Evaluate annotations for dataclasses when infer is called on param

jedi/plugins/stdlib.py

@@ -580,7 +580,10 @@ class DataclassParamName(BaseTreeParamName):
         return self._default_node
 
     def infer(self):
+        if self._annotation_node is None:
             return NO_CONTEXTS  # TODO implement
+        else:
+            return self.parent_context.eval_node(self._annotation_node)
 
 
 class ItemGetterCallable(ContextWrapper):

test/test_evaluate/test_signature.py

@@ -86,12 +86,16 @@ def test_dataclass_signature(Script, skip_pre_python37, start, start_params):
     code = dedent('''
             name: str
             foo = 3
-            unit_price: float
-            quantity_on_hand: int = 0
+            price: float
+            quantity: int = 0.0
 
         X(''')
 
     code = 'from dataclasses import dataclass\n' + start + code
 
     sig, = Script(code).call_signatures()
-    assert [p.name for p in sig.params] == start_params + ['name', 'unit_price', 'quantity_on_hand']
+    assert [p.name for p in sig.params] == start_params + ['name', 'price', 'quantity']
+    quantity, = sig.params[-1].infer()
+    assert quantity.name == 'int'
+    price, = sig.params[-2].infer()
+    assert price.name == 'float'