[Clang] FE assertion failure in CGCoroutine: "expected alloca to be emitted"

[yuxuanchen1997]

Minimum Repro:

#include <coroutine>

struct Task {
    struct promise_type {
        Task get_return_object();
        
        std::suspend_always initial_suspend() { return {}; }
        std::suspend_always final_suspend() noexcept { return {}; }
        
        void return_void() {}
        void unhandled_exception() {}
    };
    
    Task() {}
    Task(const Task&) {};
};

// Example usage
const Task example() {
    co_return;
}

Compiler Explorer: https://godbolt.org/z/9W6vEvvz4

Assertion failure here:

llvm-project/clang/lib/CodeGen/CGCoroutine.cpp

Line 712 in 1a940bf

assert(GroAlloca && "expected alloca to be emitted");

This is due to that in example()'s return type is const qualified and therefore didn't pass this check for DirectEmit, and that it's NRVO optimized. Emission of NRVO optimized variables don't have their Pointer be an AllocaInst.

The fix is either don't compare Qualified Types in DirectEmit or just don't attach the metadata when the return value is NRVO optimized.

[llvmbot]

@llvm/issue-subscribers-coroutines

Author: Yuxuan Chen (yuxuanchen1997)

Minimum Repro: ```cpp #include <coroutine>

struct Task {
struct promise_type {
Task get_return_object();

    std::suspend_always initial_suspend() { return {}; }
    std::suspend_always final_suspend() noexcept { return {}; }
    
    void return_void() {}
    void unhandled_exception() {}
};

Task() {}
Task(const Task&) {};

};

// Example usage
const Task example() {
co_return;
}

Compiler Explorer: https://godbolt.org/z/9W6vEvvz4

Assertion failure here: https://github.com/llvm/llvm-project/blob/1a940bfff9176fb38a71b0045d7ae25df9a4ca7d/clang/lib/CodeGen/CGCoroutine.cpp#L712

This is due to that in `example()`'s return type is const qualified and therefore didn't pass [this check](https://github.com/llvm/llvm-project/blob/1a940bfff9176fb38a71b0045d7ae25df9a4ca7d/clang/lib/CodeGen/CGCoroutine.cpp#L683) for `DirectEmit`, and that it's NRVO optimized. 

The fix is either don't compare Qualified Types in `DirectEmit` or just don't attach the metadata when the return value is NRVO optimized, in which case the `AllocaInst` doesn't exist. 
</details>

[llvmbot]

@llvm/issue-subscribers-clang-codegen

Author: Yuxuan Chen (yuxuanchen1997)

Minimum Repro: ```cpp #include <coroutine>

struct Task {
struct promise_type {
Task get_return_object();

    std::suspend_always initial_suspend() { return {}; }
    std::suspend_always final_suspend() noexcept { return {}; }
    
    void return_void() {}
    void unhandled_exception() {}
};

Task() {}
Task(const Task&) {};

};

// Example usage
const Task example() {
co_return;
}

Compiler Explorer: https://godbolt.org/z/9W6vEvvz4

Assertion failure here: https://github.com/llvm/llvm-project/blob/1a940bfff9176fb38a71b0045d7ae25df9a4ca7d/clang/lib/CodeGen/CGCoroutine.cpp#L712

This is due to that in `example()`'s return type is const qualified and therefore didn't pass [this check](https://github.com/llvm/llvm-project/blob/1a940bfff9176fb38a71b0045d7ae25df9a4ca7d/clang/lib/CodeGen/CGCoroutine.cpp#L683) for `DirectEmit`, and that it's NRVO optimized. Emission of NRVO optimized variables don't have their `Pointer` be an `AllocaInst`. 

The fix is either don't compare Qualified Types in `DirectEmit` or just don't attach the metadata when the return value is NRVO optimized.
</details>