spec: document fp rounding during explicit constant conversion
The gc compilers already behave this way. It was an explicit
decision during the very long constant design discussions,
but it appears not to have made it into the spec.
Fixes #4398.
R=golang-dev, r
CC=golang-dev
https://golang.org/cl/7313069
diff --git a/doc/go_spec.html b/doc/go_spec.html
index aa4330d..246a365 100644
--- a/doc/go_spec.html
+++ b/doc/go_spec.html
@@ -3470,6 +3470,14 @@
<code>x</code> is representable by a value of type <code>T</code>.
</li>
<li>
+ <code>x</code> is a floating-point constant,
+ <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.
+ The constant <code>T(x)</code> is the rounded value.
+ </li>
+ <li>
<code>x</code> is an integer constant and <code>T</code> is a
<a href="#String_types">string type</a>.
The same rule as for non-constant <code>x</code> applies in this case
@@ -3485,6 +3493,7 @@
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
string('x') // "x" of type string
string(0x266c) // "♬" of type string
MyString("foo" + "bar") // "foobar" of type MyString
