Adding `availability` attribute overrides visibility of symbols

[var-const]

Symbols annotated with __attribute__((availability)) always have default visibility, even if marked with __attribute__((__visibility__("hidden"))) and compiled with -fvisibility=hidden. Self-contained repro (courtesy of @ldionne):

// Compile with: clang++ -xc++ - -shared -o a.out -fvisibility=hidden -fvisibility-inlines-hidden
// Check symbols with: nm -omg a.out | c++filt | grep value
template <class T>
struct foo {
    __attribute__((__visibility__("hidden")))
    __attribute__((availability(macos,strict,introduced=10.13)))
    T value() const { return 0; }
};

int f(foo<int> x) {
    return x.value();
}

Running nm on the resulting binary shows it contains

a.out: 0000000000000408 (__TEXT,__text) weak external foo<int>::value() const

Annotating a symbol with availability should not affect its visibility.

Also tracked as rdar://33655115

[llvmbot]

@llvm/issue-subscribers-clang-codegen

Author: Konstantin Varlamov (var-const)

Symbols annotated with `__attribute__((availability))` always have default visibility, even if marked with `__attribute__((__visibility__("hidden")))` and compiled with `-fvisibility=hidden`. Self-contained repro (courtesy of @ldionne): ```cpp // Compile with: clang++ -xc++ - -shared -o a.out -fvisibility=hidden -fvisibility-inlines-hidden // Check symbols with: nm -omg a.out | c++filt | grep value template <class T> struct foo { __attribute__((__visibility__("hidden"))) __attribute__((availability(macos,strict,introduced=10.13))) T value() const { return 0; } };

int f(foo<int> x) {
return x.value();
}

Running `nm` on the resulting binary shows it contains

a.out: 0000000000000408 (__TEXT,__text) weak external foo<int>::value() const

Annotating a symbol with availability should not affect its visibility.
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