Type If and Match in statement position as Unit

This is an alternative fix for #5750

Author: allanrenucci

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

@@ -728,17 +728,11 @@ class Typer extends Namer
   def typedIf(tree: untpd.If, pt: Type)(implicit ctx: Context): Tree = track("typedIf") {
     if (tree.isInline) checkInInlineContext("inline if", tree.posd)
     val cond1 = typed(tree.cond, defn.BooleanType)
-
-    if (tree.elsep.isEmpty) {
-      val thenp1 = typed(tree.thenp, defn.UnitType)
-      val elsep1 = tpd.unitLiteral.withSpan(tree.span.endPos)
-      cpy.If(tree)(cond1, thenp1, elsep1).withType(defn.UnitType)
-    }
-    else {
-      val thenp1 :: elsep1 :: Nil = harmonic(harmonize, pt)(
-        (tree.thenp :: tree.elsep :: Nil).map(typed(_, pt.notApplied)))
-      assignType(cpy.If(tree)(cond1, thenp1, elsep1), thenp1, elsep1)
+    val thenp1 :: elsep1 :: Nil = harmonic(harmonize, pt) {
+      val elsep = tree.elsep.orElse(untpd.unitLiteral.withSpan(tree.span.endPos))
+      (tree.thenp :: elsep :: Nil).map(typed(_, pt.notApplied))
     }
+    assignType(cpy.If(tree)(cond1, thenp1, elsep1), thenp1, elsep1)
   }
 
   /** Decompose function prototype into a list of parameter prototypes and a result prototype
@@ -2089,7 +2083,15 @@ class Typer extends Namer
       case Thicket(stats) :: rest =>
         traverse(stats ++ rest)
       case stat :: rest =>
-        val stat1 = typed(stat)(ctx.exprContext(stat, exprOwner))
+        val pt = stat match {
+          case _: untpd.If | _: untpd.Match =>
+            // Typing `if` and `match` statement with `Unit` as expected
+            // type produces more efficient code (see #5750).
+            defn.UnitType
+          case _ =>
+            WildcardType
+        }
+        val stat1 = typed(stat, pt)(ctx.exprContext(stat, exprOwner))
         checkStatementPurity(stat1)(stat, exprOwner)
         buf += stat1
         traverse(rest)

compiler/test/dotty/tools/backend/jvm/DottyBytecodeTests.scala

@@ -627,9 +627,12 @@ class TestBCode extends DottyBytecodeTest {
   @Test def i5750 = {
     val source =
       """class Test {
-        |  def foo: String = ""
+        |  def foo(): String = ""
         |  def test(cond: Boolean): Int = {
-        |    if (cond) foo
+        |    if (cond) foo()
+        |    if (cond) () else foo()
+        |    if (cond) foo() else ()
+        |    cond match { case true => foo(); case _ => () }
         |    1
         |  }
         |}

compiler/test/dotty/tools/dotc/reporting/ErrorMessagesTests.scala

@@ -1367,7 +1367,7 @@ class ErrorMessagesTests extends ErrorMessagesTest {
         |  val z = Boo(1, "foo")
         |
         |  z match {
-        |    case Boo(a, b, c) => a
+        |    case Boo(a, b, c) => ()
         |  }
         |}
       """.stripMargin

tests/patmat/enum/patmat-enum.scala

@@ -2,9 +2,9 @@ object Test1 {
   val day: Day = ???
 
   day match {
-    case Day.MONDAY => true
-    case Day.TUESDAY => true
-    case Day.WEDNESDAY => true
+    case Day.MONDAY => ()
+    case Day.TUESDAY => ()
+    case Day.WEDNESDAY => ()
   }
 }
 
@@ -13,9 +13,9 @@ object Test2 {
   val day: Day = ???
 
   day match {
-    case MONDAY => true
-    case TUESDAY => true
-    case WEDNESDAY => true
-    case SUNDAY => true
+    case MONDAY => ()
+    case TUESDAY => ()
+    case WEDNESDAY => ()
+    case SUNDAY => ()
   }
 }

tests/patmat/t3111.scala

@@ -2,12 +2,12 @@ object Test {
   val bool: Boolean = false
 
   bool match {
-    case true => "true!"
+    case true => ()
   }
 
   bool match {
-    case true => "true!"
-    case false => "false!"
-    case _ => "cats and dogs living together... mass hysteria!"
+    case true => ()
+    case false => ()
+    case _ => ()
   }
 }

tests/patmat/tuple.scala

@@ -1,5 +1,5 @@
 object Test {
   (4, (4, 6)) match {
-    case (x, (y, z)) => true
+    case (x, (y, z)) => ()
   }
 }

tests/pos-special/isInstanceOf/3324d.scala

@@ -2,7 +2,7 @@ class Test {
   val x: Any = ???
 
   x match {
-    case _: List[Int @unchecked] => 5
-    case _: Option[Int] @unchecked => 5
+    case _: List[Int @unchecked] => ()
+    case _: Option[Int] @unchecked => ()
   }
 }

tests/run/virtpatmat_alts.decompiled

@@ -2,7 +2,7 @@
 object Test extends dotty.runtime.LegacyApp() {
   scala.Tuple2.apply[scala.Boolean, scala.Boolean](true, true) match {
     case (scala.Tuple2(true, true) | scala.Tuple2(false, false)) =>
-      1
+      ()
   }
   scala.List.apply[scala.Int](5) match {
     case (scala.::(1, scala.Nil) | scala.::(2, scala.Nil)) =>

tests/run/virtpatmat_alts.scala

@@ -3,7 +3,7 @@
  */
 object Test extends dotty.runtime.LegacyApp {
   (true, true) match {
-    case (true, true) | (false, false) => 1
+    case (true, true) | (false, false) => ()
   }
 
   List(5) match {