IRGen: Weakly link symbols for unavailable declarations #64429
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When computing linkage, the compiler would treat unavailable declarations as if they were "always available" when they lack an `introduced:` version: ``` // Library @available(macOS, unavailable) public func foo() { // … } // Client import Library @available(macOS, unavailable) func bar() { // Even though foo() and bar() are unavalable on macOS the compiler still // strongly links foo(). foo() } ``` This created an unnecessary dependency between libraries and their clients and also can interfere with back deployment, since unavailable declarations may not be present in a library on all OS versions. Developers could work around these issues by conditionally compiling the code that references an unavailable declaration, but they shouldn't have to given that unavailable code is meant to be provably unreachable at runtime. Additionally, it could improve library code size if we allowed the compiler to strip unavailable declarations from a binary completely. Resolves rdar://106673713
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This is a second attempt of #64353.
When computing linkage, the compiler would treat unavailable declarations as if they were "always available" if they also lack an
introduced:version:This created an unnecessary dependency between libraries and their clients and also can interfere with back deployment, since unavailable declarations may not be present in a library on all OS versions. Developers could work around these
issues by conditionally compiling the code that references an unavailable declaration, but they shouldn't have to given that unavailable code is meant to be provably unreachable at runtime. Additionally, in the future it could improve library code size if we allowed the compiler to strip unavailable declarations from a binary completely.
Resolves rdar://106673713