MitalAshok wrote:
Here's one scenario where it could make a difference in codegen
<https://godbolt.org/z/dxvjzsWKr>:
```c++
// Header
struct X;
extern X x;
void f(X&);
inline void g() {
f(x);
}
inline void h() {
f(*__builtin_launder(__builtin_addressof(x)));
}
// Source
struct X {
virtual void mem() { __builtin_printf("X::mem()\n"); }
};
void f(X& x) {
x.mem();
}
void(*p[2])() = { g, h }; // Force inline functions to be generated
```
Even though Clang currently doesn't do this optimization, with
`-fstrict-vtable-pointers`, `g()` is allowed to to devirtualise the call.
Also, we can't "complete" the type at all. See
<https://github.com/llvm/llvm-project/pull/91070/files#diff-a8ef8d65dd30e14c0c43287a86c22b072ad29d04e85c2807602de9bcf2a26af1R160-R164>,
where the template is not allowed to be instantiated by `launder`.
Maybe we could delay the code-gen to the end of the TU to see if it's completed
by then? I do think it's an incredibly rare situation for some incomplete type
to be `std::launder`ed and then later completed in the same TU.
https://github.com/llvm/llvm-project/pull/91070
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