Warning for ODR violation

[hfinkel]

Bugzilla Link	23159
Version	trunk
OS	Linux
CC	@DougGregor,@zygoloid

Extended Description

Richard provided to me this example showing ODR violation:

#include

// implicitly internal linkage due to 'const'
const int n = 5;

// odr violation, captures n by reference
inline auto f() { return std::make_pair(n, 3); }

// much more obvious odr violation, takes address of n
inline auto g() {
static const int *p = &n;
return p;
}

Clang compiles this, however, without issuing any warnings. Taking the address of a variable with internal linkage in an inline function is probably not a good idea.

[llvmbot]

There are additional examples of this sort of error that should produce a warning. Probably inline functions and template definitions should warn about any usage of names with internal linkage (and entities in anonymous namespaces, before those were made to have internal linkage).

namespace {
inline void foo() {}
}

static inline void bar() {}

// ODR violations if these appear in a header and that header is included in more than one implementation file:

inline void baz() { foo(); bar(); } 

template<typename T> void qux() { foo(); bar(); }

I think warning about such definitions as soon as they're seen in a header, without waiting to see if that header is included in more than one implementation file, is reasonable.

The fix-it would be to either make these static or in an anonymous namespace, or to move the definitions to an implementation file.