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In the process of answering this question on SO for C++11, I realized that in C++03 (as well as in C) the use of the comma operator is explicitly forbidden in a constant-expression . Paragraph 5.19/1 of the C++03 Standard on constant expressions says: [...] In particular, except in sizeof expressions, functions, class objects, pointers, or references shall not be used, and assignment, increment, decrement, function-call, or comma operators shall not be used . In C++11, however, that last part mentioning the comma operator seems to be vanished. And while paragraph 5.19/2 of the C++11 Standard clearly specifies that assignment, increment, decrement, and non- constexpr function call expressions shall not appear as sub-expressions of a constant-expression , the usage of the comma operator does not seem to be forbidden anymore. For instance, the following program compiles fine on GCC 4.7.2 and Clang 3.3 with std=c++11 (apart from compiler warnings saying the comma operator has no effect and the x and arr variables are unused): int main() { constexpr int x = (0, 42); int arr[(0, 42)]; } However, it must be said that even the following program compiles fine with the -std=c++03 option (both on Clang and GCC), which is clearly not correct, given the above quote from the C++03 Standard: int main() { int arr[(0, 42)]; } QUESTION: Is there a difference between C++03 and C++11 as to whether or not the comma operator is allowed in a constant expression, or am I missing something? As a bonus (non-constructive) question, I would be interest

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