Make match types with no matching cases not an error

Modify the MatchReducer to return NoType in the case of no matches, rather than throwing a MatchTypeReductionError.
This makes it consistent with the other match type reduction failures, where being stuck does not result in an error, but simply in an unreduced match type.

We still get the explanations of the underlying error in the MatchTypeTrace, but in positions which need the reduction for conformance, rather than at application site of the match type.

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

@@ -3594,8 +3594,12 @@ class MatchReducer(initctx: Context) extends TypeComparer(initctx) {
             MatchTypeTrace.emptyScrutinee(scrut)
             NoType
       case Nil =>
-        val casesText = MatchTypeTrace.noMatchesText(scrut, cases)
-        throw MatchTypeReductionError(em"Match type reduction $casesText")
+        /* TODO warn ? then re-enable warn/12974.scala:26
+        val noCasesText = MatchTypeTrace.noMatchesText(scrut, cases)
+        report.warning(reporting.MatchTypeNoCases(noCasesText), pos = ???)
+        */
+        MatchTypeTrace.noMatches(scrut, cases)
+        NoType
 
     inFrozenConstraint(recur(cases))
   }

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

@@ -53,9 +53,6 @@ object TypeError:
     def toMessage(using Context) = msg
 end TypeError
 
-class MatchTypeReductionError(msg: Message)(using Context) extends TypeError:
-  def toMessage(using Context) = msg
-
 class MalformedType(pre: Type, denot: Denotation, absMembers: Set[Name])(using Context) extends TypeError:
   def toMessage(using Context) = em"malformed type: $pre is not a legal prefix for $denot because it contains abstract type member${if (absMembers.size == 1) "" else "s"} ${absMembers.mkString(", ")}"
 

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

@@ -663,7 +663,7 @@ trait ImplicitRunInfo:
               traverseChildren(t)
             case t =>
               traverseChildren(t)
-              traverse(try t.normalized catch case _: MatchTypeReductionError => t)
+              traverse(t.normalized)
       catch case ex: Throwable => handleRecursive("collectParts of", t.show, ex)
 
       def apply(tp: Type): collection.Set[Type] =