spec: clarify numeric conversions where IEEE-754 produces -0.0
The spec defines precise numeric constants which do not overflow.
Consequently, +/-Inf and NaN values were excluded. The case was not
clear for -0.0 but they are mostly of interest to determine the sign
of infinities which don't exist.
That said, the conversion rules explicitly say that T(x) (for a numeric
x and floating-point type T) is the value after rounding per IEEE-754.
The result is constant if x is constant. Rounding per IEEE-754 can
produce a -0.0 which we cannot represent as a constant.
Thus, the spec is inconsistent. Attempt to fix the inconsistency by
adjusting the rounding rule rather than letting -0.0 into the language.
For more details, see the issue below.
Open to discussion.
Fixes #12576.
Change-Id: Ibe3c676372ab16d9229f1f9daaf316f761e074ee
Reviewed-on: https://go-review.googlesource.com/14727
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
diff --git a/doc/go_spec.html b/doc/go_spec.html
index 6c4eac1..873c127 100644
--- a/doc/go_spec.html
+++ b/doc/go_spec.html
@@ -1,6 +1,6 @@
<!--{
"Title": "The Go Programming Language Specification",
- "Subtitle": "Version of September 24, 2015",
+ "Subtitle": "Version of October 20, 2015",
"Path": "/ref/spec"
}-->
@@ -558,7 +558,9 @@
</p>
<p>
-Numeric constants represent values of arbitrary precision and do not overflow.
+Numeric constants represent exact values of arbitrary precision and do not overflow.
+Consequently, there are no constants denoting the IEEE-754 negative zero, infinity,
+and not-a-number values.
</p>
<p>
@@ -594,16 +596,6 @@
</p>
<p>
-There are no constants denoting the IEEE-754 infinity and not-a-number values,
-but the <a href="/pkg/math/"><code>math</code> package</a>'s
-<a href="/pkg/math/#Inf">Inf</a>,
-<a href="/pkg/math/#NaN">NaN</a>,
-<a href="/pkg/math/#IsInf">IsInf</a>, and
-<a href="/pkg/math/#IsNaN">IsNaN</a>
-functions return and test for those values at run time.
-</p>
-
-<p>
Implementation restriction: Although numeric constants have arbitrary
precision in the language, a compiler may implement them using an
internal representation with limited precision. That said, every
@@ -3795,7 +3787,8 @@
<code>T</code> is a floating-point type,
and <code>x</code> is representable by a value
of type <code>T</code> after rounding using
- IEEE 754 round-to-even rules.
+ IEEE 754 round-to-even rules, but with an IEEE <code>-0.0</code>
+ further rounded to an unsigned <code>0.0</code>.
The constant <code>T(x)</code> is the rounded value.
</li>
<li>
@@ -3815,6 +3808,7 @@
float32(2.718281828) // 2.718281828 of type float32
complex128(1) // 1.0 + 0.0i of type complex128
float32(0.49999999) // 0.5 of type float32
+float64(-1e-1000) // 0.0 of type float64
string('x') // "x" of type string
string(0x266c) // "♬" of type string
MyString("foo" + "bar") // "foobar" of type MyString
