Add syntax for qualified types

Co-Authored-By: Quentin Bernet <[email protected]>

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -8,7 +8,7 @@ import Symbols.*, StdNames.*, Trees.*, ContextOps.*
 import Decorators.*
 import Annotations.Annotation
 import NameKinds.{UniqueName, ContextBoundParamName, ContextFunctionParamName, DefaultGetterName, WildcardParamName}
-import typer.{Namer, Checking}
+import typer.{Namer, Checking, ErrorReporting}
 import util.{Property, SourceFile, SourcePosition, SrcPos, Chars}
 import config.{Feature, Config}
 import config.Feature.{sourceVersion, migrateTo3, enabled, betterForsEnabled}
@@ -199,9 +199,10 @@ object desugar {
   def valDef(vdef0: ValDef)(using Context): Tree =
     val vdef @ ValDef(_, tpt, rhs) = vdef0
     val valName = normalizeName(vdef, tpt).asTermName
+    val tpt1 = qualifiedType(tpt, valName)
     var mods1 = vdef.mods
 
-    val vdef1 = cpy.ValDef(vdef)(name = valName).withMods(mods1)
+    val vdef1 = cpy.ValDef(vdef)(name = valName, tpt = tpt1).withMods(mods1)
 
     if isSetterNeeded(vdef) then
       val setterParam = makeSyntheticParameter(tpt = SetterParamTree().watching(vdef))
@@ -2145,6 +2146,10 @@ object desugar {
       case PatDef(mods, pats, tpt, rhs) =>
         val pats1 = if (tpt.isEmpty) pats else pats map (Typed(_, tpt))
         flatTree(pats1 map (makePatDef(tree, mods, _, rhs)))
+      case QualifiedTypeTree(parent, None, qualifier) =>
+        ErrorReporting.errorTree(parent, em"missing parameter name in qualified type", tree.srcPos)
+      case QualifiedTypeTree(parent, Some(paramName), qualifier) =>
+        qualifiedType(parent, paramName, qualifier, tree.span)
       case ext: ExtMethods =>
         Block(List(ext), syntheticUnitLiteral.withSpan(ext.span))
       case f: FunctionWithMods if f.hasErasedParams => makeFunctionWithValDefs(f, pt)
@@ -2323,4 +2328,23 @@ object desugar {
     collect(tree)
     buf.toList
   }
+
+  /** If `tree` is a `QualifiedTypeTree`, then desugars it using `paramName` as
+   *  the qualified paramater name. Otherwise, returns `tree` unchanged.
+   */
+  def qualifiedType(tree: Tree, paramName: TermName)(using Context): Tree = tree match
+    case QualifiedTypeTree(parent, None, qualifier) => qualifiedType(parent, paramName, qualifier, tree.span)
+    case _ => tree
+
+  /** Returns the annotated type used to represent the qualified type with the
+   *  given components:
+   *  `parent @qualified[parent]((paramName: parent) => qualifier)`.
+   */
+  def qualifiedType(parent: Tree, paramName: TermName, qualifier: Tree, span: Span)(using Context): Tree =
+    val param = makeParameter(paramName, parent, EmptyModifiers) // paramName: parent
+    val predicate = WildcardFunction(List(param), qualifier) // (paramName: parent) => qualifier
+    val qualifiedAnnot = scalaAnnotationDot(nme.qualified)
+    val annot = Apply(TypeApply(qualifiedAnnot, List(parent)), predicate).withSpan(span) // @qualified[parent](predicate)
+    Annotated(parent, annot).withSpan(span) // parent @qualified[parent](predicate)
+
 }

compiler/src/dotty/tools/dotc/ast/untpd.scala

@@ -156,6 +156,9 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
    */
   case class CapturesAndResult(refs: List[Tree], parent: Tree)(implicit @constructorOnly src: SourceFile) extends TypTree
 
+  /** { x: T with p }   (only relevant under qualifiedTypes) */
+  case class QualifiedTypeTree(parent: Tree, paramName: Option[TermName], qualifier: Tree)(implicit @constructorOnly src: SourceFile) extends TypTree
+
   /** A type tree appearing somewhere in the untyped DefDef of a lambda, it will be typed using `tpFun`.
    *
    *  @param isResult  Is this the result type of the lambda? This is handled specially in `Namer#valOrDefDefSig`.
@@ -704,6 +707,10 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
       case tree: CapturesAndResult if (refs eq tree.refs) && (parent eq tree.parent) => tree
       case _ => finalize(tree, untpd.CapturesAndResult(refs, parent))
 
+    def QualifiedTypeTree(tree: Tree)(parent: Tree, paramName: Option[TermName], qualifier: Tree)(using Context): Tree = tree match
+      case tree: QualifiedTypeTree if (parent eq tree.parent) && (paramName eq tree.paramName) && (qualifier eq tree.qualifier) => tree
+      case _ => finalize(tree, untpd.QualifiedTypeTree(parent, paramName, qualifier)(tree.source))
+
     def TypedSplice(tree: Tree)(splice: tpd.Tree)(using Context): ProxyTree = tree match {
       case tree: TypedSplice if splice `eq` tree.splice => tree
       case _ => finalize(tree, untpd.TypedSplice(splice)(using ctx))
@@ -767,6 +774,8 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
         cpy.MacroTree(tree)(transform(expr))
       case CapturesAndResult(refs, parent) =>
         cpy.CapturesAndResult(tree)(transform(refs), transform(parent))
+      case QualifiedTypeTree(parent, paramName, qualifier) =>
+        cpy.QualifiedTypeTree(tree)(transform(parent), paramName, transform(qualifier))
       case _ =>
         super.transformMoreCases(tree)
     }
@@ -826,6 +835,8 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
         this(x, expr)
       case CapturesAndResult(refs, parent) =>
         this(this(x, refs), parent)
+      case QualifiedTypeTree(parent, paramName, qualifier) =>
+        this(this(x, parent), qualifier)
       case _ =>
         super.foldMoreCases(x, tree)
     }

compiler/src/dotty/tools/dotc/config/Feature.scala

@@ -33,6 +33,7 @@ object Feature:
   val clauseInterleaving = experimental("clauseInterleaving")
   val pureFunctions = experimental("pureFunctions")
   val captureChecking = experimental("captureChecking")
+  val qualifiedTypes = experimental("qualifiedTypes")
   val into = experimental("into")
   val namedTuples = experimental("namedTuples")
   val modularity = experimental("modularity")
@@ -65,6 +66,7 @@ object Feature:
     (clauseInterleaving, "Enable clause interleaving"),
     (pureFunctions, "Enable pure functions for capture checking"),
     (captureChecking, "Enable experimental capture checking"),
+    (qualifiedTypes, "Enable experimental qualified types"),
     (into, "Allow into modifier on parameter types"),
     (namedTuples, "Allow named tuples"),
     (modularity, "Enable experimental modularity features"),
@@ -158,6 +160,10 @@ object Feature:
     if ctx.run != null then ctx.run.nn.ccEnabledSomewhere
     else enabledBySetting(captureChecking)
 
+   /** Is qualifiedTypes enabled for this compilation unit? */
+  def qualifiedTypesEnabled(using Context) =
+    enabledBySetting(qualifiedTypes)
+
   def sourceVersionSetting(using Context): SourceVersion =
     SourceVersion.valueOf(ctx.settings.source.value)
 

compiler/src/dotty/tools/dotc/core/StdNames.scala

@@ -585,6 +585,7 @@ object StdNames {
     val productElementName: N   = "productElementName"
     val productIterator: N      = "productIterator"
     val productPrefix: N        = "productPrefix"
+    val qualified : N           = "qualified"
     val quotes : N              = "quotes"
     val raw_ : N                = "raw"
     val refl: N                 = "refl"

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -445,6 +445,13 @@ object Parsers {
       finally inMatchPattern = saved
     }
 
+    private var inQualifiedType = false
+    private def fromWithinQualifiedType[T](body: => T): T =
+      val saved = inQualifiedType
+      inQualifiedType = true
+      try body
+      finally inQualifiedType = saved
+
     private var staged = StageKind.None
     def withinStaged[T](kind: StageKind)(op: => T): T = {
       val saved = staged
@@ -1085,6 +1092,25 @@ object Parsers {
       || in.lookahead.token == EOF     // important for REPL completions
       || ctx.mode.is(Mode.Interactive) // in interactive mode the next tokens might be missing
 
+    /** Under `qualifiedTypes` language import: is the token sequence following
+     *  the current `{` classified as a qualified type? This is the case if the
+     *  next token is an `IDENT`, followed by `:`.
+     */
+    def followingIsQualifiedType(): Boolean =
+      in.featureEnabled(Feature.qualifiedTypes) && {
+        val lookahead = in.LookaheadScanner(allowIndent = true)
+
+        if in.token == INDENT then
+          () // The LookaheadScanner doesn't see previous indents, so no need to skip it
+        else
+          lookahead.nextToken() // skips the opening brace
+
+        lookahead.token == IDENTIFIER && {
+          lookahead.nextToken()
+          lookahead.token == COLONfollow
+        }
+      }
+
   /* --------- OPERAND/OPERATOR STACK --------------------------------------- */
 
     var opStack: List[OpInfo] = Nil
@@ -1872,12 +1898,22 @@ object Parsers {
         t
     }
 
-    /** WithType ::= AnnotType {`with' AnnotType}    (deprecated)
-     */
+    /** With qualifiedTypes enabled:
+      * WithType ::= AnnotType [`with' PostfixExpr]
+      *
+      * Otherwise:
+      * WithType ::= AnnotType {`with' AnnotType}    (deprecated)
+      */
     def withType(): Tree = withTypeRest(annotType())
 
     def withTypeRest(t: Tree): Tree =
-      if in.token == WITH then
+      if Feature.qualifiedTypesEnabled && in.token == WITH then
+        if inQualifiedType then t
+        else
+          in.nextToken()
+          val qualifier = postfixExpr()
+          QualifiedTypeTree(t, None, qualifier).withSpan(Span(t.span.start, qualifier.span.end))
+      else if in.token == WITH then
         val withOffset = in.offset
         in.nextToken()
         if in.token == LBRACE || in.token == INDENT then
@@ -2025,6 +2061,7 @@ object Parsers {
      *                     |  ‘(’ ArgTypes ‘)’
      *                     |  ‘(’ NamesAndTypes ‘)’
      *                     |  Refinement
+     *                     |  QualifiedType             -- under qualifiedTypes
      *                     |  TypeSplice                -- deprecated syntax (since 3.0.0)
      *                     |  SimpleType1 TypeArgs
      *                     |  SimpleType1 `#' id
@@ -2034,6 +2071,8 @@ object Parsers {
         atSpan(in.offset) {
           makeTupleOrParens(inParensWithCommas(argTypes(namedOK = false, wildOK = true, tupleOK = true)))
         }
+      else if in.token == LBRACE && followingIsQualifiedType() then
+        qualifiedType()
       else if in.token == LBRACE then
         atSpan(in.offset) { RefinedTypeTree(EmptyTree, refinement(indentOK = false)) }
       else if (isSplice)
@@ -2198,6 +2237,19 @@ object Parsers {
       else
         inBraces(refineStatSeq())
 
+    /** QualifiedType ::= `{` Ident `:` Type `with` Block `}`
+     */
+    def qualifiedType(): Tree =
+      val startOffset = in.offset
+      accept(LBRACE)
+      val id = ident()
+      accept(COLONfollow)
+      val tp = fromWithinQualifiedType(typ())
+      accept(WITH)
+      val qualifier = block(simplify = true)
+      accept(RBRACE)
+      QualifiedTypeTree(tp, Some(id), qualifier).withSpan(Span(startOffset, qualifier.span.end))
+
     /** TypeBounds ::= [`>:' Type] [`<:' Type]
      *              |  `^`                     -- under captureChecking
      */

compiler/src/dotty/tools/dotc/printing/RefinedPrinter.scala

@@ -808,6 +808,10 @@ class RefinedPrinter(_ctx: Context) extends PlainPrinter(_ctx) {
         prefix ~~ idx.toString ~~ "|" ~~ tpeText ~~ "|" ~~ argsText ~~ "|" ~~ contentText ~~ postfix
       case CapturesAndResult(refs, parent) =>
         changePrec(GlobalPrec)("^{" ~ Text(refs.map(toText), ", ") ~ "}" ~ toText(parent))
+      case QualifiedTypeTree(parent, paramName, predicate) =>
+        paramName match
+          case Some(name) => "{" ~ toText(name) ~  ": " ~ toText(parent) ~ " with " ~ toText(predicate) ~ "}"
+          case None => toText(parent) ~ " with " ~ toText(predicate)
       case ContextBoundTypeTree(tycon, pname, ownName) =>
         toText(pname) ~ " : " ~ toText(tycon) ~ (" as " ~ toText(ownName) `provided` !ownName.isEmpty)
       case _ =>

library/src/scala/annotation/qualified.scala

@@ -0,0 +1,4 @@
+package scala.annotation
+
+/** Annotation for qualified types. */
+@experimental class qualified[T](predicate: T => Boolean) extends StaticAnnotation

library/src/scala/runtime/stdLibPatches/language.scala

@@ -84,6 +84,10 @@ object language:
     @compileTimeOnly("`captureChecking` can only be used at compile time in import statements")
     object captureChecking
 
+    /** Experimental support for qualified types */
+    @compileTimeOnly("`qualifiedTypes` can only be used at compile time in import statements")
+    object qualifiedTypes
+
     /** Experimental support for automatic conversions of arguments, without requiring
      *  a language import `import scala.language.implicitConversions`.
      *