Deprecation warnings for operators without alpha

Operators without an alpha annotation give a deprecation warning -strict mode.

Author: odersky

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

@@ -111,6 +111,7 @@ class PostTyper extends MacroTransform with IdentityDenotTransformer { thisPhase
 
     private def processMemberDef(tree: Tree)(implicit ctx: Context): tree.type = {
       val sym = tree.symbol
+      Checking.checkValidOperator(sym)
       sym.transformAnnotations(transformAnnot)
       sym.defTree = tree
       tree

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

@@ -274,6 +274,20 @@ object Checking {
     }
   }
 
+  /** If `sym` has an operator name, check that it has an @alpha annotation under -strict */
+  def checkValidOperator(sym: Symbol)(implicit ctx: Context): Unit =
+    sym.name.toTermName match {
+      case name: SimpleName
+      if name.exists(isOperatorPart) &&
+        !sym.getAnnotation(defn.AlphaAnnot).isDefined &&
+        !sym.is(Synthetic) &&
+        !name.isConstructorName &&
+        ctx.settings.strict.value =>
+        ctx.deprecationWarning(
+          i"$sym has an operator name; it should come with an @alpha annotation", sym.sourcePos)
+      case _ =>
+    }
+
   /** Check that `info` of symbol `sym` is not cyclic.
    *  @pre     sym is not yet initialized (i.e. its type is a Completer).
    *  @return  `info` where every legal F-bounded reference is proctected

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -180,7 +180,8 @@ class CompilationTests extends ParallelTesting {
         "tests/neg-custom-args/toplevel-samesource/nested/S.scala"),
         defaultOptions),
       compileFile("tests/neg-custom-args/i6300.scala", allowDeepSubtypes),
-      compileFile("tests/neg-custom-args/infix.scala", defaultOptions.and("-strict", "-deprecation", "-Xfatal-warnings"))
+      compileFile("tests/neg-custom-args/infix.scala", defaultOptions.and("-strict", "-deprecation", "-Xfatal-warnings")),
+      compileFile("tests/neg-custom-args/alpha.scala", defaultOptions.and("-strict", "-deprecation", "-Xfatal-warnings"))
     ).checkExpectedErrors()
   }
 

tests/neg-custom-args/alpha.scala

@@ -0,0 +1,9 @@
+// Compile with -strict -Xfatal-warnings -deprecation
+import scala.annotation.alpha
+class & {   // error
+
+  @alpha("op") def *(x: Int): Int = ???  // OK
+  def / (x: Int): Int       // error
+  val frozen_& : Int = ???  // error
+  object some_???           // error
+}