x[0] == 1 constant expression in C++11 when x is const int[]?

Question

Is the following C++11 program ill-formed?

const int x[] = {1,2,3};

static_assert(x[0] == 1, "yay");

int main() {}

gcc and clang seem to think so, but why isn't x[0] == 1 a constant expression?

x[0] == 1
subscript operator
*(x+0) == 1
array-to-pointer conversion (int* p = x)
*(p+0) == 1
pointer addition
*p == 1
indirection (lvalue y = x[0])
y == 1
lvalue-to-rvalue conversion:

a non-volatile glvalue (yes, x[0] is a glvalue and non-volatile) of integral (yes it has type const int) or enumeration type that refers to a non-volatile const object (yes it has type const int) with a preceding initialization (yes initialized with 1), initialized with a constant expression (yes 1 is constant expression)

Seems true, the first element of the x array satisfies these conditions.

1 == 1

?

Is this a compiler bug, standard defect, or am i missing something?

What part of 5.19 [expr.const] says this isn't a constant expression?

Answer 1

In 5.19:

A [...]expression is a constant expression unless it involves one of the following [...]:

• an lvalue-to-rvalue conversion (4.1) unless it is applied to

  • a glvalue of integral or enumeration type that refers to a non-volatile const object with a preceding initialization, initialized with a constant expression, or
  • a glvalue of literal type that refers to a non-volatile object defined with constexpr, or that refers to a sub-object of such an object, or
  • a glvalue of literal type that refers to a non-volatile temporary object initialized with a constant expression

Putting it plainly, lvalue-to-rvalue conversion can only be done in constant expressions if:

• a constant integral (or enum) declaration initialized with a constant: const int x = 3;.
• a declaration with constexpr: constexpr int x[] = {1,2,3};.
• a temporary object initialized with a constant expression...

Your example does include lvalue-to-rvalue conversion, but has none of these exceptions, so x is not a constant expression. If, however, you change it to:

constexpr int x[] = {1,2,3};

static_assert(x[0] == 1, "yay");

int main() {}

Then all is well.