Fix recursive generic typed defs (#18809)

compiler/semtypinst.nim

@@ -378,7 +378,7 @@ proc handleGenericInvocation(cl: var TReplTypeVars, t: PType): PType =
   cl.typeMap = newTypeMapLayer(cl)
 
   for i in 1..<t.len:
-    var x = replaceTypeVarsT(cl, t[i])
+    var x = replaceTypeVarsT(cl, header[i])
     assert x.kind != tyGenericInvocation
     header[i] = x
     propagateToOwner(header, x)

tests/generics/trecursivegenerics.nim

@@ -0,0 +1,12 @@
+block: # Replicates #18728
+  type
+    FlipFlop[A, B] = ref object
+      next: FlipFlop[B, A]
+  
+    Trinary[A, B, C] = ref object
+      next: Trinary[B, C, A]
+  
+  assert typeof(FlipFlop[int, string]().next) is FlipFlop[string, int]
+  assert typeof(FlipFlop[string, int]().next) is FlipFlop[int, string]
+  assert typeof(Trinary[int, float, string]().next) is Trinary[float, string, int]
+  assert typeof(Trinary[int, float, string]().next.next) is Trinary[string, int, float]