Support SFINAE in Constant representability check#669
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Right now, we have an unconditional `UnitRatio` invocation, which produces a hard compiler error. We thought this was fine, because nobody should be invoking this check with different-dimension units. However, it somehow does get invoked in Aurora-internal code. I tried reproducing the error in the Au repo only, and couldn't do it. But in any case, we can fix it by just avoiding the hard error. The most straightforward way to fix it was to just make a new trait, `IsUnitRatioRepresentableIn<T, U1, U2>`. This doesn't seem like something I want to expose to end users, so I kept it as a library internal detail.
geoffviola
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Apr 6, 2026
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| TEST(CanStoreValueIn, ReturnsFalseRatherThanHardErrorForDifferentDimensions) { | ||
| constexpr bool result = Constant<Meters>::can_store_value_in<long>(Nano<Seconds>{}); |
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Does the size of long matter? It's 8 bytes for GCC and 4 bytes for MSVC: godbolt.
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In this specific case, definitely not. The answer doesn't depend on the ratio, because the ratio is not even well defined. It will return false because the dimensions are different.
| // same dimension. | ||
| namespace detail { | ||
| template <typename T, typename U1, typename U2> | ||
| struct IsUnitRatioRepresentableIn; |
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Q: Does it make sense to unit test this?
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I hadn't been planning on it initially, but I agree it makes sense to add them.
geoffviola
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Apr 6, 2026
geoffviola
approved these changes
Apr 6, 2026
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Right now, we have an unconditional
UnitRatioinvocation, whichproduces a hard compiler error. We thought this was fine, because
nobody should be invoking this check with different-dimension units.
However, it somehow does get invoked in Aurora-internal code.
I tried reproducing the error in the Au repo only, and couldn't do it.
But in any case, we can fix it by just avoiding the hard error.
The most straightforward way to fix it was to just make a new trait,
IsUnitRatioRepresentableIn<T, U1, U2>. This doesn't seem likesomething I want to expose to end users, so I kept it as a library
internal detail.