Make named tuples an experimental feature again (#22045)

Resolves #22042

* Reverts most of the stabilization changes introduced in
#21680 excluding bugfixes introduced
when stabilizing the name tuples
* Adapts #21823 and
#21949 warnings to make them both
syntax deprecations instead of ambiguous syntax.
* Adds automatic rewrite to #21823
to replace `(foo = bar)` into `{foo = bar}`

Author: WojciechMazur

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

@@ -34,6 +34,7 @@ object Feature:
   val pureFunctions = experimental("pureFunctions")
   val captureChecking = experimental("captureChecking")
   val into = experimental("into")
+  val namedTuples = experimental("namedTuples")
   val modularity = experimental("modularity")
   val betterMatchTypeExtractors = experimental("betterMatchTypeExtractors")
   val quotedPatternsWithPolymorphicFunctions = experimental("quotedPatternsWithPolymorphicFunctions")
@@ -65,6 +66,7 @@ object Feature:
     (pureFunctions, "Enable pure functions for capture checking"),
     (captureChecking, "Enable experimental capture checking"),
     (into, "Allow into modifier on parameter types"),
+    (namedTuples, "Allow named tuples"),
     (modularity, "Enable experimental modularity features"),
     (betterMatchTypeExtractors, "Enable better match type extractors"),
     (betterFors, "Enable improvements in `for` comprehensions")

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

@@ -26,7 +26,7 @@ enum MigrationVersion(val warnFrom: SourceVersion, val errorFrom: SourceVersion)
   case WithOperator extends MigrationVersion(`3.4`, future)
   case FunctionUnderscore extends MigrationVersion(`3.4`, future)
   case NonNamedArgumentInJavaAnnotation extends MigrationVersion(`3.6`, `3.6`)
-  case AmbiguousNamedTupleInfixApply extends MigrationVersion(`3.6`, never)
+  case AmbiguousNamedTupleSyntax extends MigrationVersion(`3.6`, future)
   case ImportWildcard extends MigrationVersion(future, future)
   case ImportRename extends MigrationVersion(future, future)
   case ParameterEnclosedByParenthesis extends MigrationVersion(future, future)

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

@@ -667,7 +667,7 @@ object Parsers {
       else leading :: Nil
 
     def maybeNamed(op: () => Tree): () => Tree = () =>
-      if isIdent && in.lookahead.token == EQUALS && sourceVersion.isAtLeast(`3.6`) then
+      if isIdent && in.lookahead.token == EQUALS && in.featureEnabled(Feature.namedTuples) then
         atSpan(in.offset):
           val name = ident()
           in.nextToken()
@@ -1149,8 +1149,8 @@ object Parsers {
       if isType then infixOp
       else infixOp.right match
         case Tuple(args) if args.exists(_.isInstanceOf[NamedArg]) && !isNamedTupleOperator =>
-          report.errorOrMigrationWarning(AmbiguousNamedTupleInfixApply(), infixOp.right.srcPos, MigrationVersion.AmbiguousNamedTupleInfixApply)
-          if MigrationVersion.AmbiguousNamedTupleInfixApply.needsPatch then
+          report.errorOrMigrationWarning(DeprecatedInfixNamedArgumentSyntax(), infixOp.right.srcPos, MigrationVersion.AmbiguousNamedTupleSyntax)
+          if MigrationVersion.AmbiguousNamedTupleSyntax.needsPatch then
             val asApply = cpy.Apply(infixOp)(Select(opInfo.operand, opInfo.operator.name), args)
             patch(source, infixOp.span, asApply.show(using ctx.withoutColors))
             asApply // allow to use pre-3.6 syntax in migration mode
@@ -2172,7 +2172,7 @@ object Parsers {
 
       if namedOK && isIdent && in.lookahead.token == EQUALS then
         commaSeparated(() => namedArgType())
-      else if tupleOK && isIdent && in.lookahead.isColon && sourceVersion.isAtLeast(`3.6`) then
+      else if tupleOK && isIdent && in.lookahead.isColon && in.featureEnabled(Feature.namedTuples) then
         commaSeparated(() => namedElem())
       else
         commaSeparated(() => argType())

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

@@ -216,8 +216,8 @@ enum ErrorMessageID(val isActive: Boolean = true) extends java.lang.Enum[ErrorMe
   case FinalLocalDefID // errorNumber: 200
   case NonNamedArgumentInJavaAnnotationID // errorNumber: 201
   case QuotedTypeMissingID // errorNumber: 202
-  case AmbiguousNamedTupleAssignmentID // errorNumber: 203
-  case AmbiguousNamedTupleInfixApplyID // errorNumber: 204
+  case DeprecatedAssignmentSyntaxID // errorNumber: 203
+  case DeprecatedInfixNamedArgumentSyntaxID // errorNumber: 204
 
   def errorNumber = ordinal - 1
 

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

@@ -3344,21 +3344,20 @@ final class QuotedTypeMissing(tpe: Type)(using Context) extends StagingMessage(Q
 
 end QuotedTypeMissing
 
-final class AmbiguousNamedTupleAssignment(key: Name, value: untpd.Tree)(using Context) extends SyntaxMsg(AmbiguousNamedTupleAssignmentID):
+final class DeprecatedAssignmentSyntax(key: Name, value: untpd.Tree)(using Context) extends SyntaxMsg(DeprecatedAssignmentSyntaxID):
   override protected def msg(using Context): String =
-    i"""Ambiguous syntax: this is interpreted as a named tuple with one element,
+    i"""Deprecated syntax: in the future it would be interpreted as a named tuple with one element,
       |not as an assignment.
       |
       |To assign a value, use curly braces: `{${key} = ${value}}`."""
-  
+      + Message.rewriteNotice("This", version = SourceVersion.`3.6-migration`)
+
   override protected def explain(using Context): String = ""
 
-class AmbiguousNamedTupleInfixApply()(using Context) extends SyntaxMsg(AmbiguousNamedTupleInfixApplyID):
+class DeprecatedInfixNamedArgumentSyntax()(using Context) extends SyntaxMsg(DeprecatedInfixNamedArgumentSyntaxID):
   def msg(using Context) =
-    "Ambigious syntax: this infix call argument list is interpreted as single named tuple argument, not as an named arguments list."
-    + Message.rewriteNotice("This", version = SourceVersion.`3.6-migration`)
+    i"""Deprecated syntax: infix named arguments lists are deprecated; in the future it would be interpreted as a single name tuple argument.
+       |To avoid this warning, either remove the argument names or use dotted selection."""
+        + Message.rewriteNotice("This", version = SourceVersion.`3.6-migration`)
 
-  def explain(using Context) =
-    i"""Starting with Scala 3.6 infix named arguments are interpretted as Named Tuple.
-        |
-        |To avoid this warning, either remove the argument names or use dotted selection."""
+  def explain(using Context) = ""

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

@@ -795,7 +795,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
     def tryNamedTupleSelection() =
       val namedTupleElems = qual.tpe.widenDealias.namedTupleElementTypes
       val nameIdx = namedTupleElems.indexWhere(_._1 == selName)
-      if nameIdx >= 0 && sourceVersion.isAtLeast(`3.6`) then
+      if nameIdx >= 0 && Feature.enabled(Feature.namedTuples) then
         typed(
           untpd.Apply(
             untpd.Select(untpd.TypedSplice(qual), nme.apply),
@@ -3404,7 +3404,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
   /** Translate tuples of all arities */
   def typedTuple(tree: untpd.Tuple, pt: Type)(using Context): Tree =
     val tree1 = desugar.tuple(tree, pt)
-    checkAmbiguousNamedTupleAssignment(tree)
+    checkDeprecatedAssignmentSyntax(tree)
     if tree1 ne tree then typed(tree1, pt)
     else
       val arity = tree.trees.length
@@ -3433,15 +3433,18 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
  /** Checks if `tree` is a named tuple with one element that could be
   *  interpreted as an assignment, such as `(x = 1)`. If so, issues a warning.
   */
-  def checkAmbiguousNamedTupleAssignment(tree: untpd.Tuple)(using Context): Unit =
+  def checkDeprecatedAssignmentSyntax(tree: untpd.Tuple)(using Context): Unit =
     tree.trees match
       case List(NamedArg(name, value)) =>
         val tmpCtx = ctx.fresh.setNewTyperState()
         typedAssign(untpd.Assign(untpd.Ident(name), value), WildcardType)(using tmpCtx)
         if !tmpCtx.reporter.hasErrors then
           // If there are no errors typing the above, then the named tuple is
           // ambiguous and we issue a warning.
-          report.migrationWarning(AmbiguousNamedTupleAssignment(name, value), tree.srcPos)
+          report.migrationWarning(DeprecatedAssignmentSyntax(name, value), tree.srcPos)
+          if MigrationVersion.AmbiguousNamedTupleSyntax.needsPatch then
+            patch(tree.source, Span(tree.span.start, tree.span.start + 1), "{")
+            patch(tree.source, Span(tree.span.end - 1, tree.span.end), "}")
       case _ => ()
 
   /** Retrieve symbol attached to given tree */

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

@@ -80,6 +80,7 @@ class CompilationTests {
       compileDir("tests/rewrites/annotation-named-pararamters", defaultOptions.and("-rewrite", "-source:3.6-migration")),
       compileFile("tests/rewrites/i21418.scala", unindentOptions.and("-rewrite", "-source:3.5-migration")),
       compileFile("tests/rewrites/infix-named-args.scala", defaultOptions.and("-rewrite", "-source:3.6-migration")),
+      compileFile("tests/rewrites/ambigious-named-tuple-assignment.scala", defaultOptions.and("-rewrite", "-source:3.6-migration")),
     ).checkRewrites()
   }
 

docs/_docs/reference/other-new-features/named-tuples.md renamed to docs/_docs/reference/experimental/named-tuples.md

@@ -1,10 +1,10 @@
 ---
 layout: doc-page
 title: "Named Tuples"
-nightlyOf: https://docs.scala-lang.org/scala3/reference/other-new-features/named-tuples.html
+nightlyOf: https://docs.scala-lang.org/scala3/reference/experimental/named-tuples.html
 ---
 
-Starting in Scala 3.6, the elements of a tuple can be named. Example:
+The elements of a tuple can now be named. Example:
 ```scala
 type Person = (name: String, age: Int)
 val Bob: Person = (name = "Bob", age = 33)

docs/sidebar.yml

@@ -72,7 +72,6 @@ subsection:
           - page: reference/other-new-features/export.md
           - page: reference/other-new-features/opaques.md
           - page: reference/other-new-features/opaques-details.md
-          - page: reference/other-new-features/named-tuples.md
           - page: reference/other-new-features/open-classes.md
           - page: reference/other-new-features/parameter-untupling.md
           - page: reference/other-new-features/parameter-untupling-spec.md
@@ -159,6 +158,7 @@ subsection:
           - page: reference/experimental/cc.md
           - page: reference/experimental/purefuns.md
           - page: reference/experimental/tupled-function.md
+          - page: reference/experimental/named-tuples.md
           - page: reference/experimental/modularity.md
           - page: reference/experimental/typeclasses.md
           - page: reference/experimental/runtimeChecked.md

library/src/scala/NamedTuple.scala

@@ -1,6 +1,8 @@
 package scala
+import annotation.experimental
 import compiletime.ops.boolean.*
 
+@experimental
 object NamedTuple:
 
   /** The type to which named tuples get mapped to. For instance,
@@ -131,6 +133,7 @@ object NamedTuple:
 end NamedTuple
 
 /** Separate from NamedTuple object so that we can match on the opaque type NamedTuple. */
+@experimental
 object NamedTupleDecomposition:
   import NamedTuple.*
   extension [N <: Tuple, V <: Tuple](x: NamedTuple[N, V])