From ada692733646af886cf09d8d07f04083f5ea9e4f Mon Sep 17 00:00:00 2001
From: sobolevn <mail@sobolevn.me>
Date: Fri, 24 Sep 2021 21:29:20 +0300
Subject: [PATCH] Better narrows `bool` in `if` statements

---
 mypy/checker.py                     |  7 ++++---
 mypy/subtypes.py                    |  7 +++++++
 test-data/unit/check-narrowing.test | 20 ++++++++++++++++++++
 3 files changed, 31 insertions(+), 3 deletions(-)

diff --git a/mypy/checker.py b/mypy/checker.py
index 102108499d7b..2059abd1a1b3 100644
--- a/mypy/checker.py
+++ b/mypy/checker.py
@@ -5307,9 +5307,10 @@ def conditional_type_map(expr: Expression,
                 return None, {}
             else:
                 # we can only restrict when the type is precise, not bounded
-                proposed_precise_type = UnionType([type_range.item
-                                          for type_range in proposed_type_ranges
-                                          if not type_range.is_upper_bound])
+                proposed_precise_type = UnionType.make_union([
+                    type_range.item
+                    for type_range in proposed_type_ranges
+                    if not type_range.is_upper_bound])
                 remaining_type = restrict_subtype_away(current_type, proposed_precise_type)
                 return {expr: proposed_type}, {expr: remaining_type}
         else:
diff --git a/mypy/subtypes.py b/mypy/subtypes.py
index 63cebc8aa483..6fadbdcd219f 100644
--- a/mypy/subtypes.py
+++ b/mypy/subtypes.py
@@ -1129,6 +1129,13 @@ def restrict_subtype_away(t: Type, s: Type, *, ignore_promotions: bool = False)
                      if (isinstance(get_proper_type(item), AnyType) or
                          not covers_at_runtime(item, s, ignore_promotions))]
         return UnionType.make_union(new_items)
+    elif isinstance(t, Instance) and t.type.fullname == 'builtins.bool':
+        # Narrowing `bool` special case.
+        # When we got `bool` and `Literal[True]` / `Literal[False]`
+        # we return the inverse boolean value.
+        if isinstance(s, LiteralType):
+            return LiteralType(False, t) if s.value is True else LiteralType(True, t)
+        return t
     else:
         return t
 
diff --git a/test-data/unit/check-narrowing.test b/test-data/unit/check-narrowing.test
index 7c8415b75fe1..9b9542a31586 100644
--- a/test-data/unit/check-narrowing.test
+++ b/test-data/unit/check-narrowing.test
@@ -1030,6 +1030,26 @@ else:
 
 [builtins fixtures/primitives.pyi]
 
+[case testNarrowingBoolLiteralIdentityCheck]
+x: bool
+if x is False:
+    reveal_type(x)  # N: Revealed type is "Literal[False]"
+else:
+    reveal_type(x)  # N: Revealed type is "Literal[True]"
+
+y: bool
+if y is True:
+    reveal_type(y)  # N: Revealed type is "Literal[True]"
+else:
+    reveal_type(y)  # N: Revealed type is "Literal[False]"
+
+z: bool
+if z:  # TODO: this can be later emulated to be `z is True` in `mypy`
+    reveal_type(z)  # N: Revealed type is "builtins.bool"
+else:
+    reveal_type(z)  # N: Revealed type is "builtins.bool"
+[builtins fixtures/primitives.pyi]
+
 [case testNarrowingTypedDictUsingEnumLiteral]
 # flags: --python-version 3.6
 from typing import Union