Initialize type params for kind-arity check

The type params might have type params of their own.

src/compiler/scala/tools/nsc/typechecker/Typers.scala

@@ -5551,15 +5551,12 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
           if (sameLength(tparams, args)) {
             // @M: kind-arity checking is done here and in adapt, full kind-checking is in checkKindBounds (in Infer)
             val args1 = map2Conserve(args, tparams) { (arg, tparam) =>
-              def ptParams = Kind.FromParams(tparam.typeParams)
-
               // if symbol hasn't been fully loaded, can't check kind-arity except when we're in a pattern,
               // where we can (we can't take part in F-Bounds) and must (scala/bug#8023)
-              val pt = if (mode.typingPatternOrTypePat) {
-                tparam.initialize; ptParams
-              }
-              else if (isComplete) ptParams
-              else Kind.Wildcard
+              val pt = if (mode.typingPatternOrTypePat || isComplete)
+                Kind.FromParams(tparam.initialize.typeParams)
+              else
+                Kind.Wildcard
 
               typedHigherKindedType(arg, mode, pt)
             }

test/files/pos/t12187.scala

@@ -0,0 +1,8 @@
+object Test {
+  trait Foo[S[_[_], _[_]]] extends Bar[S] {
+    def m[F[_]](x: S[({ type G[A] = Bar[S] })#G, F]): Unit
+  }
+  trait Bar[S[_[_], _[_]]] {
+    def m[F[_]](x: S[({ type G[A] = Bar[S] })#G, F]): Unit
+  }
+}