Narrow falsey str/bytes/int to literal type (#17818)

Closes #16891

### Before
```python
from typing import Literal

def f1(a: str) -> Literal[""]:
    return a and exit()  # E: Incompatible return value type (got "str", expected "Literal['']")

def f2(a: int) -> Literal[0]:
    return a and exit()  # E: Incompatible return value type (got "int", expected "Literal[0]")

def f3(a: bytes) -> Literal[b""]:
    return a and exit()  # E: Incompatible return value type (got "bytes", expected "Literal[b'']")
```
### After
```none
Success: no issues found in 1 source file
```

mypy/typeops.py

@@ -657,6 +657,10 @@ def false_only(t: Type) -> ProperType:
         new_items = [false_only(item) for item in t.items]
         can_be_false_items = [item for item in new_items if item.can_be_false]
         return make_simplified_union(can_be_false_items, line=t.line, column=t.column)
+    elif isinstance(t, Instance) and t.type.fullname in ("builtins.str", "builtins.bytes"):
+        return LiteralType("", fallback=t)
+    elif isinstance(t, Instance) and t.type.fullname == "builtins.int":
+        return LiteralType(0, fallback=t)
     else:
         ret_type = _get_type_method_ret_type(t, name="__bool__") or _get_type_method_ret_type(
             t, name="__len__"

test-data/unit/check-expressions.test

@@ -342,7 +342,7 @@ b: bool
 i: str
 j = b or i
 if not j:
-    reveal_type(j) # N: Revealed type is "builtins.str"
+    reveal_type(j) # N: Revealed type is "Literal['']"
 [builtins fixtures/bool.pyi]
 
 [case testAndOr]

test-data/unit/check-narrowing.test

@@ -1007,7 +1007,7 @@ str_or_false: Union[Literal[False], str]
 if str_or_false:
     reveal_type(str_or_false)   # N: Revealed type is "builtins.str"
 else:
-    reveal_type(str_or_false)   # N: Revealed type is "Union[Literal[False], builtins.str]"
+    reveal_type(str_or_false)   # N: Revealed type is "Union[Literal[False], Literal['']]"
 
 true_or_false: Literal[True, False]
 
@@ -1017,6 +1017,22 @@ else:
     reveal_type(true_or_false)  # N: Revealed type is "Literal[False]"
 [builtins fixtures/primitives.pyi]
 
+[case testNarrowingFalseyToLiteral]
+from typing import Union
+
+a: str
+b: bytes
+c: int
+d: Union[str, bytes, int]
+
+if not a:
+    reveal_type(a)  # N: Revealed type is "Literal['']"
+if not b:
+    reveal_type(b)  # N: Revealed type is "Literal[b'']"
+if not c:
+    reveal_type(c)  # N: Revealed type is "Literal[0]"
+if not d:
+    reveal_type(d)  # N: Revealed type is "Union[Literal[''], Literal[b''], Literal[0]]"
 
 [case testNarrowingIsInstanceFinalSubclass]
 # flags: --warn-unreachable

test-data/unit/check-optional.test

@@ -53,14 +53,14 @@ x = None  # type:  Optional[int]
 if x:
   reveal_type(x)  # N: Revealed type is "builtins.int"
 else:
-  reveal_type(x)  # N: Revealed type is "Union[builtins.int, None]"
+  reveal_type(x)  # N: Revealed type is "Union[Literal[0], None]"
 [builtins fixtures/bool.pyi]
 
 [case testIfNotCases]
 from typing import Optional
 x = None  # type:  Optional[int]
 if not x:
-  reveal_type(x)  # N: Revealed type is "Union[builtins.int, None]"
+  reveal_type(x)  # N: Revealed type is "Union[Literal[0], None]"
 else:
   reveal_type(x)  # N: Revealed type is "builtins.int"
 [builtins fixtures/bool.pyi]
@@ -109,13 +109,13 @@ reveal_type(z2)  # N: Revealed type is "Union[builtins.int, builtins.str, None]"
 from typing import Optional
 x = None  # type: Optional[str]
 y1 = x and 'b'
-reveal_type(y1)  # N: Revealed type is "Union[builtins.str, None]"
+reveal_type(y1)  # N: Revealed type is "Union[Literal[''], None, builtins.str]"
 y2 = x and 1  # x could be '', so...
-reveal_type(y2)  # N: Revealed type is "Union[builtins.str, None, builtins.int]"
+reveal_type(y2)  # N: Revealed type is "Union[Literal[''], None, builtins.int]"
 z1 = 'b' and x
 reveal_type(z1)  # N: Revealed type is "Union[builtins.str, None]"
 z2 = int() and x
-reveal_type(z2)  # N: Revealed type is "Union[builtins.int, builtins.str, None]"
+reveal_type(z2)  # N: Revealed type is "Union[Literal[0], builtins.str, None]"
 
 [case testLambdaReturningNone]
 f = lambda: None

test-data/unit/check-python38.test

@@ -463,9 +463,9 @@ def check_partial_list() -> None:
 if (x := 0):
     reveal_type(x)  # E: Statement is unreachable
 else:
-    reveal_type(x)  # N: Revealed type is "builtins.int"
+    reveal_type(x)  # N: Revealed type is "Literal[0]"
 
-reveal_type(x)  # N: Revealed type is "builtins.int"
+reveal_type(x)  # N: Revealed type is "Literal[0]"
 
 [case testWalrusAssignmentAndConditionScopeForProperty]
 # flags: --warn-unreachable
@@ -483,7 +483,7 @@ wrapper = PropertyWrapper()
 if x := wrapper.f:
     reveal_type(x)  # N: Revealed type is "builtins.str"
 else:
-    reveal_type(x)  # N: Revealed type is "builtins.str"
+    reveal_type(x)  # N: Revealed type is "Literal['']"
 
 reveal_type(x)  # N: Revealed type is "builtins.str"
 
@@ -505,7 +505,7 @@ def f() -> str: ...
 if x := f():
     reveal_type(x)  # N: Revealed type is "builtins.str"
 else:
-    reveal_type(x)  # N: Revealed type is "builtins.str"
+    reveal_type(x)  # N: Revealed type is "Literal['']"
 
 reveal_type(x)  # N: Revealed type is "builtins.str"