Infer typing.cast return depending on whether argument is inferable

In the case of typing.cast(A, x) where x is inferable, we prefer to
yield the type of x since we don't want to lose information. If x is
uninferable, then treating it as an instance of A is reasonable.

astroid/brain/brain_typing.py

@@ -10,6 +10,7 @@
 # Copyright (c) 2021 hippo91 <guillaume.peillex@gmail.com>
 
 """Astroid hooks for typing.py support."""
+import itertools
 import typing
 from functools import partial
 
@@ -365,8 +366,12 @@ def infer_typing_cast(
     if func.qname() != "typing.cast" or len(node.args) != 2:
         raise UseInferenceDefault
 
-    type_node = next(node.args[0].infer(context=ctx))
-    return iter([Instance(type_node)])
+    peek_value_node, value_node = itertools.tee(node.args[1].infer(context=ctx))
+    if next(peek_value_node) is Uninferable:
+        type_node = next(node.args[0].infer(context=ctx))
+        return iter([Instance(type_node)])
+    else:
+        return value_node
 
 
 AstroidManager().register_transform(

tests/unittest_brain.py

@@ -1801,7 +1801,8 @@ def test_typing_object_builtin_subscriptable(self):
             self.assertIsInstance(inferred, nodes.ClassDef)
             self.assertIsInstance(inferred.getattr("__iter__")[0], nodes.FunctionDef)
 
-    def test_typing_cast(self):
+    def test_typing_cast_uninferable(self):
+        """cast will yield instance of casted-to type if not otherwise inferable"""
         node = builder.extract_node(
             """
         from typing import cast
@@ -1817,6 +1818,23 @@ class A:
         assert isinstance(inferred, bases.Instance)
         assert inferred.name == "A"
 
+    def test_typing_cast_inferable(self):
+        """cast leaves type unchanged if it has been inferred"""
+        node = builder.extract_node(
+            """
+        from typing import cast
+        class A:
+            pass
+
+        b = list()
+        a = cast(A, b)
+        a
+        """
+        )
+        inferred = next(node.infer())
+        assert isinstance(inferred, bases.Instance)
+        assert inferred.name == "list"
+
     def test_typing_cast_attribute(self):
         node = builder.extract_node(
             """