[Issue 17748] extern(C) do nothing on struct methods

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=17748

--- Comment #5 from Simen Kjaeraas <simen.kjaras at gmail.com> ---
But the code is in D - the compiler isn't being asked to check any code that
isn't D code.

There are basically three options here:

1) Mangle S.foo as '_foo', and have it behave like an extern(C) function taking
an S as its first parameter.

2) Disallow the code, via a compilation error or at the very least a warning.

3) Ignore 'extern(C)' and generate code that behaves counter to the
programmer's expectations.

Note that none of these options require the compiler to check the correctness
of any code that is not D code.

Currently, as this bug report points out, the option chosen is 3). This is the
worst option, as it leads to an error at link-time, which might be in a dynamic
linker at some unspecified later point.

Since the compiler already knows that it won't mangle the name correctly,
implementing 2) should be little work, and provide great help to the poor
programmer who's bitten by this bug.

Since pragma(mangle) works on struct methods, implementing 1) should also not
be an insurmountable problem, but since C does not have methods, the calling
convention is basically unspecified, and something will need to be done in that
case. Not sure how much work this would be.

The current solution is basically the worst possible, and for no good reason.
The only way it could be worse is if it mangled the name as requested and still
used the D calling convention.

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