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spec: clarify expression statements

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Function and method calls are valid expression statements,
but calling certain built-in functions is not permitted.
Enumerate the built-ins.

Also: unsafe.Offsetof permits parenthesized selectors as
arguments.

This is simply documenting existing compiler behavior
(both gc and gccgo agree).

R=r, rsc, iant, ken
CC=golang-dev
https://golang.org/cl/6498138
This commit is contained in:
Robert Griesemer 2012-09-18 11:25:53 -07:00
parent 384af66984
commit 8c058b32d1
1 changed files with 16 additions and 4 deletions
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20
doc/go_spec.html
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@ -1,6 +1,6 @@
<!--{
"Title": "The Go Programming Language Specification",
"Subtitle": "Version of September 17, 2012",
"Subtitle": "Version of September 18, 2012",
"Path": "/ref/spec"
}-->
@ -3798,7 +3798,9 @@ Error: log.Panic("error encountered")
<h3 id="Expression_statements">Expression statements</h3>
<p>
Function calls, method calls, and receive operations
With the exception of specific built-in functions,
function and method <a href="#Calls">calls</a> and
<a href="#Receive_operator">receive operations</a>
can appear in statement context. Such statements may be parenthesized.
</p>
@ -3806,11 +3808,21 @@ can appear in statement context. Such statements may be parenthesized.
ExpressionStmt = Expression .
</pre>
<p>
The following built-in functions are not permitted in statement context:
</p>
<pre>
append cap complex imag len make new real
unsafe.Alignof unsafe.Offsetof unsafe.Sizeof
</pre>
<pre>
h(x+y)
f.Close()
&lt;-ch
(&lt;-ch)
len("foo") // illegal if len is the built-in function
</pre>
@ -5492,8 +5504,8 @@ of any type and return the alignment or size, respectively, of a hypothetical va
as if <code>v</code> was declared via <code>var v = x</code>.
</p>
<p>
The function <code>Offsetof</code> takes a selector (§<a href="#Selectors">Selectors</a>) denoting a struct
field of any type and returns the field offset in bytes relative to the
The function <code>Offsetof</code> takes a (possibly parenthesized) <a href="#Selectors">selector</a>
denoting a struct field of any type and returns the field offset in bytes relative to the
struct's address.
For a struct <code>s</code> with field <code>f</code>:
</p>