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

[Richard Smith]

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?
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