Fix #9432: Fail compilation on @native methods in traits

compiler/src/dotty/tools/dotc/reporting/ErrorMessageID.scala

@@ -166,7 +166,8 @@ enum ErrorMessageID extends java.lang.Enum[ErrorMessageID] {
     TypeSpliceInValPatternID,
     ModifierNotAllowedForDefinitionID,
     CannotExtendJavaEnumID,
-    InvalidReferenceInImplicitNotFoundAnnotationID
+    InvalidReferenceInImplicitNotFoundAnnotationID,
+    TraitMayNotDefineNativeMethodID
 
   def errorNumber = ordinal - 2
 }

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -1480,6 +1480,13 @@ import ast.tpd
     def explain = ""
   }
 
+  class TraitMayNotDefineNativeMethod(sym: Symbol)(
+    implicit ctx: Context)
+    extends SyntaxMsg(TraitMayNotDefineNativeMethodID) {
+    def msg = em"""A trait cannot define a ${hl("@native")} method."""
+    def explain = ""
+  }
+
   class OnlyClassesCanHaveDeclaredButUndefinedMembers(sym: Symbol)(
     implicit ctx: Context)
     extends SyntaxMsg(OnlyClassesCanHaveDeclaredButUndefinedMembersID) {

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

@@ -467,6 +467,8 @@ object Checking {
     if (sym.hasAnnotation(defn.NativeAnnot)) {
       if (!sym.is(Deferred))
         fail(NativeMembersMayNotHaveImplementation(sym))
+      else if(sym.owner.is(Trait))
+        fail(TraitMayNotDefineNativeMethod(sym))
     }
     else if (sym.is(Deferred, butNot = Param) && !sym.isType && !sym.isSelfSym) {
       if (!sym.owner.isClass || sym.owner.is(Module) || sym.owner.isAnonymousClass)

tests/neg/i9432.check

@@ -0,0 +1,4 @@
+-- [E159] Syntax Error: tests/neg/i9432.scala:2:14 ---------------------------------------------------------------------
+2 |  @native def foo(x: Int): Unit // error
+  |              ^
+  |              A trait cannot define a @native method.

tests/neg/i9432.scala

@@ -0,0 +1,3 @@
+trait CLibrary {
+  @native def foo(x: Int): Unit // error
+}