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Fix compile error for tuple size > 8 #1767

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Fix compile error for tuple size > 8 #1767

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10 changes: 5 additions & 5 deletions include/stdexec/__detail/__tuple.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -345,7 +345,11 @@ namespace STDEXEC {
constexpr auto __size = STDEXEC_REMOVE_REFERENCE(_Tuple)::__size;
static_assert(_Index < __size, "Index out of bounds in __get");

if constexpr (_Index == 0) {
// Only use __valN accessors for tuples with <= 8 elements (which have specialized storage)
// Larger tuples use __box base class and need __tup::__get
if constexpr (__size > 8) {
return __tup::__get<_Index>(static_cast<_Tuple&&>(__tupl));
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@ericniebler ericniebler Jan 16, 2026

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i'm curious why you moved this branch to the top. i expect tuples larger than 8 elements to be quite rare, which is why i put the fallback branch last.

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@lixin-wei lixin-wei Jan 16, 2026

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because tuples larger than 8 elements don't have __val fields.

if we put this to the end, the compile will fail.

} else if constexpr (_Index == 0) {
return static_cast<_Tuple&&>(__tupl).__val0;
} else if constexpr (_Index == 1) {
return static_cast<_Tuple&&>(__tupl).__val1;
Expand All @@ -361,10 +365,6 @@ namespace STDEXEC {
return static_cast<_Tuple&&>(__tupl).__val6;
} else if constexpr (_Index == 7) {
return static_cast<_Tuple&&>(__tupl).__val7;
} else if constexpr (_Index == 8) {
return static_cast<_Tuple&&>(__tupl).__val8;
} else {
return __tup::__get<_Index>(static_cast<_Tuple&&>(__tupl));
}
}

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30 changes: 30 additions & 0 deletions test/stdexec/algos/adaptors/test_when_all.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -62,6 +62,36 @@ namespace {
wait_for_value(std::move(snd), 2, 3, 5, 7, 11);
}

TEST_CASE("when_all with 8 senders", "[adaptors][when_all]") {
// 8 senders is the boundary case for the optimized tuple specializations
ex::sender auto snd = ex::when_all(
ex::just(2),
ex::just(3),
ex::just(5),
ex::just(7),
ex::just(11),
ex::just(13),
ex::just(17),
ex::just(19));
wait_for_value(std::move(snd), 2, 3, 5, 7, 11, 13, 17, 19);
}

TEST_CASE("when_all with 10 senders", "[adaptors][when_all]") {
// 10 senders uses the generic tuple with __box storage
ex::sender auto snd = ex::when_all(
ex::just(2),
ex::just(3),
ex::just(5),
ex::just(7),
ex::just(11),
ex::just(13),
ex::just(17),
ex::just(19),
ex::just(23),
ex::just(29));
wait_for_value(std::move(snd), 2, 3, 5, 7, 11, 13, 17, 19, 23, 29);
}

TEST_CASE("when_all with just one sender", "[adaptors][when_all]") {
ex::sender auto snd = ex::when_all(ex::just(2));
wait_for_value(std::move(snd), 2);
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