[PATCH] D151952: [clang] adds `__type_pack_index` so we can get a type's parameter pack index

[David Blaikie via Phabricator via cfe-commits]

dblaikie added inline comments.


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Comment at: clang/include/clang/AST/Stmt.h:796-802
+    /// If this expression is not value-dependent, this stores the value.
+    unsigned Value : 8;
 
     /// The number of arguments to this type trait. According to [implimits]
     /// 8 bits would be enough, but we require (and test for) at least 16 bits
     /// to mirror FunctionType.
+    unsigned NumArgs : 16;
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cjdb wrote:
> These numbers feel very low for how this modification is using them. Perhaps they should both be bumped to 32?
clang does, I think, have specific implementation limits for this sort of thing - 

Well, maybe we don't actually enforce a limit on number of template arguments... https://godbolt.org/z/accncYson compiles successfully, and has 2^18 parameters, beyond the 2^16 suggested here.

But maybe 2^16 is just what we test for/somewhat guarantee.

But if the `Value` is going to be the index into the args, shouldn't it be the same size as `NumArgs`, at least? (& the comment says 16, so 16 for both Value and NumArgs would seem suitably symmetric, and no larger than the current situation - since it'd just be splitting NumArgs in half, while still meeting the comment's suggestion/requirements?)

An assert, at least, when populating NumArgs to ensure it's no larger might not hurt... (which might be reachable from code/not checked prior - so it could be a hard compilation error, I guess, but I wouldn't feel super strongly about it)


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  https://reviews.llvm.org/D151952/new/

https://reviews.llvm.org/D151952