Incorrect output and segfault with std::optional<std::string> and FMT_COMPILE

[TomCrypto]

We've run into an issue that we've tracked down to formatting std::optional<std::string> with {fmt} in conjunction with the FMT_COMPILE macro. We have observed both incorrect formatted strings and also segmentation faults. I've created a minimal repro case on godbolt with clang trunk and {fmt} trunk:

https://godbolt.org/z/rE5xadqf1

The issue does not seem to occur if FMT_COMPILE is not used or if the literal passed to it is just "{}", but as soon as the literal contains more than just that, issues crop up when formatting string optionals. I haven't checked if it happens with other std types. It doesn't appear to occur when formatting plain strings, or std::optional<int>, etc...

The documentation suggests this should be a valid use of FMT_COMPILE so I'm submitting this as a bug.

[phprus]

Simple test case:
https://godbolt.org/z/qe5zK9Phx

[phprus]

@vitaut

This error has been present since the commit: a34a97c

New test:

TEST(compile_test, format_escape) {
  EXPECT_EQ(fmt::format(FMT_COMPILE("{:?}"), "string"), "\"string\"");
  EXPECT_EQ(fmt::format(FMT_COMPILE("0{:?}"), "string"), "0\"string\"");
  EXPECT_EQ(fmt::format(FMT_COMPILE("{:?}0"), "string"), "\"string\"0");
}