Fix #7380: Check type members of mirror formals

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

@@ -219,6 +219,12 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
       case _ => true
     loop(formal)
 
+  private def checkRefinement(formal: Type, name: TypeName, expected: Type, span: Span)(using Context): Unit =
+    val actual = formal.lookupRefined(name)
+    if actual.exists && !(expected =:= actual)
+    then report.error(
+      em"$name mismatch, expected: $expected, found: $actual.", ctx.source.atSpan(span))
+
   private def mkMirroredMonoType(mirroredType: HKTypeLambda)(using Context): Type =
     val monoMap = new TypeMap:
       def apply(t: Type) = t match
@@ -260,10 +266,13 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
             case _ =>
               val elems = TypeOps.nestedPairs(accessors.map(mirroredType.memberInfo(_).widenExpr))
               (mirroredType, elems)
+          val elemsLabels = TypeOps.nestedPairs(elemLabels)
+          checkRefinement(formal, tpnme.MirroredElemTypes, elemsType, span)
+          checkRefinement(formal, tpnme.MirroredElemLabels, elemsLabels, span)
           val mirrorType =
             mirrorCore(defn.Mirror_ProductClass, monoType, mirroredType, cls.name, formal)
               .refinedWith(tpnme.MirroredElemTypes, TypeAlias(elemsType))
-              .refinedWith(tpnme.MirroredElemLabels, TypeAlias(TypeOps.nestedPairs(elemLabels)))
+              .refinedWith(tpnme.MirroredElemLabels, TypeAlias(elemsLabels))
           val mirrorRef =
             if (cls.is(Scala2x)) anonymousMirror(monoType, ExtendsProductMirror, span)
             else companionPath(mirroredType, span)

tests/neg/7380.scala

@@ -0,0 +1,14 @@
+import scala.deriving.Mirror
+
+object Test {
+  summon[Mirror.Of[(Int, String)] {
+    type MirroredElemTypes = (Int, Int, Int)
+  }] // error
+  // MirroredElemTypes missmatch, expected: (Int, String), found: (Int, Int, Int).
+
+  summon[Mirror.Of[(Int, String)] {
+    type MirroredElemLabels = ("_1", "_2", "_3")
+  }]  // error
+  // MirroredElemLabels missmatch, expected: (("_1" : String), ("_2" : String)),
+  // found: (("_1" : String), ("_2" : String), ("_3" : String)).
+}