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Implement MSVC's language extension to form pointers to members of virtual bases #16085

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rnk opened this issue Apr 9, 2013 · 7 comments
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bugzilla Issues migrated from bugzilla c++ C++ language related issues

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@rnk
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rnk commented Apr 9, 2013

Bugzilla Link 15713
Resolution FIXED
Resolved on Dec 02, 2019 13:50
Version unspecified
OS Windows NT
Blocks #14079
CC @XVilka,@DougGregor,@nico,@timurrrr

Extended Description

Clang currently rejects this program, as the standard says it is ill-formed in [conv.mem], I think:

struct A { int a; };
struct B : virtual A { int b; };
int B::*memptr_b = &B::a; // error

memptr_data_virtual.cpp:8:20: error: conversion from pointer to member of class 'A' to pointer to member of class 'B' via virtual base 'A' is not allowed
int B::*memptr_b = &B::a; // error

MSVC accepts this program, and has a whole bunch of machinery baked into the ABI to support it. Data member pointers can be formed with up to 3 (!) i32s.

There's basically no way to implement this within the Itanium ABI since data memptrs there are always just one offset, so this would only be in -fms-compatibility (or -fms-extensions, I forget which is which).

Mostly I'm just filing this to have a discussion of record with a decision to implement or not. We should document this as part of http://clang.llvm.org/compatibility.html, probably.

@timurrrr
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timurrrr commented Apr 12, 2013

I think it's perfectly fine to not support it until we observe a real code that actually uses it and has some very strong reasons to use this bad pattern.

@rnk
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rnk commented Apr 12, 2013

Right, I'm mostly just filing the PR to document the state.

FWIW I think John said it best in the code review. This isn't a bad pattern, it's just a feature that MSVC implemented which was pared down during standardization.

@nico
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nico commented Jun 23, 2015

r240383, r240384 removed a few FIXMEs pointing to this bug. (The bug is still valid though.)

Since we haven't needed this yet, it's probably ok to WontFix it if it's only implementable on Windows.

@XVilka
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XVilka commented Nov 29, 2019

Is this bug still relevant? Because it is mentioned at the MSVC compatibility page and was updated 4 years ago...

https://clang.llvm.org/docs/MSVCCompatibility.html

@rnk
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rnk commented Dec 2, 2019

Yeah, we implemented this a long time ago.

@rnk
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rnk commented Nov 26, 2021

mentioned in issue llvm/llvm-bugzilla-archive#15875

@majnemer
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majnemer mannequin commented Nov 26, 2021

mentioned in issue llvm/llvm-bugzilla-archive#23452

@llvmbot llvmbot transferred this issue from llvm/llvm-bugzilla-archive Dec 4, 2021
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