https://llvm.org/bugs/show_bug.cgi?id=30592
Bug ID: 30592
Summary: void() incorrectly rejected as non-constant expression
Product: clang
Version: trunk
Hardware: PC
OS: Windows NT
Status: NEW
Severity: normal
Priority: P
Component: C++
Assignee: unassignedclangb...@nondot.org
Reporter: ca...@carter.net
CC: dgre...@apple.com, llvm-bugs@lists.llvm.org
Classification: Unclassified
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.
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