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Fix 21210: std.traits : isAssignable false positive on disabled copy struct #7612

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Aug 31, 2020
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Fix 21210: std.traits : isAssignable false positive on disabled copy struct #7612

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7 changes: 5 additions & 2 deletions std/traits.d
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Original file line number Diff line number Diff line change
Expand Up @@ -5180,10 +5180,10 @@ enum isAssignable(Lhs, Rhs = Lhs) = isRvalueAssignable!(Lhs, Rhs) && isLvalueAss
}

// 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.


// ditto
private enum isLvalueAssignable(Lhs, Rhs = Lhs) = __traits(compiles, lvalueOf!Lhs = lvalueOf!Rhs);
private enum isLvalueAssignable(Lhs, Rhs = Lhs) = __traits(compiles, { lvalueOf!Lhs = lvalueOf!Rhs; });

@safe unittest
{
Expand Down Expand Up @@ -5224,6 +5224,9 @@ private enum isLvalueAssignable(Lhs, Rhs = Lhs) = __traits(compiles, lvalueOf!Lh
static assert(!isRvalueAssignable!(S6, S5));
static assert( isLvalueAssignable!(S6, S5));
static assert( isLvalueAssignable!(S6, immutable S5));

// https://issues.dlang.org/show_bug.cgi?id=21210
static assert(!isAssignable!S5);
}


Expand Down