[cfe-dev] Linkage of static local variables in available_externally definitions

[Mehdi Amini via cfe-dev]

> On Jan 26, 2017, at 2:51 PM, David Blaikie via cfe-dev <cfe-dev at lists.llvm.org> wrote:
> 
> In implementing modular codegen, specifically verifying the behavior for static local variables in inline functions in modular headers, I came across a piece of logic in Clang's linkage calculation that seems off.
> 
> To demonstrate a real-world example:
> 
> template<typename T>
> inline void f() { static int i; }
> extern template void f<int>();
> ...
> template void f<int>();
> 
> In the TU with the extern template declaration, an available_externally definition of f<int> and f<int>::i are provided (when optimizations are enabled).
> 
> In the TU with the extern template definition, a weak_odr definition of f<int> is provided but a linkonce_odr definition of f<int>::i is provided.
> 
> This surprises me and doesn't seem correct. What if the TU with the weak_odr definition is optimized to the point of removing the f<int>::i definition - but the available_externally TU is not - it inlines f<int> but leaves a use of f<int>::i available?
> 
> What's the deal here - am I missing something that makes this safe/correct?

Seems buggy to me, either it should be (decl / def ): available_externally / weak_odr or: linkonce_odr / linkonce_odr.

Indeed based on your example, here are two source files and a header that fail to link on macOS when optimizations are enabled.

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— 
Mehdi