Merge pull request #12580 from dotty-staging/fix-12486

Fix checking of `new` to rule out singletons

Author: OlivierBlanvillain

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

@@ -303,6 +303,10 @@ class PostTyper extends MacroTransform with IdentityDenotTransformer { thisPhase
               // need to check instantiability here, because the type of the New itself
               // might be a type constructor.
               ctx.typer.checkClassType(tree.tpe, tree.srcPos, traitReq = false, stablePrefixReq = true)
+              if !nu.tpe.isLambdaSub then
+                // Check the constructor type as well; it could be an illegal singleton type
+                // which would not be reflected as `tree.tpe`
+                ctx.typer.checkClassType(nu.tpe, tree.srcPos, traitReq = false, stablePrefixReq = false)
               Checking.checkInstantiable(tree.tpe, nu.srcPos)
               withNoCheckNews(nu :: Nil)(app1)
             case _ =>

tests/neg/i12486.scala

@@ -0,0 +1,10 @@
+val hello: String = "hello"
+
+object MyObj {
+  val a: Int = 123
+  val b: Double = 456.789
+  val c: String = "ABC"
+}
+
+val stringFromSingleton: String = new hello.type()    // error: not a class type
+val myObjFromSingleton: MyObj.type = new MyObj.type() // error: not a class type