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Re: C++ PATCH for c++/65046 (ABI tags and functions/variables)

On 2015.03.19 at 15:17 -0400, Jason Merrill wrote:
> This patch makes some significant changes to attribute abi_tag.
> First, it allows explicit naming of tags on inline namespaces, which 
> previously always had a tag with the same name as the namespace itself; 
> this is still the default if no tag is specified.
> It also introduces automatic tagging of functions and variables with 
> tagged types where the tags are not already reflected in the mangled 
> name.  I feel somewhat uneasy about this change, but I think it's the 
> right answer.  -Wabi-tag will also warn about this so that people are 
> aware of it and can tag explicitly if they want to.

This breaks compatibility with other compilers. Consider the case when
a user compiles a library, that contains e.g. some member function with
a std::string return type, with clang using gcc-5's libstdc++. It will
be mangled without abi-tags, because clang doesn't support them. 
Now when the user switches back to gcc-5 and uses the libraries headers
in a new project he will get undefined symbol errors when linking with
the library, because gcc-5 adds an abi-tag to the member function

Another issue is that the -Wabi-tag warning isn't enabled by -Wall or
even by -Wextra.


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