fixes #24848; presumably-invalid discard R[[R[int]]]() with type R[C] = ref object / b: C generates Error: internal erro

compiler/semtypinst.nim

@@ -296,6 +296,11 @@ proc replaceTypeVarsN(cl: var TReplTypeVars, n: PNode; start=0; expectedType: PT
         replaceTypeVarsS(cl, n.sym, replaceTypeVarsT(cl, n.sym.typ))
     if result.sym.kind == skField and
         (cl.owner == nil or result.sym.owner == cl.owner):
+      if not cl.allowMetaTypes and result.typ != nil and result.typ.isMetaType:
+        localError(cl.c.config, result.info,
+          "'" & result.typ.typeToString & "' is not a concrete type")
+        result.typ = errorType(cl.c)
+        result.sym.typ = result.typ
       if result.sym.ast != nil:
         # instantiate default value of object/tuple field
         var n = result.sym.ast

tests/generics/t24848.nim

@@ -0,0 +1,9 @@
+discard """
+  errormsg: "'array[0..0, typedesc[R[system.int]]]' is not a concrete type"
+  line: 7
+"""
+
+type R[C] = ref object
+  b: C
+
+discard R[[R[int]]]()