Don't allow single apply's to be used as constructor proxies

The previous commits allowed them but that caused runtime errors in Specs2 since
`apply` was bound to something different from before.

Since this is a weird corner case it's better not to allow it. As long as we do
not crash we are good.

The error message we get now is:

    constructor proxy `apply` cannot be used as a value

which is also a bit strange, but since this is a truly weird corner case it's not worth it
to finetune this.

Author: odersky

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

@@ -3997,6 +3997,10 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
         pt)
         .showing(i"convert creator $tree -> $result", typr)
 
+    def isApplyProxy(tree: Tree) = tree match
+      case Select(_, nme.apply) => tree.symbol.isAllOf(ApplyProxyFlags)
+      case _ => false
+
     tree match {
       case _: MemberDef | _: PackageDef | _: Import | _: WithoutTypeOrPos[?] | _: Closure => tree
       case _ => tree.tpe.widen match {
@@ -4012,7 +4016,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
               adaptOverloaded(ref)
           }
         case poly: PolyType if !(ctx.mode is Mode.Type) =>
-          if tree.symbol.isAllOf(ApplyProxyFlags) then newExpr
+          if isApplyProxy(tree) then newExpr
           else if pt.isInstanceOf[PolyProto] then tree
           else
             var typeArgs = tree match
@@ -4026,7 +4030,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
             readaptSimplified(handleStructural(tree))
           else pt match {
             case pt: FunProto =>
-              if tree.symbol.isAllOf(ApplyProxyFlags) then newExpr
+              if isApplyProxy(tree) then newExpr
               else adaptToArgs(wtp, pt)
             case pt: PolyProto if !wtp.isImplicitMethod =>
               tryInsertApplyOrImplicit(tree, pt, locked)(tree) // error will be reported in typedTypeApply

tests/neg/i15414.scala

@@ -0,0 +1,4 @@
+class C()
+object C {
+  val c = apply() // error
+}