Fix patterns containing wildcards; enhance test

Author: noti0na1

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

@@ -1489,13 +1489,18 @@ object desugar {
         case _ => -1
       }
 
+      val nonWildcardVars = pat match {
+        case Tuple(pats) => pats.filterNot(isWildcardPattern).length
+        case _ => -1
+      }
+
       val isMatchingTuple: Tree => Boolean = {
         case Tuple(es) => varTuplePatternArity == es.length && !hasNamedArg(es)
         case _ => false
       }
 
       // We can only optimize `val pat = if (...) e1 else e2` if:
-      // - `e1` and `e2` are both tuples of arity N
+      // - `e1` and `e2` are both literal tuples of arity N
       // - `pat` is a tuple of N variables or wildcard patterns like `(x1, x2, ..., xN)`
       val tupleOptimizable = forallResults(rhs, isMatchingTuple)
 
@@ -1504,7 +1509,7 @@ object desugar {
         case _ => false
 
       val vars =
-        if (tupleOptimizable) // include `_`
+        if varTuplePatternArity > 0 && nonWildcardVars > 1 then // include `_`
           pat match
             case Tuple(pats) => pats.map { case id: Ident => id -> TypeTree() }
         else
@@ -1522,7 +1527,7 @@ object desugar {
         if tupleOptimizable then rhs
         else
           val caseDef =
-            if varTuplePatternArity >= 0 && ids.length > 1 then
+            if varTuplePatternArity > 0 && ids.length > 1 then
               // If the pattern contains only simple variables or wildcards,
               // we don't need to create a new tuple.
               // If there is only one variable (ids.length == 1),

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

@@ -207,6 +207,12 @@ trait TreeInfo[T <: Untyped] { self: Trees.Instance[T] =>
     case _  => false
   }
 
+  /** Is tree a wildcard pattern? Not including `x @ _` */
+  def isWildcardPattern(pat: Tree): Boolean = unsplice(pat) match {
+    case x: Ident => x.name == nme.WILDCARD && !isBackquoted(x)
+    case _  => false
+  }
+
   /** The first constructor definition in `stats` */
   def firstConstructor(stats: List[Tree]): Tree = stats match {
     case (meth: DefDef) :: _ if meth.name.isConstructorName => meth

tests/pos/simple-tuple-extract.scala

@@ -1,34 +1,37 @@
 
 class Test:
-  def f1: (Int, Int, Int) = (1, 2, 3)
-  def f2: (x: Int, y: Int) = (3, 4)
+  def f1: (Int, String, AnyRef) = (1, "2", "3")
+  def f2: (x: Int, y: String) = (0, "y")
 
   def test1 =
     val (a, b, c) = f1
     // Desugared to:
-    // val $2$: (Int, Int, Int) =
-    //   this.f1:(Int, Int, Int) @unchecked match
+    // val $2$: (Int, String, AnyRef) =
+    //   this.f1:(Int, String, AnyRef) @unchecked match
     //     {
-    //       case $1$ @ Tuple3.unapply[Int, Int, Int](_, _, _) =>
-    //         $1$:(Int, Int, Int)
+    //       case $1$ @ Tuple3.unapply[Int, String, Object](_, _, _) =>
+    //         $1$:(Int, String, AnyRef)
     //     }
     // val a: Int = $2$._1
-    // val b: Int = $2$._2
-    // val c: Int = $2$._3
-    a + b + c
+    // val b: String = $2$._2
+    // val c: AnyRef = $2$._3
+    a + b.length() + c.toString.length()
 
   def test2 =
-    val (_, d, e) = f1
-    e + e
+    val (_, b, c) = f1
+    b.length() + c.toString.length()
+
+    val (a2, _, c2) = f1
+    a2 + c2.toString.length()
 
   def test3 =
-    val (_, f, _) = f1
-    f + f
+    val (_, b, _) = f1
+    b.length() + 1
 
   def test4 =
     val (x, y) = f2
-    x + y
+    x + y.length()
 
   def test5 =
-    val (_, a) = f2
-    a + a
+    val (_, b) = f2
+    b.length() + 1