[llvm] [ADT] Fix llvm::join on containers of char*s (PR #67113)

[David Blaikie via llvm-commits]

================
@@ -417,8 +418,13 @@ inline std::string join_impl(IteratorT Begin, IteratorT End,
     return S;
 
   size_t Len = (std::distance(Begin, End) - 1) * Separator.size();
-  for (IteratorT I = Begin; I != End; ++I)
-    Len += (*I).size();
+  for (IteratorT I = Begin; I != End; ++I) {
+    if constexpr (std::is_same_v<std::remove_reference_t<decltype(*I)>,
+                                 const char *>)
+      Len += strlen(*I);
+    else
+      Len += (*I).size();
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dwblaikie wrote:

> Sure, constructing a temporary StringRef would work too, what's the difference?

It does allow the implementation to remain generic, if somewhat more subtle than it currently is.

> > any chance of using StringRef in the callers? Rather than having more places dealing with null terminated strings?
> 
> Converting Driver and others to use StringRef seems like a helpful project, though not one I personally have time for. Having a few functions not work with `char*` doesn't seem like a useful state in the meanwhile, but I already wrote the concat-loop where this was actually needed, so I can drop this if we'd rather not have it.

*nod* I'm more comfortable with it with the StringRef/generic implementation. "supports anything that's StringRef-able" sounds OK to me.

https://github.com/llvm/llvm-project/pull/67113