[suggest] Support refining existing type annotations (#7838)

mypy/suggestions.py

diff --git a/mypy/suggestions.py b/mypy/suggestions.py
index f01dc9b..7bb4a58 100644
--- a/mypy/suggestions.py
+++ b/mypy/suggestions.py
@@ -288,6 +288,12 @@
             AnyType(TypeOfAny.special_form),
             self.builtin_type('builtins.function'))
 
+    def get_starting_type(self, fdef: FuncDef) -> CallableType:
+        if isinstance(fdef.type, CallableType):
+            return fdef.type
+        else:
+            return self.get_trivial_type(fdef)
+
     def get_args(self, is_method: bool,
                  base: CallableType, defaults: List[Optional[Type]],
                  callsites: List[Callsite],
@@ -356,11 +362,12 @@
         """
         options = self.get_args(is_method, base, defaults, callsites, uses)
         options = [self.add_adjustments(tps) for tps in options]
-        return [base.copy_modified(arg_types=list(x)) for x in itertools.product(*options)]
+        return [refine_callable(base, base.copy_modified(arg_types=list(x)))
+                for x in itertools.product(*options)]
 
     def get_callsites(self, func: FuncDef) -> Tuple[List[Callsite], List[str]]:
         """Find all call sites of a function."""
-        new_type = self.get_trivial_type(func)
+        new_type = self.get_starting_type(func)
 
         collector_plugin = SuggestionPlugin(func.fullname())
 
@@ -413,7 +420,7 @@
         with strict_optional_set(graph[mod].options.strict_optional):
             guesses = self.get_guesses(
                 is_method,
-                self.get_trivial_type(node),
+                self.get_starting_type(node),
                 self.get_default_arg_types(graph[mod], node),
                 callsites,
                 uses,
@@ -432,7 +439,7 @@
             else:
                 ret_types = [NoneType()]
 
-        guesses = [best.copy_modified(ret_type=t) for t in ret_types]
+        guesses = [best.copy_modified(ret_type=refine_type(best.ret_type, t)) for t in ret_types]
         guesses = self.filter_options(guesses, is_method)
         best, errors = self.find_best(node, guesses)
 
@@ -593,8 +600,9 @@
         """
         old = func.unanalyzed_type
         # During reprocessing, unanalyzed_type gets copied to type (by aststrip).
-        # We don't modify type because it isn't necessary and it
-        # would mess up the snapshotting.
+        # We set type to None to ensure that the type always changes during
+        # reprocessing.
+        func.type = None
         func.unanalyzed_type = typ
         try:
             res = self.fgmanager.trigger(func.fullname())
@@ -682,6 +690,8 @@
         if isinstance(t, UnionType):
             if any(isinstance(x, AnyType) for x in t.items):
                 return 20
+            if any(has_any_type(x) for x in t.items):
+                return 15
             if not is_optional(t):
                 return 10
         if isinstance(t, CallableType) and (has_any_type(t) or is_tricky_callable(t)):
@@ -840,6 +850,105 @@
     return len([x for x in msgs if ' error: ' in x])
 
 
+def refine_type(ti: Type, si: Type) -> Type:
+    """Refine `ti` by replacing Anys in it with information taken from `si`
+
+    This basically works by, when the types have the same structure,
+    traversing both of them in parallel and replacing Any on the left
+    with whatever the type on the right is. If the types don't have the
+    same structure (or aren't supported), the left type is chosen.
+
+    For example:
+      refine(Any, T) = T,  for all T
+      refine(float, int) = float
+      refine(List[Any], List[int]) = List[int]
+      refine(Dict[int, Any], Dict[Any, int]) = Dict[int, int]
+      refine(Tuple[int, Any], Tuple[Any, int]) = Tuple[int, int]
+
+      refine(Callable[[Any], Any], Callable[[int], int]) = Callable[[int], int]
+      refine(Callable[..., int], Callable[[int, float], Any]) = Callable[[int, float], int]
+
+      refine(Optional[Any], int) = Optional[int]
+      refine(Optional[Any], Optional[int]) = Optional[int]
+      refine(Optional[Any], Union[int, str]) = Optional[Union[int, str]]
+      refine(Optional[List[Any]], List[int]) = List[int]
+
+    """
+    t = get_proper_type(ti)
+    s = get_proper_type(si)
+
+    if isinstance(t, AnyType):
+        return s
+
+    if isinstance(t, Instance) and isinstance(s, Instance) and t.type == s.type:
+        return t.copy_modified(args=[refine_type(ta, sa) for ta, sa in zip(t.args, s.args)])
+
+    if (
+        isinstance(t, TupleType)
+        and isinstance(s, TupleType)
+        and t.partial_fallback == s.partial_fallback
+        and len(t.items) == len(s.items)
+    ):
+        return t.copy_modified(items=[refine_type(ta, sa) for ta, sa in zip(t.items, s.items)])
+
+    if isinstance(t, CallableType) and isinstance(s, CallableType):
+        return refine_callable(t, s)
+
+    if isinstance(t, UnionType):
+        return refine_union(t, s)
+
+    # TODO: Refining of builtins.tuple, Type?
+
+    return t
+
+
+def refine_union(t: UnionType, s: ProperType) -> Type:
+    """Refine a union type based on another type.
+
+    This is done by refining every component of the union against the
+    right hand side type (or every component of its union if it is
+    one). If an element of the union is succesfully refined, we drop it
+    from the union in favor of the refined versions.
+    """
+    rhs_items = s.items if isinstance(s, UnionType) else [s]
+
+    new_items = []
+    for lhs in t.items:
+        refined = False
+        for rhs in rhs_items:
+            new = refine_type(lhs, rhs)
+            if new != lhs:
+                new_items.append(new)
+                refined = True
+        if not refined:
+            new_items.append(lhs)
+
+    # Turn strict optional on when simplifying the union since we
+    # don't want to drop Nones.
+    with strict_optional_set(True):
+        return make_simplified_union(new_items)
+
+
+def refine_callable(t: CallableType, s: CallableType) -> CallableType:
+    """Refine a callable based on another.
+
+    See comments for refine_type.
+    """
+    if t.fallback != s.fallback:
+        return t
+
+    if t.is_ellipsis_args and not is_tricky_callable(s):
+        return s.copy_modified(ret_type=refine_type(t.ret_type, s.ret_type))
+
+    if is_tricky_callable(t) or t.arg_kinds != s.arg_kinds:
+        return t
+
+    return t.copy_modified(
+        arg_types=[refine_type(ta, sa) for ta, sa in zip(t.arg_types, s.arg_types)],
+        ret_type=refine_type(t.ret_type, s.ret_type),
+    )
+
+
 T = TypeVar('T')
 
 

test-data/unit/fine-grained-suggest.test

diff --git a/test-data/unit/fine-grained-suggest.test b/test-data/unit/fine-grained-suggest.test
index 8968641..c2b5a4c 100644
--- a/test-data/unit/fine-grained-suggest.test
+++ b/test-data/unit/fine-grained-suggest.test
@@ -906,3 +906,144 @@
 ==
 foo.py:4: error: unexpected EOF while parsing
 -- )
+
+[case testSuggestRefine]
+# suggest: foo.foo
+# suggest: foo.spam
+# suggest: foo.eggs
+# suggest: foo.take_l
+# suggest: foo.union
+# suggest: foo.callable1
+# suggest: foo.callable2
+# suggest: foo.optional1
+# suggest: foo.optional2
+# suggest: foo.optional3
+# suggest: foo.optional4
+# suggest: foo.optional5
+# suggest: foo.dict1
+# suggest: foo.tuple1
+[file foo.py]
+from typing import Any, List, Union, Callable, Optional, Set, Dict, Tuple
+
+def bar():
+    return 10
+
+def foo(x: int, y):
+    return x + y
+
+foo(bar(), 10)
+
+def spam(x: int, y: Any) -> Any:
+    return x + y
+
+spam(bar(), 20)
+
+def eggs(x: int) -> List[Any]:
+    a = [x]
+    return a
+
+def take_l(x: List[Any]) -> Any:
+    return x[0]
+
+test = [10, 20]
+take_l(test)
+
+def union(x: Union[int, str]):
+    pass
+
+union(10)
+
+def add1(x: float) -> int:
+    pass
+
+def callable1(f: Callable[[int], Any]):
+    return f(10)
+
+callable1(add1)
+
+def callable2(f: Callable[..., Any]):
+    return f(10)
+
+callable2(add1)
+
+def optional1(x: Optional[Any]):
+    pass
+
+optional1(10)
+
+def optional2(x: Union[None, int, Any]):
+    if x is None:
+        pass
+    elif isinstance(x, str):
+        pass
+    else:
+        add1(x)
+
+optional2(10)
+optional2('test')
+
+def optional3(x: Optional[List[Any]]):
+    assert not x
+    return x[0]
+
+optional3(test)
+
+set_test = {1, 2}
+
+def optional4(x: Union[Set[Any], List[Any]]):
+    pass
+
+optional4(test)
+optional4(set_test)
+
+def optional5(x: Optional[Any]):
+    pass
+
+optional5(10)
+optional5(None)
+
+def dict1(d: Dict[int, Any]):
+    pass
+
+d: Dict[Any, int]
+dict1(d)
+
+def tuple1(d: Tuple[int, Any]):
+    pass
+
+t: Tuple[Any, int]
+tuple1(t)
+
+[builtins fixtures/isinstancelist.pyi]
+[out]
+(int, int) -> int
+(int, int) -> int
+(int) -> foo.List[int]
+(foo.List[int]) -> int
+(Union[int, str]) -> None
+(Callable[[int], int]) -> int
+(Callable[[float], int]) -> int
+(Optional[int]) -> None
+(Union[None, int, str]) -> None
+(Optional[foo.List[int]]) -> int
+(Union[foo.Set[int], foo.List[int]]) -> None
+(Optional[int]) -> None
+(foo.Dict[int, int]) -> None
+(Tuple[int, int]) -> None
+==
+
+[case testSuggestRefine2]
+# suggest: foo.optional5
+[file foo.py]
+from typing import Optional, Any
+
+def optional5(x: Optional[Any]):
+    pass
+
+optional5(10)
+optional5(None)
+
+[builtins fixtures/isinstancelist.pyi]
+[out]
+(Optional[int]) -> None
+==