Normalize mirrorType before reducing to mirrorSource

Author: EugeneFlesselle

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

@@ -339,7 +339,7 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
      *  or a TermRef to a singleton value. These are
      *  the base elements required to generate a mirror.
      */
-    def reduce(mirroredType: Type)(using Context): Either[String, MirrorSource] = mirroredType match
+    def reduce(mirroredType: Type)(using Context): Either[String, MirrorSource] = mirroredType.normalized match
       case tp: TypeRef =>
         val sym = tp.symbol
         if sym.isClass then // direct ref to a class, not an alias

tests/pos/i19198.scala

@@ -0,0 +1,13 @@
+import deriving.Mirror
+import compiletime.summonInline
+
+inline def check1[Tps <: NonEmptyTuple]: Unit =
+  summonInline[Mirror.Of[Tuple.Head[Tps]]]
+
+inline def check2[Tps <: NonEmptyTuple]: Unit =
+  type FromType = Tuple.Head[Tps]
+  summonInline[Mirror.Of[FromType]]
+
+@main def Test: Unit =
+  check1[Option[Int] *: EmptyTuple] // Ok
+  check2[Option[Int] *: EmptyTuple] // Error: FromType is widened to Any in Syntheziser