Make it possible for users to infer annotations/defaults

Fixes #1039

jedi/api/classes.py

@@ -618,7 +618,7 @@ class Signature(Definition):
 
     @property
     def params(self):
-        return [Definition(self._evaluator, n)
+        return [ParamDefinition(self._evaluator, n)
                 for n in self._signature.get_param_names(resolve_stars=True)]
 
 
@@ -660,6 +660,14 @@ class CallSignature(Signature):
         )
 
 
+class ParamDefinition(Definition):
+    def infer_default(self):
+        return [Definition(self._evaluator, d.name) for d in self._name.infer_default()]
+
+    def infer_annotation(self):
+        return [Definition(self._evaluator, d.name) for d in self._name.infer_annotation()]
+
+
 def _format_signatures(context):
     return '\n'.join(
         signature.to_string()

jedi/evaluate/names.py

@@ -3,7 +3,7 @@ from abc import abstractmethod
 from parso.tree import search_ancestor
 
 from jedi._compatibility import Parameter
-from jedi.evaluate.base_context import ContextSet
+from jedi.evaluate.base_context import ContextSet, NO_CONTEXTS
 from jedi.cache import memoize_method
 
 
@@ -220,6 +220,18 @@ class ParamName(BaseTreeParamName):
     def annotation_node(self):
         return self._get_param_node().annotation
 
+    def infer_annotation(self):
+        node = self.annotation_node
+        if node is None:
+            return NO_CONTEXTS
+        return self.parent_context.parent_context.eval_node(node)
+
+    def infer_default(self):
+        node = self.default_node
+        if node is None:
+            return NO_CONTEXTS
+        return self.parent_context.parent_context.eval_node(node)
+
     @property
     def default_node(self):
         return self._get_param_node().default