[libcxx-commits] [libcxx] [libc++] Prevent calling the projection more than three times (PR #66315)

[A. Jiang via libcxx-commits]

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@@ -37,9 +37,10 @@ struct __fn {
     _LIBCPP_ASSERT_UNCATEGORIZED(!bool(std::invoke(__comp, std::invoke(__proj, __high), std::invoke(__proj, __low))),
                                  "Bad bounds passed to std::ranges::clamp");
 
-    if (std::invoke(__comp, std::invoke(__proj, __value), std::invoke(__proj, __low)))
+    auto& ValueProjection = std::invoke(__proj, __value);
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frederick-vs-ja wrote:

A reserved identifier like `__projected_value` is needed. A well-defined program can `#define ValueProjection 42`, because `ValueProjection` is unreserved, and then causes error in this line.

Moreover, `auto&` is incorrect because `std::invoke(__proj, __value)` may be an rvalue. `auto&&` or `decltype(auto)` should be used and then the result should be `std::forward`'d in later invocations.

https://github.com/llvm/llvm-project/pull/66315