Fix #5328: Disallow repeated type application

Author: odersky

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

@@ -881,6 +881,8 @@ trait Applications extends Compatibility { self: Typer with Dynamic =>
     typedExpr(tree.fun, PolyProto(typedArgs, pt)) match {
       case ExtMethodApply(app) =>
         app
+      case typedFn: TypeApply =>
+        errorTree(tree, "illegal repeated type application")
       case typedFn =>
         typedFn.tpe.widen match {
           case pt: PolyType =>

tests/neg/i5328.scala

@@ -0,0 +1,8 @@
+class C[X, Y] { def apply[Z](x: X, y: Y, z: Z) = (x, y, z) }
+
+object Test {
+  def f[X, Y]: C[X, Y] = new C[X, Y]
+  f[Int, Boolean][String](1, true, "") // OK
+  f[X = Int](1, true, "") // OK, Y and Z are inferred
+  f[X = Int][String](1, true, "") // error: illegal repeated type application
+}