fix #19882 Improve error message when instantiating generics that lac… (#20356)

* fix #19882 Improve error message when instantiating generics that lack a type

* Update tests/errmsgs/t19882.nim

Co-authored-by: Clay Sweetser <Varriount@users.noreply.github.com>

compiler/seminst.nim

@@ -54,7 +54,8 @@ iterator instantiateGenericParamList(c: PContext, n: PNode, pt: TIdTable): PSym
           # later by semAsgn in return type inference scenario
           t = q.typ
         else:
-          localError(c.config, a.info, errCannotInstantiateX % s.name.s)
+          if q.typ.kind != tyCompositeTypeClass:
+            localError(c.config, a.info, errCannotInstantiateX % s.name.s)
           t = errorType(c)
       elif t.kind in {tyGenericParam, tyConcept}:
         localError(c.config, a.info, errCannotInstantiateX % q.name.s)

tests/errmsgs/t19882.nim

@@ -0,0 +1,10 @@
+
+discard """
+  errormsg: "cannot instantiate 'A[T, P]' inside of type definition: 'init'; Maybe generic arguments are missing?"
+"""
+type A[T,P] = object
+  b:T
+  c:P
+proc init(): ref A =      
+  new(result)
+var a = init()