[PATCH] D23692: Interpret strlen as constexpr for GCC Compatibility

[Richard Smith via cfe-commits]

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This is not a conforming extension. We are explicitly not allowed to make standard library functions `constexpr` if the standard doesn't say they are; see [constexpr.functions] (17.6.5.6) in the C++ standard. Clang previously used to allow constant-folding `strlen` calls, but we removed that support intentionally for this reason.

It would seem reasonable for this constant-folding to be enabled for `strlen` if the callee was explicitly marked as `constexpr` in the source code. That way, if the standard library chooses to mark it as `constexpr` as a (currently) non-conforming extension, we would provide an optimized implementation.


Repository:
  rL LLVM

https://reviews.llvm.org/D23692