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I'm working on some code that compares two different types with std::upper_bound and does so with a custom lambda, which takes both parameters as type "auto". When building normally, this compiles fine. When building with -D_LIBCPP_DEBUG=1, this fails and somewhere
in the template magic, libc++ seems to confuse the two types and pass them into the lambda backwards.</div>
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Here is some minimal code to reproduce, and the corresponding compilation error:</div>
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<div><span style="color: rgb(197, 134, 192)">#include</span><span style="color: rgb(86, 156, 214)"> </span><span style="color: rgb(206, 145, 120)"><algorithm></span></div>
<div><span style="color: rgb(197, 134, 192)">#include</span><span style="color: rgb(86, 156, 214)"> </span><span style="color: rgb(206, 145, 120)"><vector></span></div>
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<div><span style="color: rgb(86, 156, 214)">struct</span> <span style="color: rgb(78, 201, 176)">T1</span> {</div>
<div> <span style="color: rgb(86, 156, 214)">int</span> i;</div>
<div>};</div>
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<div><span style="color: rgb(86, 156, 214)">int</span> <span style="color: rgb(220, 220, 170)">main</span>(<span style="color: rgb(86, 156, 214)">int</span> argc,
<span style="color: rgb(86, 156, 214)">char</span> <span style="color: rgb(86, 156, 214)">const</span>* argv[]) {</div>
<div> std::vector<T1> t1_vec = {{.<span style="color: rgb(156, 220, 254)">i</span> =
<span style="color: rgb(181, 206, 168)">1</span>}, {.<span style="color: rgb(156, 220, 254)">i</span> =
<span style="color: rgb(181, 206, 168)">3</span>}};</div>
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<div> <span style="color: rgb(220, 220, 170)">std::upper_bound</span>(t1_vec.<span style="color: rgb(220, 220, 170)">begin</span>(), t1_vec.<span style="color: rgb(220, 220, 170)">end</span>(),
<span style="color: rgb(181, 206, 168)">2</span>,</div>
<div> [](<span style="color: rgb(86, 156, 214)">const</span> <span style="color: rgb(86, 156, 214)">auto</span>& a,
<span style="color: rgb(86, 156, 214)">const</span> <span style="color: rgb(86, 156, 214)">auto</span>& b) {
<span style="color: rgb(197, 134, 192)">return</span> a < b.<span style="color: rgb(156, 220, 254)">i</span>; });</div>
<div> <span style="color: rgb(197, 134, 192)">return</span> <span style="color: rgb(181, 206, 168)">0</span>;</div>
<div>}</div>
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<span style="font-family: Consolas, Courier, monospace; font-size: 9pt; background-color: rgb(204, 204, 204);">experimental/upper_bound.cpp:14:67: error: member reference base type</span><span><br>
</span>
<div><span style="font-family: Consolas, Courier, monospace; font-size: 9pt; background-color: rgb(204, 204, 204);"> 'const int' is not a structure or union</span><br>
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<div><span style="font-family: Consolas, Courier, monospace; font-size: 9pt; background-color: rgb(204, 204, 204);"> [](const auto& a, const auto& b) { return a < b.i; });</span><br>
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<div><span style="font-family: Consolas, Courier, monospace; font-size: 9pt; background-color: rgb(204, 204, 204);"> ~^~</span><br>
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<div><span style="font-family: Consolas, Courier, monospace; font-size: 9pt; background-color: rgb(204, 204, 204);">external/llvm_toolchain/bin/../include/c++/v1/algorithm:777:20: note: in instantiation of function template specialization 'main(int, const char
**)::(anonymous class)::operator()<T1, int>' requested here</span><br>
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<div><span style="font-family: Consolas, Courier, monospace; font-size: 9pt; background-color: rgb(204, 204, 204);"> decltype((void)_VSTD::declval<_Compare&>()(</span><br>
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<div><span style="font-family: Consolas, Courier, monospace; font-size: 9pt; background-color: rgb(204, 204, 204);"> ^</span><br>
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<div><span style="font-family: Consolas, Courier, monospace; font-size: 9pt; background-color: rgb(204, 204, 204);">external/llvm_toolchain/bin/../include/c++/v1/__config:473:15: note: expanded from macro '_VSTD'</span><br>
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<div><span style="font-family: Consolas, Courier, monospace; font-size: 9pt; background-color: rgb(204, 204, 204);">#define _VSTD std::_LIBCPP_NAMESPACE</span><br>
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<div><span style="font-family: Consolas, Courier, monospace; font-size: 9pt; background-color: rgb(204, 204, 204);"> ^</span><br>
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<div><span style="font-family: Consolas, Courier, monospace; font-size: 9pt; background-color: rgb(204, 204, 204);">external/llvm_toolchain/bin/../include/c++/v1/algorithm:771:13: note: while substituting deduced template arguments into function template '__do_compare_assert'
[with _LHS = T1, _RHS = int]</span><br>
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<div><span style="font-family: Consolas, Courier, monospace; font-size: 9pt; background-color: rgb(204, 204, 204);"> __do_compare_assert(0, __y, __x);</span><br>
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<div><span style="font-family: Consolas, Courier, monospace; font-size: 9pt; background-color: rgb(204, 204, 204);"> ^</span><br>
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<div><span style="font-family: Consolas, Courier, monospace; font-size: 9pt; background-color: rgb(204, 204, 204);">external/llvm_toolchain/bin/../include/c++/v1/algorithm:4249:13: note: in instantiation of function template specialization 'std::__1::__debug_less<(lambda
at experimental/upper_bound.cpp:14:20)>::operator()<int, T1>' requested here</span><br>
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<div><span style="font-family: Consolas, Courier, monospace; font-size: 9pt; background-color: rgb(204, 204, 204);"> if (__comp(__value_, *__m))</span><br>
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<div><span style="font-family: Consolas, Courier, monospace; font-size: 9pt; background-color: rgb(204, 204, 204);"> ^</span><br>
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<div><span style="font-family: Consolas, Courier, monospace; font-size: 9pt; background-color: rgb(204, 204, 204);">external/llvm_toolchain/bin/../include/c++/v1/algorithm:4268:12: note: in instantiation of function template specialization 'std::__1::__upper_bound<std::__1::__debug_less<(lambda
at experimental/upper_bound.cpp:14:20)> &, std::__1::__wrap_iter<T1 *>, int>' requested here</span><br>
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<div><span style="font-family: Consolas, Courier, monospace; font-size: 9pt; background-color: rgb(204, 204, 204);"> return __upper_bound<_Comp_ref>(__first, __last, __value_, __c);</span><br>
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<div><span style="font-family: Consolas, Courier, monospace; font-size: 9pt; background-color: rgb(204, 204, 204);"> ^</span><br>
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<div><span style="font-family: Consolas, Courier, monospace; font-size: 9pt; background-color: rgb(204, 204, 204);">experimental/upper_bound.cpp:13:8: note: in instantiation of function template specialization 'std::__1::upper_bound<std::__1::__wrap_iter<T1
*>, int, (lambda at experimental/upper_bound.cpp:14:20)>' requested here</span><br>
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<span style="font-family: Consolas, Courier, monospace; font-size: 9pt; background-color: rgb(204, 204, 204);"> std::upper_bound(t1_vec.begin(), t1_vec.end(), 2,</span><br>
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