Fix union simplification performance regression (#12519)

#11962 can generate large unions with many Instance types with
last_known_value set. This caused our union simplification algorithm
to be extremely slow, as it hit an O(n**2) code path.

We already had a fast code path for unions of regular literal types. This
generalizes it for unions containing Instance types with last known
values (which behave similarly to literals in a literal type context).

Also fix a union simplification bug that I encountered while writing tests
for this change.

Work on #12408.

mypy/test/testtypes.py

@@ -465,7 +465,12 @@ def test_simplified_union(self) -> None:
         self.assert_simplified_union([fx.a, UnionType([fx.a])], fx.a)
         self.assert_simplified_union([fx.b, UnionType([fx.c, UnionType([fx.d])])],
                                      UnionType([fx.b, fx.c, fx.d]))
+
+    def test_simplified_union_with_literals(self) -> None:
+        fx = self.fx
+
         self.assert_simplified_union([fx.lit1, fx.a], fx.a)
+        self.assert_simplified_union([fx.lit1, fx.lit2, fx.a], fx.a)
         self.assert_simplified_union([fx.lit1, fx.lit1], fx.lit1)
         self.assert_simplified_union([fx.lit1, fx.lit2], UnionType([fx.lit1, fx.lit2]))
         self.assert_simplified_union([fx.lit1, fx.lit3], UnionType([fx.lit1, fx.lit3]))
@@ -481,6 +486,40 @@ def test_simplified_union(self) -> None:
         self.assert_simplified_union([fx.lit1, fx.lit2_inst], UnionType([fx.lit1, fx.lit2_inst]))
         self.assert_simplified_union([fx.lit1, fx.lit3_inst], UnionType([fx.lit1, fx.lit3_inst]))
 
+    def test_simplified_union_with_str_literals(self) -> None:
+        fx = self.fx
+
+        self.assert_simplified_union([fx.lit_str1, fx.lit_str2, fx.str_type], fx.str_type)
+        self.assert_simplified_union([fx.lit_str1, fx.lit_str1, fx.lit_str1], fx.lit_str1)
+        self.assert_simplified_union([fx.lit_str1, fx.lit_str2, fx.lit_str3],
+                                     UnionType([fx.lit_str1, fx.lit_str2, fx.lit_str3]))
+        self.assert_simplified_union([fx.lit_str1, fx.lit_str2, fx.uninhabited],
+                                     UnionType([fx.lit_str1, fx.lit_str2]))
+
+    def test_simplified_union_with_str_instance_literals(self) -> None:
+        fx = self.fx
+
+        self.assert_simplified_union([fx.lit_str1_inst, fx.lit_str2_inst, fx.str_type],
+                                     fx.str_type)
+        self.assert_simplified_union([fx.lit_str1_inst, fx.lit_str1_inst, fx.lit_str1_inst],
+                                     fx.lit_str1_inst)
+        self.assert_simplified_union([fx.lit_str1_inst, fx.lit_str2_inst, fx.lit_str3_inst],
+                                     UnionType([fx.lit_str1_inst,
+                                                fx.lit_str2_inst,
+                                                fx.lit_str3_inst]))
+        self.assert_simplified_union([fx.lit_str1_inst, fx.lit_str2_inst, fx.uninhabited],
+                                     UnionType([fx.lit_str1_inst, fx.lit_str2_inst]))
+
+    def test_simplified_union_with_mixed_str_literals(self) -> None:
+        fx = self.fx
+
+        self.assert_simplified_union([fx.lit_str1, fx.lit_str2, fx.lit_str3_inst],
+                                     UnionType([fx.lit_str1,
+                                                fx.lit_str2,
+                                                fx.lit_str3_inst]))
+        self.assert_simplified_union([fx.lit_str1, fx.lit_str1, fx.lit_str1_inst],
+                                     UnionType([fx.lit_str1, fx.lit_str1_inst]))
+
     def assert_simplified_union(self, original: List[Type], union: Type) -> None:
         assert_equal(make_simplified_union(original), union)
         assert_equal(make_simplified_union(list(reversed(original))), union)

mypy/test/typefixture.py

@@ -65,6 +65,7 @@ def make_type_var(name: str, id: int, values: List[Type], upper_bound: Type,
                                               variances=[COVARIANT])   # class tuple
         self.type_typei = self.make_type_info('builtins.type')         # class type
         self.bool_type_info = self.make_type_info('builtins.bool')
+        self.str_type_info = self.make_type_info('builtins.str')
         self.functioni = self.make_type_info('builtins.function')  # function TODO
         self.ai = self.make_type_info('A', mro=[self.oi])              # class A
         self.bi = self.make_type_info('B', mro=[self.ai, self.oi])     # class B(A)
@@ -109,6 +110,7 @@ def make_type_var(name: str, id: int, values: List[Type], upper_bound: Type,
         self.std_tuple = Instance(self.std_tuplei, [self.anyt])        # tuple
         self.type_type = Instance(self.type_typei, [])        # type
         self.function = Instance(self.functioni, [])  # function TODO
+        self.str_type = Instance(self.str_type_info, [])
         self.a = Instance(self.ai, [])          # A
         self.b = Instance(self.bi, [])          # B
         self.c = Instance(self.ci, [])          # C
@@ -163,6 +165,13 @@ def make_type_var(name: str, id: int, values: List[Type], upper_bound: Type,
         self.lit3_inst = Instance(self.di, [], last_known_value=self.lit3)
         self.lit4_inst = Instance(self.ai, [], last_known_value=self.lit4)
 
+        self.lit_str1 = LiteralType("x", self.str_type)
+        self.lit_str2 = LiteralType("y", self.str_type)
+        self.lit_str3 = LiteralType("z", self.str_type)
+        self.lit_str1_inst = Instance(self.str_type_info, [], last_known_value=self.lit_str1)
+        self.lit_str2_inst = Instance(self.str_type_info, [], last_known_value=self.lit_str2)
+        self.lit_str3_inst = Instance(self.str_type_info, [], last_known_value=self.lit_str3)
+
         self.type_a = TypeType.make_normalized(self.a)
         self.type_b = TypeType.make_normalized(self.b)
         self.type_c = TypeType.make_normalized(self.c)

mypy/typeops.py

@@ -299,15 +299,23 @@ def callable_corresponding_argument(typ: CallableType,
     return by_name if by_name is not None else by_pos
 
 
-def is_simple_literal(t: ProperType) -> bool:
-    """
-    Whether a type is a simple enough literal to allow for fast Union simplification
+def simple_literal_value_key(t: ProperType) -> Optional[Tuple[str, ...]]:
+    """Return a hashable description of simple literal type.
+
+    Return None if not a simple literal type.
 
-    For now this means enum or string
+    The return value can be used to simplify away duplicate types in
+    unions by comparing keys for equality. For now enum, string or
+    Instance with string last_known_value are supported.
     """
-    return isinstance(t, LiteralType) and (
-            t.fallback.type.is_enum or t.fallback.type.fullname == 'builtins.str'
-    )
+    if isinstance(t, LiteralType):
+        if t.fallback.type.is_enum or t.fallback.type.fullname == 'builtins.str':
+            assert isinstance(t.value, str)
+            return 'literal', t.value, t.fallback.type.fullname
+    if isinstance(t, Instance):
+        if t.last_known_value is not None and isinstance(t.last_known_value.value, str):
+            return 'instance', t.last_known_value.value, t.type.fullname
+    return None
 
 
 def make_simplified_union(items: Sequence[Type],
@@ -341,10 +349,20 @@ def make_simplified_union(items: Sequence[Type],
                 all_items.append(typ)
         items = all_items
 
+    simplified_set = _remove_redundant_union_items(items, keep_erased)
+
+    # If more than one literal exists in the union, try to simplify
+    if (contract_literals and sum(isinstance(item, LiteralType) for item in simplified_set) > 1):
+        simplified_set = try_contracting_literals_in_union(simplified_set)
+
+    return UnionType.make_union(simplified_set, line, column)
+
+
+def _remove_redundant_union_items(items: List[ProperType], keep_erased: bool) -> List[ProperType]:
     from mypy.subtypes import is_proper_subtype
 
     removed: Set[int] = set()
-    seen: Set[Tuple[str, str]] = set()
+    seen: Set[Tuple[str, ...]] = set()
 
     # NB: having a separate fast path for Union of Literal and slow path for other things
     # would arguably be cleaner, however it breaks down when simplifying the Union of two
@@ -354,10 +372,8 @@ def make_simplified_union(items: Sequence[Type],
         if i in removed:
             continue
         # Avoid slow nested for loop for Union of Literal of strings/enums (issue #9169)
-        if is_simple_literal(item):
-            assert isinstance(item, LiteralType)
-            assert isinstance(item.value, str)
-            k = (item.value, item.fallback.type.fullname)
+        k = simple_literal_value_key(item)
+        if k is not None:
             if k in seen:
                 removed.add(i)
                 continue
@@ -373,13 +389,13 @@ def make_simplified_union(items: Sequence[Type],
             seen.add(k)
             if safe_skip:
                 continue
+
         # Keep track of the truishness info for deleted subtypes which can be relevant
         cbt = cbf = False
         for j, tj in enumerate(items):
             # NB: we don't need to check literals as the fast path above takes care of that
             if (
                     i != j
-                    and not is_simple_literal(tj)
                     and is_proper_subtype(tj, item, keep_erased_types=keep_erased)
                     and is_redundant_literal_instance(item, tj)  # XXX?
             ):
@@ -393,13 +409,7 @@ def make_simplified_union(items: Sequence[Type],
         elif not item.can_be_false and cbf:
             items[i] = true_or_false(item)
 
-    simplified_set = [items[i] for i in range(len(items)) if i not in removed]
-
-    # If more than one literal exists in the union, try to simplify
-    if (contract_literals and sum(isinstance(item, LiteralType) for item in simplified_set) > 1):
-        simplified_set = try_contracting_literals_in_union(simplified_set)
-
-    return UnionType.make_union(simplified_set, line, column)
+    return [items[i] for i in range(len(items)) if i not in removed]
 
 
 def _get_type_special_method_bool_ret_type(t: Type) -> Optional[Type]: