diff --git a/conformance/results/mypy/specialtypes_promotions.toml b/conformance/results/mypy/specialtypes_promotions.toml
index 864bc73e..04499e86 100644
--- a/conformance/results/mypy/specialtypes_promotions.toml
+++ b/conformance/results/mypy/specialtypes_promotions.toml
@@ -1,7 +1,12 @@
-conformant = "Pass"
+conformant = "Partial"
+notes = """
+Does not narrow from float to int after isinstance() check
+"""
output = """
-specialtypes_promotions.py:13: error: "float" has no attribute "numerator" [attr-defined]
+specialtypes_promotions.py:17: error: "float" has no attribute "numerator" [attr-defined]
+specialtypes_promotions.py:33: error: Incompatible return value type (got "complex", expected "float") [return-value]
"""
-conformance_automated = "Pass"
+conformance_automated = "Fail"
errors_diff = """
+Line 26: Expected 1 errors
"""
diff --git a/conformance/results/mypy/version.toml b/conformance/results/mypy/version.toml
index 268bd31a..dc9b55ed 100644
--- a/conformance/results/mypy/version.toml
+++ b/conformance/results/mypy/version.toml
@@ -1,2 +1,2 @@
version = "mypy 1.10.0"
-test_duration = 1.4
+test_duration = 1.5
diff --git a/conformance/results/pyre/specialtypes_promotions.toml b/conformance/results/pyre/specialtypes_promotions.toml
index fb9055fb..f17ad77f 100644
--- a/conformance/results/pyre/specialtypes_promotions.toml
+++ b/conformance/results/pyre/specialtypes_promotions.toml
@@ -1,10 +1,13 @@
conformant = "Partial"
notes = """
Does not reject use of attribute that is compatible only with float.
+Does not narrow from float to int after isinstance() check
"""
output = """
+specialtypes_promotions.py:33:8 Incompatible return type [7]: Expected `float` but got `complex`.
"""
conformance_automated = "Fail"
errors_diff = """
-Line 13: Expected 1 errors
+Line 17: Expected 1 errors
+Line 26: Expected 1 errors
"""
diff --git a/conformance/results/pyre/version.toml b/conformance/results/pyre/version.toml
index 57ed5696..1fec89de 100644
--- a/conformance/results/pyre/version.toml
+++ b/conformance/results/pyre/version.toml
@@ -1,2 +1,2 @@
version = "pyre 0.9.21"
-test_duration = 3.4
+test_duration = 2.9
diff --git a/conformance/results/pyright/specialtypes_promotions.toml b/conformance/results/pyright/specialtypes_promotions.toml
index 20daf3a3..560cb119 100644
--- a/conformance/results/pyright/specialtypes_promotions.toml
+++ b/conformance/results/pyright/specialtypes_promotions.toml
@@ -1,7 +1,11 @@
conformant = "Pass"
output = """
-specialtypes_promotions.py:13:7 - error: Cannot access attribute "numerator" for class "float"
+specialtypes_promotions.py:17:7 - error: Cannot access attribute "numerator" for class "float"
Attribute "numerator" is unknown (reportAttributeAccessIssue)
+specialtypes_promotions.py:26:16 - error: Expression of type "Literal['x']" is incompatible with return type "int"
+ "Literal['x']" is incompatible with "int" (reportReturnType)
+specialtypes_promotions.py:33:16 - error: Expression of type "complex" is incompatible with return type "float"
+ "complex" is incompatible with "float" (reportReturnType)
"""
conformance_automated = "Pass"
errors_diff = """
diff --git a/conformance/results/pyright/version.toml b/conformance/results/pyright/version.toml
index 6302edbf..789c07a2 100644
--- a/conformance/results/pyright/version.toml
+++ b/conformance/results/pyright/version.toml
@@ -1,2 +1,2 @@
version = "pyright 1.1.364"
-test_duration = 1.4
+test_duration = 1.6
diff --git a/conformance/results/pytype/specialtypes_promotions.toml b/conformance/results/pytype/specialtypes_promotions.toml
index dc20943f..3d6ee073 100644
--- a/conformance/results/pytype/specialtypes_promotions.toml
+++ b/conformance/results/pytype/specialtypes_promotions.toml
@@ -1,7 +1,12 @@
-conformant = "Pass"
+conformant = "Partial"
+notes = """
+Does not narrow from float to int after isinstance() check
+"""
output = """
-File "specialtypes_promotions.py", line 13, in func1: No attribute 'numerator' on float [attribute-error]
+File "specialtypes_promotions.py", line 17, in func1: No attribute 'numerator' on float [attribute-error]
+File "specialtypes_promotions.py", line 33, in func2: bad return type [bad-return-type]
"""
-conformance_automated = "Pass"
+conformance_automated = "Fail"
errors_diff = """
+Line 26: Expected 1 errors
"""
diff --git a/conformance/results/pytype/version.toml b/conformance/results/pytype/version.toml
index 68b8ac60..bbfb29cd 100644
--- a/conformance/results/pytype/version.toml
+++ b/conformance/results/pytype/version.toml
@@ -1,2 +1,2 @@
version = "pytype 2024.04.11"
-test_duration = 30.1
+test_duration = 49.8
diff --git a/conformance/results/results.html b/conformance/results/results.html
index 9693b2e8..bc90bed9 100644
--- a/conformance/results/results.html
+++ b/conformance/results/results.html
@@ -159,16 +159,16 @@ <h3>Python Type System Conformance Test Results</h3>
<div class="table_container"><table><tbody>
<tr><th class="col1"> </th>
<th class='tc-header'><div class='tc-name'>mypy 1.10.0</div>
-<div class='tc-time'>1.4sec</div>
+<div class='tc-time'>1.5sec</div>
</th>
<th class='tc-header'><div class='tc-name'>pyright 1.1.364</div>
-<div class='tc-time'>1.4sec</div>
+<div class='tc-time'>1.6sec</div>
</th>
<th class='tc-header'><div class='tc-name'>pyre 0.9.21</div>
-<div class='tc-time'>3.4sec</div>
+<div class='tc-time'>2.9sec</div>
</th>
<th class='tc-header'><div class='tc-name'>pytype 2024.04.11</div>
-<div class='tc-time'>30.1sec</div>
+<div class='tc-time'>49.8sec</div>
</th>
</tr>
<tr><th class="column" colspan="5">
diff --git a/conformance/tests/specialtypes_promotions.py b/conformance/tests/specialtypes_promotions.py
index fc51e133..0216337d 100644
--- a/conformance/tests/specialtypes_promotions.py
+++ b/conformance/tests/specialtypes_promotions.py
@@ -2,15 +2,36 @@
Tests "type promotions" for float and complex when they appear in annotations.
"""
+from typing import assert_type
+
# Specification: https://typing.readthedocs.io/en/latest/spec/special-types.html#special-cases-for-float-and-complex
-v1: float = 1
-v2: complex = 1.2
-v2 = 1
+v1: int = 1
+v2: float = 1
+v3: float = v1
+v4: complex = 1.2
+v4 = 1
+
+
+def func1(f: float) -> int:
+ f.numerator # E: attribute exists on int but not float
+ if isinstance(f, float):
+ f.hex() # OK (attribute exists on float but not int)
+ return 1
+ else:
+ assert_type(f, int)
+ # Make sure type checkers don't treat this branch as unreachable
+ # and skip checking it.
+ return "x" # E
-def func1(f: float):
- f.numerator # E
- if not isinstance(f, float):
- f.numerator # OK
+def func2(x: int) -> float:
+ if x == 0:
+ return 1
+ elif x == 1:
+ return 1j # E
+ elif x > 10:
+ return x
+ else:
+ return 1.0
diff --git a/docs/spec/special-types.rst b/docs/spec/special-types.rst
index b94222e6..b0339ae0 100644
--- a/docs/spec/special-types.rst
+++ b/docs/spec/special-types.rst
@@ -113,11 +113,25 @@ Special cases for ``float`` and ``complex``
Python's numeric types ``complex``, ``float`` and ``int`` are not
subtypes of each other, but to support common use cases, the type
-system contains a straightforward shortcut:
-when an argument is annotated as having
-type ``float``, an argument of type ``int`` is acceptable; similar,
-for an argument annotated as having type ``complex``, arguments of
-type ``float`` or ``int`` are acceptable.
+system contains a special case.
+
+When a reference to the built-in type ``float`` appears in a :term:`type expression`,
+it is interpreted as if it were a union of the built-in types ``float`` and ``int``.
+Similarly, when a reference to the type ``complex`` appears, it is interpreted as
+a union of the built-in types ``complex``, ``float`` and ``int``.
+These implicit unions behave exactly like the corresponding explicit union types,
+but type checkers may choose to display them differently in user-visible output
+for clarity.
+
+Type checkers should support narrowing the type of a variable to exactly ``float``
+or ``int``, without the implicit union, through a call to ``isinstance()``::
+
+ def f(x: float) -> None:
+ reveal_type(x) # float | int, but type checkers may display just "float"
+ if isinstance(x, float):
+ reveal_type(x) # float
+ else:
+ reveal_type(x) # int
.. _`type-brackets`: