Show undesired anon apply rewrite

Author: som-snytt

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

@@ -1250,7 +1250,8 @@ trait Applications extends Compatibility {
             }
           else
             tryEither(simpleApply(fun1, proto)): (failedVal, failedState) =>
-              // a bug allowed empty parens to expand to implicit args, offer rewrite only on migration, then retry
+              // a bug allowed empty parens to expand to implicit args, offer rewrite only on migration,
+              // then retry with using to emulate the bug since rewrites are ignored on error.
               if proto.args.isEmpty && maybePatchBadParensForImplicit(failedState) then
                 tryWithUsing(fun1, proto).getOrElse:
                   failedState.commit()

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

@@ -617,7 +617,8 @@ object ProtoTypes {
       else new FunProto(args, resType)(typer, applyKind, state)(using newCtx)
 
     def withApplyKind(applyKind: ApplyKind) =
-      new FunProto(args, resType)(typer, applyKind, state)
+      if applyKind == this.applyKind then this
+      else new FunProto(args, resType)(typer, applyKind, state)
   }
 
   /** A prototype for expressions that appear in function position

tests/rewrites/i22792.check

@@ -3,6 +3,9 @@
 trait Permit
 class Foo:
   def run(implicit ev: Permit): Unit = ???
+  def apply(implicit ev: Permit): Unit = ???
 
 given Permit = ???
 @main def Test = new Foo().run
+
+def otherSyntax = Foo()

tests/rewrites/i22792.scala

@@ -3,6 +3,9 @@
 trait Permit
 class Foo:
   def run(implicit ev: Permit): Unit = ???
+  def apply(implicit ev: Permit): Unit = ???
 
 given Permit = ???
 @main def Test = new Foo().run()
+
+def otherSyntax = Foo()()