Clang is diagnosing under these paragraphs
[stmt.if] (emphasis mine)
2 If the if statement is of the form if constexpr, the value of
the condition shall be a contextually converted constant expression of
type bool; this form is called a constexpr if statement.[expr.const]
4 A converted constant expression of type T is an expression,
implicitly converted to type T, where the converted expression is a
constant expression and the implicit conversion sequence contains only
- integral conversions other than narrowing conversions,
Now, when it comes to integral conversions, a conversion to bool is listed as an integral conversion. And it is narrowing, in the strictest sense of the word, since a bool cannot represent all the values of an int. So the diagnostic is not without grounds.
But I think it’s also quite reasonable to consider the fact a conversion to bool is usually intended to check for “truthiness”, and so the narrowing nature of it shouldn’t matter. It looks like a minor bug in the standard1, with GCC taking the common-sense route, and Clang adhering to the dry letter of the law in the strictest sense.
1 – And a proposal exists to change it.