Are C++ standard library implementations allowed to strengthen noexcept specifications?

Question

According to the C++ standard, are implementations of the C++ standard library allowed to strengthen noexcept specifications of methods and other functions of t

Answer 1

The Standard says yes:

§ 17.6.5.12.1 Restrictions on exception handling [res.on.exception.handling]

1. Any of the functions defined in the C++ standard library can report a failure by throwing an exception of a type described in its Throws: paragraph. An implementation may strengthen the exception specification for a non-virtual function by adding a non-throwing noexcept-specification.

[...]

4. Destructor operations defined in the C++ standard library shall not throw exceptions. Every destructor in the C++ standard library shall behave as if it had a non-throwing exception specification. Any other functions defined in the C++ standard library that do not have an exception-specification may throw implementation-defined exceptions unless otherwise specified. An implementation may strengthen this implicit exception-specification by adding an explicit one.

(Comma 4 seems to just allow to be explicit about the exception specification, and to warn that the lack of an explicit exception specification means that the implementation is allowed to throw anything).

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To be honest, I don't understand why this is allowed and adding constexpr is not (§ 17.6.5.6). They look like the two sides of the same medal -- by using type traits and SFINAE you can have code which shows different behaviours depending on which Standard Library implementation you use (if it marks some functions as noexcept/constexpr, or if it doesn't), and that defeats the purposes of having a standard in the first place...