Avoid crash arising from trying to find conversions from polymorphic …

…singleton types

This is an alternative fix for #18695, which already got fixed in a different way by #18719.
This PR adds the actual tests, and leaves in the fix as a defensive measure in case the situation
arises by some other means than the one foxed in #18719.

compiler/src/dotty/tools/dotc/ast/tpd.scala

@@ -45,21 +45,19 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
   def Apply(fn: Tree, args: List[Tree])(using Context): Apply = fn match
     case Block(Nil, expr) =>
       Apply(expr, args)
+    case _: RefTree | _: GenericApply | _: Inlined | _: Hole =>
+      ta.assignType(untpd.Apply(fn, args), fn, args)
     case _ =>
-      assert(
-        fn.isInstanceOf[RefTree | GenericApply | Inlined | Hole] || ctx.reporter.errorsReported,
-        s"Illegal Apply function prefix: $fn"
-      )
+      assert(ctx.reporter.errorsReported)
       ta.assignType(untpd.Apply(fn, args), fn, args)
 
   def TypeApply(fn: Tree, args: List[Tree])(using Context): TypeApply = fn match
     case Block(Nil, expr) =>
       TypeApply(expr, args)
+    case _: RefTree | _: GenericApply =>
+      ta.assignType(untpd.TypeApply(fn, args), fn, args)
     case _ =>
-      assert(
-        fn.isInstanceOf[RefTree | GenericApply] || ctx.reporter.errorsReported,
-        s"Illegal TypeApply function prefix: $fn"
-      )
+      assert(ctx.reporter.errorsReported, s"unexpected tree for type application: $fn")
       ta.assignType(untpd.TypeApply(fn, args), fn, args)
 
   def Literal(const: Constant)(using Context): Literal =

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

@@ -4366,7 +4366,13 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
             case _ =>
               adaptOverloaded(ref)
           }
-        case poly: PolyType if !(ctx.mode is Mode.Type) =>
+        case poly: PolyType
+        if !(ctx.mode is Mode.Type) && dummyTreeOfType.unapply(tree).isEmpty =>
+            // If we are in a conversion from a TermRef with polymorphic underlying
+        	  // type, give up. In this case the typed `null` literal cannot be instantiated.
+            // Test case was but i18695.scala, but it got fixed by a different tweak in #18719.
+            // We leave test for this condition in as a defensive measure in case
+            // it arises somewhere else.
           if isApplyProxy(tree) then newExpr
           else if pt.isInstanceOf[PolyProto] then tree
           else