6 +119,18 @@ object Inferencing {
     }
   }
 
+  /** If `tree` has a PolyType, infer its type parameters by comparing with expected type `pt` */
+  def inferTypeParams(tree: Tree, pt: Type)(implicit ctx: Context): Tree = tree.tpe match {
+    case poly: PolyType =>
+      val (poly1, tvars) = constrained(poly, tree)
+      val tree1 = tree.withType(poly1).appliedToTypeTrees(tvars)
+      tree1.tpe <:< pt
+      fullyDefinedType(tree1.tpe, "template parent", tree.pos)
+      tree1
+    case _ =>
+      tree
+  }
+
   /** The list of uninstantiated type variables bound by some prefix of type `T` which
    *  occur in at least one formal parameter type of a prefix application.
    *  Considered prefixes are:
diff --git a/compiler/src/dotty/tools/dotc/typer/Namer.scala b/compiler/src/dotty/tools/dotc/typer/Namer.scala
index 4bcdd5071..1b6e437b5 100644
--- a/compiler/src/dotty/tools/dotc/typer/Namer.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Namer.scala
@@ -852,23 +852,23 @@ class Namer { typer: Typer =>
     }
   }
 
-  /** Typecheck tree during completion, and remember result in typedtree map */
-  private def typedAheadImpl(tree: Tree, pt: Type)(implicit ctx: Context): tpd.Tree = {
+  /** Typecheck `tree` during completion using `typed`, and remember result in TypedAhead map */
+  def typedAheadImpl(tree: Tree, typed: untpd.Tree => tpd.Tree)(implicit ctx: Context): tpd.Tree = {
     val xtree = expanded(tree)
     xtree.getAttachment(TypedAhead) match {
       case Some(ttree) => ttree
       case none =>
-        val ttree = typer.typed(tree, pt)
+        val ttree = typed(tree)
         xtree.putAttachment(TypedAhead, ttree)
         ttree
     }
   }
 
   def typedAheadType(tree: Tree, pt: Type = WildcardType)(implicit ctx: Context): tpd.Tree =
-    typedAheadImpl(tree, pt)(ctx retractMode Mode.PatternOrType addMode Mode.Type)
+    typedAheadImpl(tree, typer.typed(_, pt)(ctx retractMode Mode.PatternOrType addMode Mode.Type))
 
   def typedAheadExpr(tree: Tree, pt: Type = WildcardType)(implicit ctx: Context): tpd.Tree =
-    typedAheadImpl(tree, pt)(ctx retractMode Mode.PatternOrType)
+    typedAheadImpl(tree, typer.typed(_, pt)(ctx retractMode Mode.PatternOrType))
 
   def typedAheadAnnotation(tree: Tree)(implicit ctx: Context): Symbol = tree match {
     case Apply(fn, _) => typedAheadAnnotation(fn)
diff --git a/compiler/src/dotty/tools/dotc/typer/Typer.scala b/compiler/src/dotty/tools/dotc/typer/Typer.scala
index eec3859f9..73084bd86 100644
--- a/compiler/src/dotty/tools/dotc/typer/Typer.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Typer.scala
@@ -454,6 +454,11 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
     tree.tpt match {
       case templ: untpd.Template =>
         import untpd._
+        templ.parents foreach {
+          case parent: RefTree =>
+            typedAheadImpl(parent, tree => inferTypeParams(typedType(tree), pt))
+          case _ =>
+        }
         val x = tpnme.ANON_CLASS
         val clsDef = TypeDef(x, templ).withFlags(Final)
         typed(cpy.Block(tree)(clsDef :: Nil, New(Ident(x), Nil)), pt)
diff --git a/tests/pos/i1803.scala b/tests/pos/i1803.scala
new file mode 100644
index 000000000..19bf21918
--- /dev/null
+++ b/tests/pos/i1803.scala
@@ -0,0 +1,7 @@
+class C[T]
+
+object Test {
+  def f(x: C[Int]) = ???
+
+  f(new C {})
+}