Restore MatchTypeNoCases warning with correct source position

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

@@ -3974,10 +3974,19 @@ class MatchReducer(initctx: Context) extends TypeComparer(initctx) {
             MatchTypeTrace.emptyScrutinee(scrut)
             NoType
       case Nil =>
-        /* TODO warn ? then re-enable warn/12974.scala:26
         val noCasesText = MatchTypeTrace.noMatchesText(scrut, cases)
-        report.warning(reporting.MatchTypeNoCases(noCasesText), pos = ???)
-        */
+        val pos = ctx.source.atSpan(ctx.owner.span)
+        val isStableScrut = !scrut.isInstanceOf[TypeVar]
+
+        // Emit warn for user-facing term-level typing, and avoid intermediate / redundant reports
+        val doEmit =
+          pos.exists
+            && ctx.owner.isTerm
+            && isStableScrut
+            && ctx.phase.isTyper
+        if doEmit then
+          report.warning(reporting.MatchTypeNoCases(noCasesText), pos)
+
         MatchTypeTrace.noMatches(scrut, cases)
         NoType
 

tests/neg/i12049.check

@@ -103,3 +103,19 @@
    |          Therefore, reduction cannot advance to the remaining case
    |
    |            case B => String
+-- [E184] Type Warning: tests/neg/i12049.scala:14:4 --------------------------------------------------------------------
+14 |val y3: String = ??? : Last[Int *: Int *: Boolean *: String *: EmptyTuple]  // error
+   |    ^
+   |    Match type reduction failed since selector EmptyTuple
+   |    matches none of the cases
+   |
+   |        case _ *: _ *: t => Last[t]
+   |        case t *: EmptyTuple => t
+-- [E184] Type Warning: tests/neg/i12049.scala:22:4 --------------------------------------------------------------------
+22 |val z3: (A, B, A) = ??? : Reverse[(A, B, A)] // error
+   |    ^
+   |    Match type reduction failed since selector A *: EmptyTuple.type
+   |    matches none of the cases
+   |
+   |        case t1 *: t2 *: ts => Tuple.Concat[Reverse[ts], (t2, t1)]
+   |        case EmptyTuple => EmptyTuple

tests/warn/12974.scala

@@ -23,7 +23,7 @@ object RecMap {
   def main(args: Array[String]) =
     import Record._
 
-    val foo: Any = Rec.empty.fetch("foo") // TODO
+    val foo: Any = Rec.empty.fetch("foo") // warn
     //                            ^
     // Match type reduction failed since selector  EmptyTuple.type
     // matches none of the cases

tests/warn/i23822.scala

@@ -0,0 +1,16 @@
+object MatchTypes:
+
+  type ConstituentPartOf[A] = A match
+    case BigInt => Int
+    case String => Char
+
+  def lastPartOf[A](thing: A): ConstituentPartOf[A] = thing match
+    case number: BigInt => (number % 10).toInt
+    case string: String => string.charAt(string.length - 1)
+
+  def main(args: Array[String]): Unit =
+    val lastPartOfSomethingElse = lastPartOf(BigDecimal("10")) // warn
+    //                                                ^
+    // Match type reduction failed since selector BigDecimal
+    // matches none of the cases
+    println(lastPartOfSomethingElse)