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Richard Smith
71ba07239f
Previously, we picked a single Carbon parameter pattern for each C++ parameter pattern. This doesn't work well in cases where the Carbon semantics and the C++ semantics are not perfectly aligned. In particular, when a parameter is passed by value in C++, that might mean either pass-by-move (which in Carbon would best be modeled by a `var` pattern, as no other form of parameter would perform a move) or pass-by-copy (which in Carbon would best be modeled by a value parameter, as a `var` parameter would force an extra copy). After this change, we compute a passing mode for each parameter based on the implicit conversion sequence from the argument to the parameter as determined by C++ overload resolution, and use that to determine the Carbon pattern corresponding to each C++ parameter. This results in potentially generating multiple different thunks for the same C++ function if it's called in different ways, but we already did that to handle default arguments and list-initialization. The passing modes are included in the thunk mangling. Add a new value store for clang decl signatures, which capture the information about parameter passing mode as well as the other existing information about different ways that a C++ function might be imported to Carbon. Most of the rules for computing passing modes are the same as before: const references use pass by value, non-const lvalue references use pass-by-ref, non-const rvalue references use pass-by-var. But for C++ non-reference parameters, pick between pass-by-value and pass-by-var based on whether the implicit conversion sequence was effectively performing a copy. Prefer pass-by-value if either would work and they'd do the same thing. We still use pass-by-value for const references, even when the argument is an lvalue and we could pass a reference; we may want to change this in future. For virtual functions, we try to pick a worst-case passing mode, as we can only pick a single signature for what goes in the vtable. Calls to virtual functions will still use a thunk to C++, allowing variance in the calling convention at call sites. We don't allow variance in the overriders as we don't implement support for thunks for virtual functions yet. We currently use pass-by-value for const reference parameters here, but that should probably change at some point. Assisted-by: Gemini via Antigravity