Add test

Author: odersky

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

@@ -740,8 +740,9 @@ trait Checking {
          !infixOKSinceFollowedBy(tree.right) &&
          ctx.settings.strict.value =>
         ctx.deprecationWarning(
-          i"""alphanumeric method $name is not declared @infix; should not be used as infix operator.
-             |The operation can be rewritten automatically to `$name` under -deprecation -rewrite""",
+          i"""Alphanumeric method $name is not declared @infix; it should not be used as infix operator.
+             |The operation can be rewritten automatically to `$name` under -deprecation -rewrite.
+             |Or rewrite to method syntax .$name(...) manually.""",
           tree.op.sourcePos)
         if (ctx.settings.deprecation.value) {
           patch(Span(tree.op.span.start, tree.op.span.start), "`")

tests/neg-custom-args/infix.scala

@@ -0,0 +1,15 @@
+// Compile with -strict -Xfatal-warnings -deprecation
+import scala.annotation.infix
+class C {
+  @infix def op(x: Int): Int = ???
+  def meth(x: Int): Int = ???
+}
+
+val c = C()
+def test() = {
+  c op 2
+  c.meth(2)
+
+  c.op(2)
+  c meth 2    // error: should not be used as infix operator
+}