Check validity of infix operations

Author: odersky

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

@@ -18,17 +18,20 @@ import CheckRealizable._
 import ErrorReporting.errorTree
 
 import util.SourcePosition
+import util.Spans.Span
+import rewrites.Rewrites.patch
 import transform.SymUtils._
+import transform.ValueClasses._
 import Decorators._
 import ErrorReporting.{err, errorType}
 import config.Printers.typr
 import NameKinds.DefaultGetterName
+import SymDenotations.{NoCompleter, NoDenotation}
 
 import collection.mutable
-import SymDenotations.{NoCompleter, NoDenotation}
-import dotty.tools.dotc.reporting.diagnostic.Message
-import dotty.tools.dotc.reporting.diagnostic.messages._
-import dotty.tools.dotc.transform.ValueClasses._
+import reporting.diagnostic.Message
+import reporting.diagnostic.messages._
+import scala.tasty.util.Chars.isOperatorPart
 
 object Checking {
   import tpd._
@@ -657,6 +660,24 @@ trait Checking {
           i"Use of implicit conversion ${conv.showLocated}", NoSymbol, posd.sourcePos)
     }
 
+  /** Check that `tree` is a valid infix operation. That is, if the
+   *  operator is alphanumeric, it must be declared `@infix`.
+   */
+  def checkValidInfix(tree: untpd.InfixOp, app: Tree)(implicit ctx: Context): Unit =
+    tree.op match {
+      case Ident(name: SimpleName)
+      if !name.exists(isOperatorPart) && !app.symbol.hasAnnotation(defn.InfixAnnot) && false =>
+        ctx.deprecationWarning(
+          i"""alphanumeric method $name is not declared @infix; should not be used as infix operator.
+             |The operation can be rewritten automatically under -migration -rewrite""",
+          tree.op.sourcePos)
+        if (ctx.scala2Mode) {
+          patch(Span(tree.op.span.start, tree.op.span.start), "`")
+          patch(Span(tree.op.span.end, tree.op.span.end), "`")
+        }
+      case _ =>
+    }
+
   /** Issue a feature warning if feature is not enabled */
   def checkFeature(base: ClassSymbol,
                    name: TermName,
@@ -1034,5 +1055,6 @@ trait NoChecking extends ReChecking {
   override def checkNoForwardDependencies(vparams: List[ValDef])(implicit ctx: Context): Unit = ()
   override def checkMembersOK(tp: Type, pos: SourcePosition)(implicit ctx: Context): Type = tp
   override def checkInInlineContext(what: String, posd: Positioned)(implicit ctx: Context): Unit = ()
+  override def checkValidInfix(tree: untpd.InfixOp, app: Tree)(implicit ctx: Context): Unit = ()
   override def checkFeature(base: ClassSymbol, name: TermName, description: => String, featureUseSite: Symbol, pos: SourcePosition)(implicit ctx: Context): Unit = ()
 }

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

@@ -1883,6 +1883,7 @@ class Typer extends Namer
   def typedInfixOp(tree: untpd.InfixOp, pt: Type)(implicit ctx: Context): Tree = {
     val untpd.InfixOp(l, op, r) = tree
     val app = typedApply(desugar.binop(l, op, r), pt)
+    checkValidInfix(tree, app)
     if (untpd.isLeftAssoc(op.name)) app
     else {
       val defs = new mutable.ListBuffer[Tree]