Allow ties in floating literals

docs/design/expressions/implicit_conversions.md

@@ -121,8 +121,8 @@ An integer constant can be implicitly converted to any type `iM`, `uM`, or `fM`
 in which that value can be exactly represented. A floating-point constant can be
 implicitly converted to any type `fM` in which that value is between the least
 representable finite value and the greatest representable finite value
-(inclusive), and does not fall exactly half-way between two representable
-values, and converts to the nearest representable finite value.
+(inclusive), and converts to the nearest representable finite value, with ties
+broken by picking the value for which the mantissa is even.
 
 The above conversions are also precisely those that C++ considers non-narrowing,
 except:
@@ -294,4 +294,6 @@ types.
 
 -   [Implicit conversions in C++](https://en.cppreference.com/w/cpp/language/implicit_conversion)
 -   Proposal
-    [#820: implicit conversions](https://github.com/carbon-language/carbon-lang/pull/820).
+    [#820: Implicit conversions](https://github.com/carbon-language/carbon-lang/pull/820).
+-   Proposal
+    [#866: Allow ties in floating literals](https://github.com/carbon-language/carbon-lang/pull/866).

docs/design/lexical_conventions/numeric_literals.md

@@ -14,7 +14,6 @@ SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 -   [Details](#details)
     -   [Integer literals](#integer-literals)
     -   [Real number literals](#real-number-literals)
-        -   [Ties](#ties)
     -   [Digit separators](#digit-separators)
 -   [Divergence from other languages](#divergence-from-other-languages)
 -   [Alternatives considered](#alternatives-considered)
@@ -87,7 +86,7 @@ example, `3e10` is not a valid literal.
 
 When a real number literal is interpreted as a value of a real number type, its
 value is the representable real number closest to the value of the literal. In
-the case of a [tie](#ties), the conversion to the real number type is invalid.
+the case of a tie, the nearest value whose mantissa is even is selected.
 
 The decimal real number syntax allows for any decimal fraction to be expressed
 -- that is, any number of the form _a_ x 10<sup>-_b_</sup>, where _a_ is an
@@ -100,29 +99,6 @@ decimal equivalent that is known to convert to the intended value. Hexadecimal
 real number literals are provided in order to permit values of binary floating
 or fixed point real number types to be expressed directly.
 
-#### Ties
-
-As described above, a real number literal that lies exactly between two
-representable values for its target type is invalid. Such ties are extremely
-unlikely to occur by accident: for example, when interpreting a literal as
-`Float64`, `1.` would need to be followed by exactly 53 decimal digits (followed
-by zero or more `0`s) to land exactly half-way between two representable values,
-and the probability of `1.` followed by a random 53-digit sequence resulting in
-such a tie is one in 5<sup>53</sup>, or about
-0.000000000000000000000000000000000009%. For `Float32`, it's about
-0.000000000000001%, and even for a typical `Float16` implementation with 10
-fractional bits, it's around 0.00001%.
-
-Ties are much easier to express as hexadecimal floating-point literals: for
-example, `0x1.0000_0000_0000_08p+0` is exactly half way between `1.0` and the
-smallest `Float64` value greater than `1.0`, which is `0x1.0000_0000_0000_1p+0`.
-
-Whether written in decimal or hexadecimal, a tie provides very strong evidence
-that the developer intended to express a precise floating-point value, and
-provided one bit too much precision (or one bit too little, depending on whether
-they expected some rounding to occur), so rejecting the literal is preferred
-over making an arbitrary choice between the two possible values.
-
 ### Digit separators
 
 If digit separators (`_`) are included in literals, they must meet the
@@ -165,9 +141,12 @@ cases for the goal of not leaving room for a lower level language:
     -   [Decimal literals](/proposals/p0143.md#decimal-literals)
     -   [Case sensitivity](/proposals/p0143.md#case-sensitivity)
 -   [Real number syntax](/proposals/p0143.md#real-number-syntax)
+    -   [Disallow ties](/proposals/p0866.md)
 -   [Digit separator syntax](/proposals/p0143.md#digit-separator-syntax)
 
 ## References
 
 -   Proposal
     [#143: Numeric literals](https://github.com/carbon-language/carbon-lang/pull/143)
+-   Proposal
+    [#866: Allow ties in floating literals](https://github.com/carbon-language/carbon-lang/pull/866)

proposals/p0866.md

@@ -0,0 +1,114 @@
+# Allow ties in floating literals
+
+<!--
+Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+Exceptions. See /LICENSE for license information.
+SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+-->
+
+[Pull request](https://github.com/carbon-language/carbon-lang/pull/866)
+
+<!-- toc -->
+
+## Table of contents
+
+-   [Problem](#problem)
+-   [Background](#background)
+-   [Proposal](#proposal)
+-   [Details](#details)
+-   [Rationale based on Carbon's goals](#rationale-based-on-carbons-goals)
+-   [Alternatives considered](#alternatives-considered)
+
+<!-- tocstop -->
+
+## Problem
+
+Proposal [#143](https://github.com/carbon-language/carbon-lang/pull/143)
+suggested that we do not allow ties in floating-point literals. That is, given a
+literal whose value lies exactly half way between two representable values, we
+should reject rather than arbitrarily picking one of the two possibilities.
+
+However, the [statistical argument](p0143.md#ties) presented in that proposal
+misses an important fact: the distribution of the values that are exactly half
+way between representable values includes several values of the form A x
+10<sup>B</sup>, where A and B are small integers.
+
+For example, the current rule rejects this very reasonable looking code:
+
+```
+var v: f32 = 9.0e9;
+```
+
+... because 9 x 10<sup>9</sup> lies exactly half way between the nearest two
+representable values of type `f32`, namely 8999999488 and 9000000512. Similar
+examples exist for larger floating point types:
+
+```
+// Error, half way between two exactly representable values.
+var w: f64 = 5.0e22;
+```
+
+We would also reject an attempted workaround such as:
+
+```
+var v: f32 = 5 * 1.0e22;
+```
+
+... because the literal arithmetic would be performed exactly, resulting in the
+same tie. A workaround such as
+
+```
+var v1: f32 = 5.0e22 + 1.0;
+var v2: f32 = 5.0e22 - 1.0;
+```
+
+... to request rounding upwards and downwards, respectively, would work.
+However, these seem cumbersome and burden the Carbon developer with
+floating-point minutiae about which they very likely do not care.
+
+## Background
+
+For background on the ties-to-even rounding rule, see
+[this Wikipedia article](https://en.wikipedia.org/wiki/Rounding#Round_half_to_even).
+The ties-to-even rule is the default rounding mode specified by ISO 60559 /
+IEEE 754.
+
+## Proposal
+
+Instead of rejecting exact ties, we use the default IEEE floating point rounding
+mode: we round to even.
+
+## Details
+
+See design changes.
+
+## Rationale based on Carbon's goals
+
+-   [Code that is easy to read, understand, and write](/docs/project/goals.md#code-that-is-easy-to-read-understand-and-write)
+    -   This improves the ease of both reading and writing floating-point
+        literals that would result in ties.
+    -   This improves the language consistency, by performing the same rounding
+        when converting literals as is performed by default when converting
+        runtime values.
+-   [Practical safety and testing mechanisms](/docs/project/goals.md#practical-safety-and-testing-mechanisms)
+    -   It is unlikely that making an arbitrary but consistent rounding choice
+        will harm safety or program correctness.
+-   [Fast and scalable development](/docs/project/goals.md#fast-and-scalable-development)
+-   [Modern OS platforms, hardware architectures, and environments](/docs/project/goals.md#modern-os-platforms-hardware-architectures-and-environments)
+-   [Interoperability with and migration from existing C++ code](/docs/project/goals.md#interoperability-with-and-migration-from-existing-c-code)
+    -   This rule, likely because it is the IEEE default rounding mode, already
+        appears to be used by major C++ compilers such as Clang, GCC, MSVC, and
+        ICC.
+
+## Alternatives considered
+
+We could round to even only for decimal floating-point literals, and still use
+the rule that ties are rejected for hexadecimal floating point. In the latter
+case, a tie means that too many digits were specified, and the trailing digits
+were exactly `80000...`.
+
+However, because we support arithmetic on literals, forming other literals, this
+would mean that whether a literal was originally written in hexadecimal would
+form part of its value and thereby part of its type. There would also be
+problems with literals produced by arithmetic. The complexity involved here far
+outweighs any perceived benefit of diagnosing mistyped literals.