[llvm-bugs] [Bug 30592] New: void() incorrectly rejected as non-constant expression

[via llvm-bugs]

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: unassignedclangbugs at nondot.org
          Reporter: Casey at Carter.net
                CC: dgregor at apple.com, llvm-bugs at 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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