Backport "Improve assertion error message for Apply and TypeApply…

…" to LTS (#20729)

Backports #18700 to the LTS branch.

PR submitted by the release tooling.
[skip ci]

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

@@ -45,19 +45,21 @@ 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(ctx.reporter.errorsReported)
+      assert(
+        fn.isInstanceOf[RefTree | GenericApply | Inlined | Hole] || ctx.reporter.errorsReported,
+        s"Illegal Apply function prefix: $fn"
+      )
       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(ctx.reporter.errorsReported)
+      assert(
+        fn.isInstanceOf[RefTree | GenericApply] || ctx.reporter.errorsReported,
+        s"Illegal TypeApply function prefix: $fn"
+      )
       ta.assignType(untpd.TypeApply(fn, args), fn, args)
 
   def Literal(const: Constant)(using Context): Literal =