<html>
<head>
<base href="https://llvm.org/bugs/" />
</head>
<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - void() incorrectly rejected as non-constant expression"
href="https://llvm.org/bugs/show_bug.cgi?id=30592">30592</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>void() incorrectly rejected as non-constant expression
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Windows NT
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>C++
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>Casey@Carter.net
</td>
</tr>
<tr>
<th>CC</th>
<td>dgregor@apple.com, llvm-bugs@lists.llvm.org
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>r283037 miscompiles this program:
int main() {
constexpr int foo = (void(), 0);
return foo;
}
with diagnosis:
prog.cc:2:26: error: constexpr variable 'foo' must be initialized by a constant
expression
constexpr int foo = (void(), 0);
~^~~~~~~~~~
Conversions to void, e.g. void(42) or static_cast<void>(42), are acceptable,
but the prvalue void() is not. It should be, since:
* void is a simple-type-specifier,
* simple-type-specifier ( expression-list_opt ) is a postfix-expression, the
behavior of which is specified in [expr.type.conv]/1 "If the type is (possibly
cv-qualified) void and the initializer is (), the expression is a prvalue of
the specified type that performs no initialization."
* postfix-expression (through a long chain of intermediaries) is a
conditional-expression,
* a conditional-expression is a core constant expression unless it violates one
of the rules listed in [expr.const]/2, which void() does not, nor does it
"evaluate an operation that has undefined behavior,"
* [expr.const]/5 defines "A constant expression is either a glvalue core
constant expression [...stuff that doesn't apply to prvalues...], or a prvalue
core constant expression whose value satisfies the following constraints:
* if the value is an object of class type [...stuff that does not apply to
void()...]
* if the value is of pointer type [...ibid...]
* if the value is an object of class or array type [...ibid...]
To the best of my ability to determine, void() is a constant expression.</pre>
</div>
</p>
<hr>
<span>You are receiving this mail because:</span>
<ul>
<li>You are on the CC list for the bug.</li>
</ul>
</body>
</html>