Feature request: __traits(canInstantiate), like __traits(compiles) but without suppressing syntax errors
feepingcreature at gmail.com
Sat Jan 18 15:21:47 UTC 2020
On Friday, 17 January 2020 at 16:01:29 UTC, Steven Schveighoffer
> However, the real problem is things like typos that cause the
> function not to compile for reasons other than the intended
> For example:
> T foo(T)(T x)
> return X + 5; // typo on parameter name
> This will NEVER compile, because X will never exist. The
> intention was for it to compile with types that support
> addition to integers (i.e. it's EXPECTED to fail for strings),
> but instead it fails for all types. The result is that your
> code selects some other path when you don't want it to.
> Sometimes this just means it works but is slower, which is even
> harder to figure out.
> But it's hard for the compiler to deduce intention from such
> typos. It would be great if the compiler could figure out this
> type of error, but I don't think it can.
Yes it can!
D already has support for exactly this kind of thing, with
std.traits and `if()` constraints. If you have a metafunction
that tries to see if "an alias is usable with string", and it
checks that with __traits(canInstantiateCall, foo, string.init)
or whatever, then you *should* get a syntax error even in the
"return x + 5" case. If you want to avoid that, we don't need
language changes, you just need to use the *already existing*
mechanism of "if (isNumeric!T)".
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