Improve error message when instantiating generics with object constructor (#20358)

* Improve error message when instantiating generics with object constructor

* follow suggestion

* Update compiler/semobjconstr.nim

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

* Update tests/errmsgs/t19882_2.nim

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

Author: bung87

compiler/semobjconstr.nim

@@ -398,8 +398,12 @@ proc semObjConstr(c: PContext, n: PNode, flags: TExprFlags; expectedType: PType
       # multiple times as long as they don't have closures.
       result.typ.flags.incl tfHasOwned
   if t.kind != tyObject:
-    return localErrorNode(c, result,
-      "object constructor needs an object type".dup(addDeclaredLoc(c.config, t)))
+    return localErrorNode(c, result, if t.kind != tyGenericBody:
+      "object constructor needs an object type".dup(addDeclaredLoc(c.config, t))
+      else: "cannot instantiate: '" &
+        typeToString(t, preferDesc) &
+        "'; the object's generic parameters cannot be inferred and must be explicitly given"
+      )
 
   # Check if the object is fully initialized by recursively testing each
   # field (if this is a case object, initialized fields in two different

tests/errmsgs/t19882_2.nim

@@ -0,0 +1,5 @@
+discard """
+  errormsg: "cannot instantiate: 'A[T]'; the object's generic parameters cannot be inferred and must be explicitly given"
+"""
+type A[T] = object
+var a = A()