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>

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 @@ and are discussed in that section.
 </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>
@@ -593,16 +595,6 @@ respectively, depending on whether it is a boolean, rune, integer, floating-poin
 complex, or string constant.
 </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
@@ -3795,7 +3787,8 @@ type <code>T</code> in any of these cases:
 	<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 @@ uint(iota)               // iota value of type uint
 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