Fix failure to parse subclass added via add_argument and required arg…

… as link target.

CHANGELOG.rst

@@ -12,6 +12,15 @@ The semantic versioning only considers the public API as described in
 paths are considered internals and can change in minor and patch releases.
 
 
+v4.26.2 (2023-10-??)
+--------------------
+
+Fixed
+^^^^^
+- Failure to parse subclass added via add_argument and required arg as link
+  target.
+
+
 v4.26.1 (2023-10-23)
 --------------------
 

jsonargparse/_link_arguments.py

@@ -177,7 +177,7 @@ def __init__(
         if target in parser.required_args:
             parser.required_args.remove(target)
         if is_target_subclass and not valid_target_leaf:
-            sub_add_kwargs = getattr(self.target[1], "sub_add_kwargs", {})
+            sub_add_kwargs = self.target[1].sub_add_kwargs  # type: ignore
             if "linked_targets" not in sub_add_kwargs:
                 sub_add_kwargs["linked_targets"] = set()
             subtarget = target.split(".init_args.", 1)[1]

jsonargparse/_typehints.py

@@ -184,6 +184,7 @@ def __init__(self, typehint: Optional[Type] = None, enable_path: bool = False, *
         else:
             self._typehint = kwargs.pop("_typehint")
             self._enable_path = kwargs.pop("_enable_path")
+            self.sub_add_kwargs: dict = {}
             if "metavar" not in kwargs:
                 kwargs["metavar"] = typehint_metavar(self._typehint)
             super().__init__(**kwargs)

jsonargparse_tests/test_link_arguments.py

@@ -637,6 +637,38 @@ def test_on_instantiate_subclass_link_ignored_missing_param(parser, caplog):
     assert "'v.init_args.v1 --> w.init_args.w2' ignored since target" in caplog.text
 
 
+class SubRequired:
+    def __init__(self):
+        pass
+
+
+class RequiredTarget:
+    def __init__(
+        self,
+        a: int,
+        b: SubRequired,
+    ):
+        self.a = a
+        self.b = b
+
+
+class RequiredSource:
+    def __init__(self, a: int):
+        self.a = a
+
+
+def test_on_instantiate_add_argument_subclass_required_params(parser):
+    parser.add_argument("--cls1", type=RequiredSource)
+    parser.add_argument("--cls2", type=RequiredTarget)
+    parser.link_arguments("cls1.a", "cls2.init_args.a", apply_on="instantiate")
+    cfg = parser.parse_args(["--cls1=RequiredSource", "--cls1.a=1", "--cls2=RequiredTarget", "--cls2.b=SubRequired"])
+    init = parser.instantiate_classes(cfg)
+    assert isinstance(init.cls1, RequiredSource)
+    assert isinstance(init.cls2, RequiredTarget)
+    assert isinstance(init.cls2.b, SubRequired)
+    assert init.cls2.a == 1
+
+
 # link creation failures