Use ??? instead of EmptyTree for StopMacroExpansion.

[Cherry-picked 0fe0490]

Author: dwijnand

compiler/src/dotty/tools/dotc/inlines/Inlines.scala

@@ -451,7 +451,7 @@ object Inlines:
       // different for bindings from arguments and bindings from body.
       val inlined = tpd.Inlined(call, bindings, expansion)
 
-      if !hasOpaqueProxies || !inlined.tpe.exists then inlined
+      if !hasOpaqueProxies then inlined
       else
         val target =
           if inlinedMethod.is(Transparent) then call.tpe & inlined.tpe

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

@@ -72,7 +72,7 @@ object Splicer {
           if !ctx.reporter.hasErrors then
             report.error("Macro expansion was aborted by the macro without any errors reported. Macros should issue errors to end-users when aborting a macro expansion with StopMacroExpansion.", splicePos)
           // errors have been emitted
-          EmptyTree
+          ref(defn.Predef_undefined).withType(ErrorType(em"macro expansion was stopped"))
         case ex: StopInterpretation =>
           report.error(ex.msg, ex.pos)
           ref(defn.Predef_undefined).withType(ErrorType(ex.msg))

tests/neg-macros/i9014b.check

@@ -7,4 +7,4 @@
   |
   |                           given_Bar
   |
-  |                       But given instance given_Bar does not match type Bar.
+  |                       But macro expansion was stopped.