P0980 Making std::string constexpr

[tituswinters]

P0980 Making std::string constexpr (Louis Dionne)

[tituswinters]

Forward to LWG for C++20.
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This matches the similar operation for std::vector, but isn't a breaking change for C++20.

[JohelEGP]

Things added since R0 that should probably be marked constexpr too:

• u8string UDL.
• erase and erase_if.

[wg21bot]

P0980R1 Making std::string constexpr (Louis Dionne)

[wg21bot]

Adopted 2019-07.