[cfe-dev] PR15105: __attribute__((noreturn)) and noreturn function types

[Richard Smith]

Hi,

As PR15105 notes, our implementation of __attribute__((noreturn)) is
not entirely GCC-compatible. As far as I can tell, the situation in
GCC is:

GCC does not have a plain 'noreturn function' type.
__attribute__((noreturn)) on a function declaration makes that
declaration declare a noreturn function. It does not affect the
function's type.
__attribute__((noreturn)) on a function pointer or function reference
produces a 'pointer/reference to noreturn function' type.
GCC allows implicit conversions in either direction between
pointer-to-noreturn-function and pointer-to-function.

In Clang:

Clang *does* have a plain 'noreturn function' type.
__attribute__((noreturn)) on a function declaration makes that
declaration declare a noreturn function, and *adjusts the function
type* to be a noreturn function.
__attribute__((noreturn)) on a function pointer or function reference
produces a 'pointer/reference to noreturn function' type.
Clang allows a pointer-to-noreturn-function to implicitly convert to a
pointer-to-function, but not vice versa.

So... Clang's approach is more sound and type-safe, but it's not
GCC-compatible. The difference reaches into the language semantics
(C++ templates can tell the difference between noreturn and
not-noreturn functions), and the ABI (GCC has a different mangling for
pointer-to-noreturn-function which Clang does not implement).

What do people think? Is our improved behavior here sufficient to
justify the GCC incompatibility, or should we switch to being GCC
compatible? Or pick some middle-ground -- perhaps we could continue to
diagnose pointer-to-function to pointer-to-noreturn-function
conversions, except where the source expression is directly taking the
address of a noreturn function?

Thanks!
Richard