Fix tuple member selection so it works with GADT healing (#16766)

Author: dwijnand

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

@@ -1675,6 +1675,15 @@ class Definitions {
     rec(tp.stripTypeVar, Nil, bound)
   }
 
+  def isSmallGenericTuple(tp: Type)(using Context): Boolean =
+    if tp.derivesFrom(defn.PairClass) && !defn.isTupleNType(tp.widenDealias) then
+      // If this is a generic tuple we need to cast it to make the TupleN/ members accessible.
+      // This works only for generic tuples of known size up to 22.
+      defn.tupleTypes(tp.widenTermRefExpr) match
+        case Some(elems) if elems.length <= Definitions.MaxTupleArity => true
+        case _ => false
+    else false
+
   def isProductSubType(tp: Type)(using Context): Boolean = tp.derivesFrom(ProductClass)
 
   /** Is `tp` (an alias) of either a scala.FunctionN or a scala.ContextFunctionN

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

@@ -635,6 +635,9 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
        // There's a second trial where we try to instantiate all type variables in `qual.tpe.widen`,
        // but that is done only after we search for extension methods or conversions.
       typedSelect(tree, pt, qual)
+    else if defn.isSmallGenericTuple(qual.tpe) then
+      val elems = defn.tupleTypes(qual.tpe.widenTermRefExpr).getOrElse(Nil)
+      typedSelect(tree, pt, qual.cast(defn.tupleType(elems)))
     else
       val tree1 = tryExtensionOrConversion(
           tree, pt, IgnoredProto(pt), qual, ctx.typerState.ownedVars, this, inSelect = true)
@@ -654,6 +657,9 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
             if checkedType1.exists then
               gadts.println(i"Member selection healed by GADT approximation")
               finish(tree1, qual1, checkedType1)
+            else if defn.isSmallGenericTuple(qual1.tpe) then
+              gadts.println(i"Tuple member selection healed by GADT approximation")
+              typedSelect(tree, pt, qual1)
             else
               tryExtensionOrConversion(tree1, pt, IgnoredProto(pt), qual1, ctx.typerState.ownedVars, this, inSelect = true)
           else EmptyTree
@@ -4010,15 +4016,8 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
         else err.typeMismatch(tree, pt, failure)
 
       pt match
-        case pt: SelectionProto =>
-          if tree.tpe.derivesFrom(defn.PairClass) && !defn.isTupleNType(tree.tpe.widenDealias) then
-            // If this is a generic tuple we need to cast it to make the TupleN/ members accessible.
-            // This works only for generic tuples of known size up to 22.
-            defn.tupleTypes(tree.tpe.widenTermRefExpr) match
-              case Some(elems) if elems.length <= Definitions.MaxTupleArity =>
-                tree.cast(defn.tupleType(elems))
-              case _ => tree
-          else tree // other adaptations for selections are handled in typedSelect
+        case _: SelectionProto =>
+          tree // adaptations for selections are handled in typedSelect
         case _ if ctx.mode.is(Mode.ImplicitsEnabled) && tree.tpe.isValueType =>
           if pt.isRef(defn.AnyValClass, skipRefined = false)
               || pt.isRef(defn.ObjectClass, skipRefined = false)

tests/pos/i16590.scala

@@ -0,0 +1,13 @@
+enum Tag[A]:
+  case MyTuple extends Tag[(String, String)]
+
+def printIt[A](t: Tag[A], a: A): Unit =
+  t match
+    case Tag.MyTuple => println(a._1)
+
+enum Tag2[A]:
+  case MyTuple extends Tag2[String *: String *: EmptyTuple]
+
+def printIt2[A](t: Tag2[A], a: A): Unit =
+  t match
+    case Tag2.MyTuple => println(a._1)