Optimize captured type in splice hole (#17070)

If we have a quote that defined a type `T` and use it in the contents of
the splice we want to pass a `Type[T]` to the hole.

This improves a case where we had a `List[T]` and the hole could end up
with a `Type[T]` and `Type[List[T]]`. We should only pass the `Type[T]`.

Author: nicolasstucki

compiler/src/dotty/tools/dotc/transform/Splicing.scala

@@ -246,7 +246,7 @@ class Splicing extends MacroTransform:
         if tree.symbol == defn.QuotedTypeModule_of && containsCapturedType(tpt.tpe) =>
           val newContent = capturedPartTypes(tpt)
           newContent match
-            case block: Block => 
+            case block: Block =>
               inContext(ctx.withSource(tree.source)) {
                 Apply(TypeApply(typeof, List(newContent)), List(quotes)).withSpan(tree.span)
               }
@@ -342,7 +342,7 @@ class Splicing extends MacroTransform:
       val bindingSym = refBindingMap.getOrElseUpdate(tree.symbol, (tree, newBinding))._2
       ref(bindingSym)
 
-    private def newQuotedTypeClassBinding(tpe: Type)(using Context) = 
+    private def newQuotedTypeClassBinding(tpe: Type)(using Context) =
       newSymbol(
         spliceOwner,
         UniqueName.fresh(nme.Type).toTermName,
@@ -361,7 +361,7 @@ class Splicing extends MacroTransform:
       healedTypes = PCPCheckAndHeal.QuoteTypeTags(tpt.span)
       val capturePartTypes = new TypeMap {
         def apply(tp: Type) = tp match {
-          case typeRef @ TypeRef(prefix, _) if isCaptured(prefix.typeSymbol) || isCaptured(prefix.termSymbol) =>
+          case typeRef: TypeRef if containsCapturedType(typeRef) =>
             val termRef = refBindingMap
               .getOrElseUpdate(typeRef.symbol, (TypeTree(typeRef), newQuotedTypeClassBinding(typeRef)))._2.termRef
             val tagRef = healedTypes.nn.getTagRef(termRef)
@@ -376,7 +376,7 @@ class Splicing extends MacroTransform:
       tpt match
         case block: Block =>
           cpy.Block(block)(newHealedTypes ::: block.stats, TypeTree(captured))
-        case _ => 
+        case _ =>
           if newHealedTypes.nonEmpty then
             cpy.Block(tpt)(newHealedTypes, TypeTree(captured))
           else

compiler/src/dotty/tools/dotc/transform/TreeChecker.scala

@@ -655,6 +655,11 @@ object TreeChecker {
     override def typedHole(tree: untpd.Hole, pt: Type)(using Context): Tree = {
       val tree1 @ Hole(isTermHole, _, args, content, tpt) = super.typedHole(tree, pt): @unchecked
 
+      // Check that we only add the captured type `T` instead of a more complex type like `List[T]`.
+      // If we have `F[T]` with captured `F` and `T`, we should list `F` and `T` separately in the args.
+      for arg <- args do
+        assert(arg.isTerm || arg.tpe.isInstanceOf[TypeRef], "Expected TypeRef in Hole type args but got: " + arg.tpe)
+
       // Check result type of the hole
       if isTermHole then assert(tpt.typeOpt <:< pt)
       else assert(tpt.typeOpt =:= pt)

tests/pos-macros/i12440.scala

@@ -5,6 +5,12 @@ trait Mirror:
 
 class Eq:
 
+  def test0(using Quotes): Unit = '{
+    type T
+    ${ summonType[T]; ??? }
+    ${ summonType[List[T]]; ??? }
+  }
+
   def test1(using Quotes): Unit = '{
     val m: Mirror = ???
     ${ summonType[m.ElemTypes]; ??? }