Support implicitNotFound annotation

3 files changed, 24 insertions, 1 deletions

diff --git a/src/dotty/tools/dotc/core/Definitions.scala b/src/dotty/tools/dotc/core/Definitions.scala
index 5376ed591..9128bd3a5 100644
--- a/src/dotty/tools/dotc/core/Definitions.scala
+++ b/src/dotty/tools/dotc/core/Definitions.scala
@@ -449,6 +449,8 @@ class Definitions {
   def ContravariantBetweenAnnot(implicit ctx: Context) = ContravariantBetweenAnnotType.symbol.asClass
   lazy val DeprecatedAnnotType = ctx.requiredClassRef("scala.deprecated")
   def DeprecatedAnnot(implicit ctx: Context) = DeprecatedAnnotType.symbol.asClass
+  lazy val ImplicitNotFoundAnnotType = ctx.requiredClassRef("scala.annotation.implicitNotFound")
+  def ImplicitNotFoundAnnot(implicit ctx: Context) = ImplicitNotFoundAnnotType.symbol.asClass
   lazy val InvariantBetweenAnnotType = ctx.requiredClassRef("dotty.annotation.internal.InvariantBetween")
   def InvariantBetweenAnnot(implicit ctx: Context) = InvariantBetweenAnnotType.symbol.asClass
   lazy val MigrationAnnotType = ctx.requiredClassRef("scala.annotation.migration")
diff --git a/src/dotty/tools/dotc/typer/ErrorReporting.scala b/src/dotty/tools/dotc/typer/ErrorReporting.scala
index d6a87acf6..69c12e5f5 100644
--- a/src/dotty/tools/dotc/typer/ErrorReporting.scala
+++ b/src/dotty/tools/dotc/typer/ErrorReporting.scala
@@ -116,6 +116,18 @@ object ErrorReporting {
            | found   : $found
            | required: $expected""".stripMargin + whyNoMatchStr(found, expected)
     }
+
+    /** Format `raw` implicitNotFound argument, replacing all
+     *  occurrences of `${X}` where `X` is in `paramNames` with the
+     *  corresponding shown type in `args`.
+     */
+    def implicitNotFoundString(raw: String, paramNames: List[String], args: List[Type]): String = {
+      def translate(name: String): Option[String] = {
+        val idx = paramNames.indexOf(name)
+        if (idx >= 0) Some(args(idx).show) else None
+      }
+      """\$\{\w*\}""".r.replaceSomeIn(raw, m => translate(m.matched.drop(2).init))
+    }
   }
 
   def err(implicit ctx: Context): Errors = new Errors
diff --git a/src/dotty/tools/dotc/typer/Typer.scala b/src/dotty/tools/dotc/typer/Typer.scala
index b95acc7ca..f263d3093 100644
--- a/src/dotty/tools/dotc/typer/Typer.scala
+++ b/src/dotty/tools/dotc/typer/Typer.scala
@@ -1541,7 +1541,16 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
               case failure: SearchFailure =>
                 val arg = synthesizedClassTag(formal)
                 if (!arg.isEmpty) arg
-                else implicitArgError(d"no implicit argument of type $formal found for $where" + failure.postscript)
+                else {
+                  var msg = d"no implicit argument of type $formal found for $where" + failure.postscript
+                  for (notFound <- formal.typeSymbol.getAnnotation(defn.ImplicitNotFoundAnnot);
+                       Literal(Constant(raw: String)) <- notFound.argument(0))
+                    msg = err.implicitNotFoundString(
+                        raw,
+                        formal.typeSymbol.typeParams.map(_.name.unexpandedName.toString),
+                        formal.argInfos)
+                  implicitArgError(msg)
+                }
             }
           }
           if (errors.nonEmpty) {