Interaction between unsigned and pointer/array

[danakj]

Using an operator[] on a type that takes size_t works great with unsigned types. They convert to primitives.

However operator[] on a pointer is built in and takes a signed ptrdiff_t. Converting from an unsigned value to signed ptrdiff_t could overflow and thus needs to check(). But we want implicit signed conversions to not compile.

There's no way I can think of to limit the conversions to only happening inside ptr[x]. If the compiler would convert that to *(ptr + x) then it would work fine, but it doesn't. It needs x to become ptrdiff_t.

Allowing conversion to ptrdiff_t also allows conversion to long (on linux) and int64_t which is much too broad.

What do?

• A method on unsigned types to check+convert?
• Expect code with pointer indexing to be converted to span/array/vector/etc first? This should be happening regardless but ordering dependencies are not good.
• Rewrite it to *(ptr + x)?
• Introduce -funsigned-pointer-index which adds a size_t overload???
• ???

[danakj]

TODO: Make the better problem description doc public, and bring some kind of proposal to Clang.