Fix #19745 type alias anonymous class instantiation

compiler/src/dotty/tools/dotc/typer/Namer.scala

@@ -1629,9 +1629,22 @@ class Namer { typer: Typer =>
         def typedParentType(tree: untpd.Tree): tpd.Tree =
           val parentTpt = typer.typedType(parent, AnyTypeConstructorProto)
           val ptpe = parentTpt.tpe.dealias.etaCollapse
-          if ptpe.typeParams.nonEmpty
-              && ptpe.underlyingClassRef(refinementOK = false).exists
-          then
+          // Check if type params need to be inferred. This applies when:
+          // 1. The type has type params AND
+          // 2. Either it directly underlies a class (for class/trait refs), OR
+          //    it's a HKTypeLambda whose result is a class application and params appear in result
+          //    (the latter handles type aliases like `type Foo[T] = Bar[T]` but not
+          //    type-lambda-as-result cases like `type Foo[X] = [Z] =>> Bar[X]`)
+          val needsTypeParamInference = ptpe.typeParams.nonEmpty && (
+            ptpe.underlyingClassRef(refinementOK = false).exists
+            || (ptpe match
+                case tl: HKTypeLambda =>
+                  tl.paramRefs.exists(pref => tl.resType.existsPart(_ == pref))
+                  && !tl.resType.isInstanceOf[TypeLambda]
+                  && tl.resType.underlyingClassRef(refinementOK = false).exists
+                case _ => false)
+          )
+          if needsTypeParamInference then
             // Try to infer type parameters from a synthetic application.
             // This might yield new info if implicit parameters are resolved.
             // A test case is i16778.scala.

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -1232,6 +1232,36 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
     else tree1
   }
 
+  /** Type a parent type reference in `new T { ... }` expressions.
+   *  This handles type aliases that need type parameter inference
+   *  by creating a synthetic constructor application, similar to
+   *  how class parents are handled in Namer.
+   */
+  def typedNewTemplateParent(tree: untpd.Tree, pt: Type)(using Context): Tree =
+    // Type with AnyTypeConstructorProto to detect type aliases (HKTypeLambda)
+    val parentTpt = typedType(tree, AnyTypeConstructorProto)
+    val ptpe = parentTpt.tpe.dealias
+
+    ptpe match
+      case tl: HKTypeLambda
+          // Only handle type aliases where params appear in result (not type-lambda-as-result cases)
+          if tl.paramRefs.exists(pref => tl.resType.existsPart(_ == pref))
+             && !tl.resType.isInstanceOf[TypeLambda]
+             && tl.resType.underlyingClassRef(refinementOK = false).exists =>
+        // Found a type alias with type params whose result underlies a class.
+        // Create a synthetic constructor application to infer type arguments.
+        val app = untpd.Apply(untpd.Select(untpd.New(parentTpt), nme.CONSTRUCTOR), Nil)
+        val typedApp = typedExpr(app, pt)
+        TypeTree(typedApp.tpe).withSpan(tree.span)
+      case _ =>
+        // For regular class/trait references, reuse the typed tree.
+        // If type has params but isn't a TypeLambda, eta-expand for inferTypeParams.
+        val resultTpe = parentTpt.tpe
+        if resultTpe.typeParams.nonEmpty && !resultTpe.isInstanceOf[TypeLambda] then
+          inferTypeParams(parentTpt.withType(resultTpe.etaExpand), pt)
+        else
+          inferTypeParams(parentTpt, pt)
+
   def typedNew(tree: untpd.New, pt: Type)(using Context): Tree =
     tree.tpt match {
       case templ: untpd.Template =>
@@ -1248,7 +1278,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
         then
           templ1 = cpy.Template(templ)(parents = untpd.TypeTree(pt) :: Nil)
         for case parent: RefTree <- templ1.parents do
-          typedAhead(parent, tree => inferTypeParams(typedType(tree), pt))
+          typedAhead(parent, typedNewTemplateParent(_, pt))
         val anon = tpnme.ANON_CLASS
         val clsDef = TypeDef(anon, templ1).withFlags(Final | Synthetic)
         typed(

tests/pos/i19745.scala

@@ -0,0 +1,15 @@
+final abstract class ForcedRecompilationToken[T]
+object ForcedRecompilationToken {
+  implicit def default: ForcedRecompilationToken["abc"] = null
+}
+
+object x {
+  abstract class GoodNoParens[T](implicit ev: ForcedRecompilationToken[T])
+}
+type BadNoParens[T] = x.GoodNoParens[T]
+
+object App extends App {
+  new BadNoParens {}
+  new BadNoParens() {}
+  new x.GoodNoParens {}
+}

tests/pos/i19745b.scala

@@ -0,0 +1,10 @@
+object x {
+  trait GoodNoParens[T]
+}
+export x.GoodNoParens as BadNoParens
+
+object App extends App {
+  new BadNoParens {}
+  new BadNoParens() {}
+  new x.GoodNoParens {}
+}