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

[David Vandevoorde]

> 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 <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.

	Daveed


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