[cxx-abi-dev] manglings for exception specifications in function types

[Richard Smith]

On 11 October 2016 at 15:07, David Vandevoorde <daveed at edg.com> wrote:

>
> On Oct 11, 2016, at 5:11 PM, Richard Smith <richardsmith at google.com>
> wrote:
>
> Under
>   http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0012r1.html
>
> the noexceptness of a function type is now part of the type. As a result,
> we need manglings for exception-specifications on function
> pointer/reference types:
>
> void f(void()) {}
> void f(void() noexcept) {} // ok, overload not redefinition
>
> (It's not clear to me whether or not this was also necessary prior to
> C++17 to handle dependent exception specifications that appear lexically
> within the parameter list of a function template, and actual implementation
> practice varies as to whether such exception specifications are SFINAEable.)
>
>
> In order to handle overloading/SFINAE on exception specifications in
> dependent cases, we need to be able to mangle not only "noexcept", but also
> "noexcept(expression)" and "throw(<types>)". Suggestion for manglings:
>
> <exception-spec> ::=
>   nx  -- non-throwing exception specification
>   nX <expression> E  -- computed (value-dependent) noexcept
>   tw <type>* E  -- throw (types)
>
> <function-type> ::= [<CV-qualifiers>] [<exception-spec>] [Dx] F [Y]
> <bare-function-type> [<ref-qualifier>] E
>
> In the case of throw(a, b, c), we could omit types that are neither
> instantiation-dependent nor pack expansions (if that omits all types, we
> can use the 'nx' mangling instead), since C++17 says you can't overload on
> the actual types in the dynamic exception specification, and we otherwise
> only need them to be present if they might result in a substitution failure.
>
> Thoughts?
>
>
>
> That seems reasonable to me.  I don’t think we need to optimize the “throw
> (a, b, c)” case: It’s deprecated anyway, and having all the types makes for
> nicer demangling.
>

OK. As a special case, we do still need to mangle a non-dependent 'throw()'
as nx rather than twE, since 'void () throw()' and 'void () noexcept' are
the same type, but I think that falls out naturally if the nX and tw
manglings are only used for dependent exception-specifications.
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