Restore unreachable warnings by converting literals

The fix implemented to deal with primitive types auto-converting was
suppressing a series of legitimate unreachable warnings.  So we removed
that fix and tried to mirror the auto-conversion of the type, such that
the space intersection calculation ends up with the correct results.

In doing so we triggered reachability warnings being emitted by
sealed trait companion object (or enum) `ordinal` synthetic methods, in
some weird test cases.  So we added an `@unchecked` annotation to
suppress those.

Co-authored-by: Seth Tisue <[email protected]>

Author: dwijnand

compiler/src/dotty/tools/dotc/transform/patmat/Space.scala

@@ -503,8 +503,21 @@ class SpaceEngine(using Context) extends SpaceLogic {
     }
   }
 
+  /** Numeric literals, while being constant values of unrelated types (e.g. Char and Int),
+   *  when used in a case may end up matching at runtime, because their equals may returns true.
+   *  Because these are universally available, general purpose types, it would be good to avoid
+   *  returning false positive warnings, such as in `(c: Char) match { case 67 => ... }` emitting a
+   *  reachability warning on the case.  So the type `ConstantType(Constant(67, IntTag))` is
+   *  converted to `ConstantType(Constant(67, CharTag))`.  #12805 */
+  def convertConstantType(tp: Type, pt: Type): Type = tp match
+    case tp @ ConstantType(const) =>
+      val converted = const.convertTo(pt)
+      if converted == null then tp else ConstantType(converted)
+    case _ => tp
+
   /** Is `tp1` a subtype of `tp2`?  */
-  def isSubType(tp1: Type, tp2: Type): Boolean = {
+  def isSubType(_tp1: Type, tp2: Type): Boolean = {
+    val tp1 = convertConstantType(_tp1, tp2)
     debug.println(TypeComparer.explained(_.isSubType(tp1, tp2)))
     val res = if (ctx.explicitNulls) {
       tp1 <:< tp2
@@ -750,6 +763,7 @@ class SpaceEngine(using Context) extends SpaceLogic {
   }
 
   def show(ss: Seq[Space]): String = ss.map(show).mkString(", ")
+
   /** Display spaces */
   def show(s: Space): String = {
     def params(tp: Type): List[Type] = tp.classSymbol.primaryConstructor.info.firstParamTypes
@@ -894,13 +908,15 @@ class SpaceEngine(using Context) extends SpaceLogic {
       else
         project(OrType(selTyp, constantNullType, soft = false))
 
+    debug.println(s"targetSpace: ${show(targetSpace)}")
+
     // in redundancy check, take guard as false in order to soundly approximate
     val spaces = cases.map { x =>
       val res =
         if (x.guard.isEmpty) project(x.pat)
         else Empty
 
-      debug.println(s"${x.pat.show} ====> ${res}")
+      debug.println(s"${x.pat.show} ====> ${show(res)}")
       res
     }
 
@@ -914,12 +930,8 @@ class SpaceEngine(using Context) extends SpaceLogic {
         debug.println(s"---------------reachable? ${show(curr)}")
         debug.println(s"prev: ${show(prevs)}")
 
-        var covered = simplify(intersect(curr, targetSpace))
-        debug.println(s"covered: $covered")
-
-        // `covered == Empty` may happen for primitive types with auto-conversion
-        // see tests/patmat/reader.scala  tests/patmat/byte.scala
-        if (covered == Empty && !isNullLit(pat)) covered = curr
+        val covered = simplify(intersect(curr, targetSpace))
+        debug.println(s"covered: ${show(covered)}")
 
         if (isSubspace(covered, prevs)) {
           if i == cases.length - 1

tests/patmat/i12805.check

@@ -0,0 +1,3 @@
+10: Match case Unreachable
+16: Match case Unreachable
+22: Match case Unreachable

tests/patmat/i12805.scala

@@ -0,0 +1,22 @@
+import scala.language.implicitConversions
+
+type Timeframe = "1m" | "2m" | "1H"
+type TimeframeN = 1 | 2 | 60
+
+def manualConvertToN(tf: Timeframe): TimeframeN = tf match
+  case "1m" => 1
+  case "2m" => 2
+  case "1H" => 60
+  case "4H" => ??? // was: no reachability warning
+
+given Conversion[Timeframe, TimeframeN] =
+  case "1m" => 1
+  case "2m" => 2
+  case "1H" => 60
+  case "4H" => ??? // was: no reachability warning
+
+given Conversion[TimeframeN, Timeframe] =
+  case 1 => "1m"
+  case 2 => "2m"
+  case 60 => "1H"
+  case 240 => ??? // was: no reachability warning

tests/patmat/i12805b.check

@@ -0,0 +1,3 @@
+4: Match case Unreachable
+9: Match case Unreachable
+14: Match case Unreachable

tests/patmat/i12805b.scala

@@ -0,0 +1,14 @@
+def test1(a: 1 | 2) = a match
+  case 1 => true
+  case 2 => false
+  case 4 => ??? // unreachable case, was: no warning
+
+def test2(a: 1 | 2) = a match
+  case 1 => true
+  case 2 => false
+  case _ => ??? // unreachable
+
+def test3(a: 1 | 2) = a match
+  case 1 => true
+  case 2 => false
+  case a if a < 0 => ??? // unreachable