Allow @implicitNotFound messages as explanations

A problem of the @implicitNotFOund mechanism so far was that the user defined
message replaced the compiler-generated one, which might lose valuable information.

This commit adds an alternative where an @implicitNotFound message that starts
with `...` is taken as an explanation (without the ...) enabled under -explain.
The compiler-generated message is then kept as the explicit error message.

We apply the mechanism for an @implicitNotFound message for `boundary.Label`.
This now produces messages like this one:
```
-- [E172] Type Error: tests/neg-custom-args/explain/labelNotFound.scala:2:30 -------------------------------------------
2 |  scala.util.boundary.break(1) // error
  |                              ^
  |No given instance of type scala.util.boundary.Label[Int] was found for parameter label of method break in object boundary
  |---------------------------------------------------------------------------------------------------------------------
  | Explanation (enabled by `-explain`)
  |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
  | A Label is generated from an enclosing `scala.util.boundary` call.
  | Maybe that boundary is missing?
   ---------------------------------------------------------------------------------------------------------------------
```

Author: odersky

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

@@ -2562,6 +2562,106 @@ class MissingImplicitArgument(
     case ambi: AmbiguousImplicits => withoutDisambiguation()
     case _ =>
 
+  /** Format `raw` implicitNotFound or implicitAmbiguous argument, replacing
+   *  all occurrences of `${X}` where `X` is in `paramNames` with the
+   *  corresponding shown type in `args`.
+   */
+  def userDefinedErrorString(raw: String, paramNames: List[String], args: List[Type])(using Context): String =
+    def translate(name: String): Option[String] =
+      val idx = paramNames.indexOf(name)
+      if (idx >= 0) Some(i"${args(idx)}") else None
+    """\$\{\s*([^}\s]+)\s*\}""".r.replaceAllIn(raw, (_: Regex.Match) match
+      case Regex.Groups(v) => quoteReplacement(translate(v).getOrElse("")).nn
+    )
+
+  /** @param rawMsg           Message template with variables, e.g. "Variable A is ${A}"
+   *  @param sym              Symbol of the annotated type or of the method whose parameter was annotated
+   *  @param substituteType   Function substituting specific types for abstract types associated with variables, e.g A -> Int
+   */
+  def formatAnnotationMessage(rawMsg: String, sym: Symbol, substituteType: Type => Type)(using Context): String =
+    val substitutableTypesSymbols = substitutableTypeSymbolsInScope(sym)
+    userDefinedErrorString(
+      rawMsg,
+      paramNames = substitutableTypesSymbols.map(_.name.unexpandedName.toString),
+      args = substitutableTypesSymbols.map(_.typeRef).map(substituteType)
+    )
+
+  /** Extract a user defined error message from a symbol `sym`
+   *  with an annotation matching the given class symbol `cls`.
+   */
+  def userDefinedMsg(sym: Symbol, cls: Symbol)(using Context) =
+    for
+      ann <- sym.getAnnotation(cls)
+      msg <- ann.argumentConstantString(0)
+    yield msg
+
+  def userDefinedImplicitNotFoundTypeMessageFor(sym: Symbol)(using Context): Option[String] =
+    for
+      rawMsg <- userDefinedMsg(sym, defn.ImplicitNotFoundAnnot)
+      if Feature.migrateTo3 || sym != defn.Function1
+        // Don't inherit "No implicit view available..." message if subtypes of Function1 are not treated as implicit conversions anymore
+    yield
+      val substituteType = (_: Type).asSeenFrom(pt, sym)
+      formatAnnotationMessage(rawMsg, sym, substituteType)
+
+  /** Extracting the message from a method parameter, e.g. in
+   *
+   *  trait Foo
+   *
+   *  def foo(implicit @annotation.implicitNotFound("Foo is missing") foo: Foo): Any = ???
+   */
+  def userDefinedImplicitNotFoundParamMessage(using Context): Option[String] =
+    paramSymWithMethodCallTree.flatMap: (sym, applTree) =>
+      userDefinedMsg(sym, defn.ImplicitNotFoundAnnot).map: rawMsg =>
+        val fn = tpd.funPart(applTree)
+        val targs = tpd.typeArgss(applTree).flatten
+        val methodOwner = fn.symbol.owner
+        val methodOwnerType = tpd.qualifier(fn).tpe
+        val methodTypeParams = fn.symbol.paramSymss.flatten.filter(_.isType)
+        val methodTypeArgs = targs.map(_.tpe)
+        val substituteType = (_: Type).asSeenFrom(methodOwnerType, methodOwner).subst(methodTypeParams, methodTypeArgs)
+        formatAnnotationMessage(rawMsg, sym.owner, substituteType)
+
+  def userDefinedImplicitNotFoundTypeMessage(using Context): Option[String] =
+    def recur(tp: Type): Option[String] = tp match
+      case tp: TypeRef =>
+        val sym = tp.symbol
+        userDefinedImplicitNotFoundTypeMessageFor(sym).orElse(recur(tp.info))
+      case tp: ClassInfo =>
+        tp.baseClasses.iterator
+          .map(userDefinedImplicitNotFoundTypeMessageFor)
+          .find(_.isDefined).flatten
+      case tp: TypeProxy =>
+        recur(tp.superType)
+      case tp: AndType =>
+        recur(tp.tp1).orElse(recur(tp.tp2))
+      case _ =>
+        None
+    recur(pt)
+
+  /** The implicitNotFound annotation on the parameter, or else on the type.
+   *  implicitNotFound message strings starting with `...` are intended for
+   *  additional explanations, not the message proper. The leading `...` is
+   *  dropped in this case.
+   *  @param explain  The message is used for an additional explanation, not
+   *                  the message proper.
+   */
+  def userDefinedImplicitNotFoundMessage(explain: Boolean)(using Context): Option[String] =
+    def filter(msg: Option[String]) = msg match
+      case Some(str) =>
+        if str.startsWith("...") then
+          if explain then Some(str.drop(3)) else None
+        else if explain then None
+        else msg
+      case None => None
+    filter(userDefinedImplicitNotFoundParamMessage)
+      .orElse(filter(userDefinedImplicitNotFoundTypeMessage))
+
+  object AmbiguousImplicitMsg {
+    def unapply(search: SearchSuccess): Option[String] =
+      userDefinedMsg(search.ref.symbol, defn.ImplicitAmbiguousAnnot)
+  }
+
   def msg(using Context): String =
 
     def formatMsg(shortForm: String)(headline: String = shortForm) = arg match
@@ -2585,29 +2685,6 @@ class MissingImplicitArgument(
               |But ${tpe.explanation}."""
           case _ => headline
 
-    /** Format `raw` implicitNotFound or implicitAmbiguous argument, replacing
-     *  all occurrences of `${X}` where `X` is in `paramNames` with the
-     *  corresponding shown type in `args`.
-     */
-    def userDefinedErrorString(raw: String, paramNames: List[String], args: List[Type]): String = {
-      def translate(name: String): Option[String] = {
-        val idx = paramNames.indexOf(name)
-        if (idx >= 0) Some(i"${args(idx)}") else None
-      }
-
-      """\$\{\s*([^}\s]+)\s*\}""".r.replaceAllIn(raw, (_: Regex.Match) match {
-        case Regex.Groups(v) => quoteReplacement(translate(v).getOrElse("")).nn
-      })
-    }
-
-    /** Extract a user defined error message from a symbol `sym`
-     *  with an annotation matching the given class symbol `cls`.
-     */
-    def userDefinedMsg(sym: Symbol, cls: Symbol) = for {
-      ann <- sym.getAnnotation(cls)
-      msg <- ann.argumentConstantString(0)
-    } yield msg
-
     def location(preposition: String) = if (where.isEmpty) "" else s" $preposition $where"
 
     def defaultAmbiguousImplicitMsg(ambi: AmbiguousImplicits) =
@@ -2644,77 +2721,13 @@ class MissingImplicitArgument(
       userDefinedErrorString(raw, params, args)
     }
 
-    /** @param rawMsg           Message template with variables, e.g. "Variable A is ${A}"
-     *  @param sym              Symbol of the annotated type or of the method whose parameter was annotated
-     *  @param substituteType   Function substituting specific types for abstract types associated with variables, e.g A -> Int
-     */
-    def formatAnnotationMessage(rawMsg: String, sym: Symbol, substituteType: Type => Type): String = {
-      val substitutableTypesSymbols = substitutableTypeSymbolsInScope(sym)
-
-      userDefinedErrorString(
-        rawMsg,
-        paramNames = substitutableTypesSymbols.map(_.name.unexpandedName.toString),
-        args = substitutableTypesSymbols.map(_.typeRef).map(substituteType)
-      )
-    }
-
-    /** Extracting the message from a method parameter, e.g. in
-     *
-     *  trait Foo
-     *
-     *  def foo(implicit @annotation.implicitNotFound("Foo is missing") foo: Foo): Any = ???
-     */
-    def userDefinedImplicitNotFoundParamMessage: Option[String] = paramSymWithMethodCallTree.flatMap { (sym, applTree) =>
-      userDefinedMsg(sym, defn.ImplicitNotFoundAnnot).map { rawMsg =>
-        val fn = tpd.funPart(applTree)
-        val targs = tpd.typeArgss(applTree).flatten
-        val methodOwner = fn.symbol.owner
-        val methodOwnerType = tpd.qualifier(fn).tpe
-        val methodTypeParams = fn.symbol.paramSymss.flatten.filter(_.isType)
-        val methodTypeArgs = targs.map(_.tpe)
-        val substituteType = (_: Type).asSeenFrom(methodOwnerType, methodOwner).subst(methodTypeParams, methodTypeArgs)
-        formatAnnotationMessage(rawMsg, sym.owner, substituteType)
-      }
-    }
-
     /** Extracting the message from a type, e.g. in
      *
      *  @annotation.implicitNotFound("Foo is missing")
      *  trait Foo
      *
      *  def foo(implicit foo: Foo): Any = ???
      */
-    def userDefinedImplicitNotFoundTypeMessage: Option[String] =
-      def recur(tp: Type): Option[String] = tp match
-        case tp: TypeRef =>
-          val sym = tp.symbol
-          userDefinedImplicitNotFoundTypeMessageFor(sym).orElse(recur(tp.info))
-        case tp: ClassInfo =>
-          tp.baseClasses.iterator
-            .map(userDefinedImplicitNotFoundTypeMessageFor)
-            .find(_.isDefined).flatten
-        case tp: TypeProxy =>
-          recur(tp.superType)
-        case tp: AndType =>
-          recur(tp.tp1).orElse(recur(tp.tp2))
-        case _ =>
-          None
-      recur(pt)
-
-    def userDefinedImplicitNotFoundTypeMessageFor(sym: Symbol): Option[String] =
-      for
-        rawMsg <- userDefinedMsg(sym, defn.ImplicitNotFoundAnnot)
-        if Feature.migrateTo3 || sym != defn.Function1
-          // Don't inherit "No implicit view available..." message if subtypes of Function1 are not treated as implicit conversions anymore
-      yield
-        val substituteType = (_: Type).asSeenFrom(pt, sym)
-        formatAnnotationMessage(rawMsg, sym, substituteType)
-
-    object AmbiguousImplicitMsg {
-      def unapply(search: SearchSuccess): Option[String] =
-        userDefinedMsg(search.ref.symbol, defn.ImplicitAmbiguousAnnot)
-    }
-
     arg.tpe match
       case ambi: AmbiguousImplicits =>
         (ambi.alt1, ambi.alt2) match
@@ -2728,8 +2741,7 @@ class MissingImplicitArgument(
         i"""No implicit search was attempted${location("for")}
             |since the expected type $target is not specific enough"""
       case _ =>
-        val shortMessage = userDefinedImplicitNotFoundParamMessage
-          .orElse(userDefinedImplicitNotFoundTypeMessage)
+        val shortMessage = userDefinedImplicitNotFoundMessage(explain = false)
           .getOrElse(defaultImplicitNotFoundMessage)
         formatMsg(shortMessage)()
   end msg
@@ -2758,7 +2770,8 @@ class MissingImplicitArgument(
             .orElse(noChainConversionsNote(ignoredConvertibleImplicits))
             .getOrElse(ctx.typer.importSuggestionAddendum(pt))
 
-  def explain(using Context) = ""
+  def explain(using Context) = userDefinedImplicitNotFoundMessage(explain = true)
+    .getOrElse("")
 end MissingImplicitArgument
 
 class CannotBeAccessed(tpe: NamedType, superAccess: Boolean)(using Context)

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

@@ -143,6 +143,7 @@ class CompilationTests {
       compileFilesInDir("tests/neg-custom-args/feature", defaultOptions.and("-Xfatal-warnings", "-feature")),
       compileFilesInDir("tests/neg-custom-args/no-experimental", defaultOptions.and("-Yno-experimental")),
       compileFilesInDir("tests/neg-custom-args/captures", defaultOptions.and("-language:experimental.captureChecking")),
+      compileFilesInDir("tests/neg-custom-args/explain", defaultOptions.and("-explain")),
       compileFile("tests/neg-custom-args/avoid-warn-deprecation.scala", defaultOptions.and("-Xfatal-warnings", "-feature")),
       compileFile("tests/neg-custom-args/i3246.scala", scala2CompatMode),
       compileFile("tests/neg-custom-args/overrideClass.scala", scala2CompatMode),
@@ -155,9 +156,6 @@ class CompilationTests {
       compileFile("tests/neg-custom-args/i1754.scala", allowDeepSubtypes),
       compileFile("tests/neg-custom-args/i12650.scala", allowDeepSubtypes),
       compileFile("tests/neg-custom-args/i9517.scala", defaultOptions.and("-Xprint-types")),
-      compileFile("tests/neg-custom-args/i11637.scala", defaultOptions.and("-explain")),
-      compileFile("tests/neg-custom-args/i15575.scala", defaultOptions.and("-explain")),
-      compileFile("tests/neg-custom-args/i16601a.scala", defaultOptions.and("-explain")),
       compileFile("tests/neg-custom-args/interop-polytypes.scala", allowDeepSubtypes.and("-Yexplicit-nulls")),
       compileFile("tests/neg-custom-args/conditionalWarnings.scala", allowDeepSubtypes.and("-deprecation").and("-Xfatal-warnings")),
       compileFilesInDir("tests/neg-custom-args/isInstanceOf", allowDeepSubtypes and "-Xfatal-warnings"),
@@ -182,7 +180,6 @@ class CompilationTests {
       compileFile("tests/neg-custom-args/matchable.scala", defaultOptions.and("-Xfatal-warnings", "-source", "future")),
       compileFile("tests/neg-custom-args/i7314.scala", defaultOptions.and("-Xfatal-warnings", "-source", "future")),
       compileFile("tests/neg-custom-args/capt-wf.scala", defaultOptions.and("-language:experimental.captureChecking", "-Xfatal-warnings")),
-      compileDir("tests/neg-custom-args/hidden-type-errors", defaultOptions.and("-explain")),
       compileFile("tests/neg-custom-args/i13026.scala", defaultOptions.and("-print-lines")),
       compileFile("tests/neg-custom-args/i13838.scala", defaultOptions.and("-Ximplicit-search-limit", "1000")),
       compileFile("tests/neg-custom-args/jdk-9-app.scala", defaultOptions.and("-release:8")),

library/src/scala/util/boundary.scala

@@ -1,4 +1,5 @@
 package scala.util
+import scala.annotation.implicitNotFound
 
 /** A boundary that can be exited by `break` calls.
  *  `boundary` and `break` represent a unified and superior alternative for the
@@ -34,6 +35,7 @@ object boundary:
 
   /** Labels are targets indicating which boundary will be exited by a `break`.
    */
+  @implicitNotFound("...A Label is generated from an enclosing `scala.util.boundary` call.\nMaybe that boundary is missing?")
   final class Label[-T]
 
   /** Abort current computation and instead return `value` as the value of

tests/neg-custom-args/explain/hidden-type-errors.check

@@ -0,0 +1,23 @@
+-- [E007] Type Mismatch Error: tests/neg-custom-args/explain/hidden-type-errors/Test.scala:6:24 ------------------------
+6 |   val x = X.doSomething("XXX") // error
+  |           ^^^^^^^^^^^^^^^^^^^^
+  |           Found:    String
+  |           Required: Int
+  |---------------------------------------------------------------------------------------------------------------------
+  | Explanation (enabled by `-explain`)
+  |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+  |
+  | Tree: t12717.A.bar("XXX")
+  | I tried to show that
+  |   String
+  | conforms to
+  |   Int
+  | but the comparison trace ended with `false`:
+  |
+  |   ==> String  <:  Int
+  |     ==> String  <:  Int
+  |     <== String  <:  Int = false
+  |   <== String  <:  Int = false
+  |
+  | The tests were made under the empty constraint
+   ---------------------------------------------------------------------------------------------------------------------

tests/neg-custom-args/hidden-type-errors/Macro.scala renamed to tests/neg-custom-args/explain/hidden-type-errors/Macro.scala

@@ -1,4 +1,4 @@
--- [E057] Type Mismatch Error: tests/neg-custom-args/i11637.scala:11:33 ------------------------------------------------
+-- [E057] Type Mismatch Error: tests/neg-custom-args/explain/i11637.scala:11:33 ----------------------------------------
 11 |  var h = new HKT3_1[FunctorImpl]();  // error // error
    |                                 ^
    |       Type argument test2.FunctorImpl does not conform to upper bound [Generic2[T <: String] <: Set[T]] =>> Any
@@ -26,7 +26,7 @@
    |
    | The tests were made under the empty constraint
     --------------------------------------------------------------------------------------------------------------------
--- [E057] Type Mismatch Error: tests/neg-custom-args/i11637.scala:11:21 ------------------------------------------------
+-- [E057] Type Mismatch Error: tests/neg-custom-args/explain/i11637.scala:11:21 ----------------------------------------
 11 |  var h = new HKT3_1[FunctorImpl]();  // error // error
    |                     ^
    |       Type argument test2.FunctorImpl does not conform to upper bound [Generic2[T <: String] <: Set[T]] =>> Any

tests/neg-custom-args/hidden-type-errors/Test.scala renamed to tests/neg-custom-args/explain/hidden-type-errors/Test.scala

@@ -1,4 +1,4 @@
--- [E057] Type Mismatch Error: tests/neg-custom-args/i15575.scala:3:27 -------------------------------------------------
+-- [E057] Type Mismatch Error: tests/neg-custom-args/explain/i15575.scala:3:27 -----------------------------------------
 3 |  def bar[T]: Unit = foo[T & Any] // error
   |                           ^
   |                           Type argument T & Any does not conform to lower bound Any
@@ -18,7 +18,7 @@
   |
   | The tests were made under the empty constraint
    ---------------------------------------------------------------------------------------------------------------------
--- [E057] Type Mismatch Error: tests/neg-custom-args/i15575.scala:7:14 -------------------------------------------------
+-- [E057] Type Mismatch Error: tests/neg-custom-args/explain/i15575.scala:7:14 -----------------------------------------
 7 |  val _ = foo[String]  // error
   |              ^
   |              Type argument String does not conform to lower bound CharSequence

tests/neg-custom-args/i11637.check renamed to tests/neg-custom-args/explain/i11637.check

@@ -1,4 +1,4 @@
--- [E042] Type Error: tests/neg-custom-args/i16601a.scala:1:27 ---------------------------------------------------------
+-- [E042] Type Error: tests/neg-custom-args/explain/i16601a.scala:1:27 -------------------------------------------------
 1 |@main def Test: Unit = new concurrent.ExecutionContext  // error
   |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |                           ExecutionContext is a trait; it cannot be instantiated

tests/neg-custom-args/i11637.scala renamed to tests/neg-custom-args/explain/i11637.scala

@@ -0,0 +1,10 @@
+-- [E172] Type Error: tests/neg-custom-args/explain/labelNotFound.scala:2:30 -------------------------------------------
+2 |  scala.util.boundary.break(1) // error
+  |                              ^
+  |No given instance of type scala.util.boundary.Label[Int] was found for parameter label of method break in object boundary
+  |---------------------------------------------------------------------------------------------------------------------
+  | Explanation (enabled by `-explain`)
+  |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+  | A Label is generated from an enclosing `scala.util.boundary` call.
+  | Maybe that boundary is missing?
+   ---------------------------------------------------------------------------------------------------------------------

tests/neg-custom-args/i15575.check renamed to tests/neg-custom-args/explain/i15575.check

@@ -0,0 +1,4 @@
+object Test:
+  scala.util.boundary.break(1) // error
+
+