Fix 21210: std.traits : isAssignable false positive on disabled copy struct #7612
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…struct `isAssignable` would previously return `true` for non-copyable types, even though code that tried to use an lvalue would not compile. This behavior was originally found when implementing `-preview=in`. With the new -preview=in check, the const-folding seemed to be a bit too aggressive when an rvalue is passed, meaning that the check might fail (probably due to the code that initialize the temporary).
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Thanks for your pull request, @Geod24! Bugzilla references
Testing this PR locallyIf you don't have a local development environment setup, you can use Digger to test this PR: dub run digger -- build "master + phobos#7612" |
| @@ -5180,10 +5180,10 @@ enum isAssignable(Lhs, Rhs = Lhs) = isRvalueAssignable!(Lhs, Rhs) && isLvalueAss | |||
| } | |||
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| // ditto | |||
| private enum isRvalueAssignable(Lhs, Rhs = Lhs) = __traits(compiles, lvalueOf!Lhs = rvalueOf!Rhs); | |||
| private enum isRvalueAssignable(Lhs, Rhs = Lhs) = __traits(compiles, { lvalueOf!Lhs = rvalueOf!Rhs; }); | |||
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Isn't this a more of a workaround for a compiler issue?
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I'm conflicted about this, because I'm not exactly sure of what code path is taken within the compiler.
However, I can see why this behavior (CTFE-ing the expression) would make sense. But it's also a fatal flaw with __traits(compiles, ...), which is not shared by is(typeof(...)). On a related not, I never understood why we added the former if we had the latter.
wilzbach
approved these changes
Aug 30, 2020
thewilsonator
approved these changes
Aug 31, 2020
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The failure with SumType has been fixed.