Backport "Fix infinite loop in Mirror synthesis of unreducible match type" to LTS (#21042)

Backports #20133 to the LTS branch.

PR submitted by the release tooling.
[skip ci]

Author: WojciechMazur

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

@@ -378,7 +378,9 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
                 // avoid type aliases for tuples
                 Right(MirrorSource.GenericTuple(types))
               case _ => reduce(tp.underlying)
-          case tp: MatchType => reduce(tp.normalized)
+          case tp: MatchType =>
+            val n = tp.tryNormalize
+            if n.exists then reduce(n) else Left(i"its subpart `$tp` is an unreducible match type.")
           case _ => reduce(tp.superType)
       case tp @ AndType(l, r) =>
         for

tests/neg/i19198.scala

@@ -0,0 +1,13 @@
+import deriving.Mirror
+import compiletime.summonInline
+
+type DoesNotReduce[T] = T match
+  case String => Any
+
+type DoesNotReduce2[T] <: T = T match
+  case String => T
+
+class Foo
+@main def Test: Unit =
+  summonInline[Mirror.Of[DoesNotReduce[Option[Int]]]] // error
+  summonInline[Mirror.Of[DoesNotReduce2[Option[Int]]]] // error