[Bug c++/97553] New: [missed optimization] constexprness not noticed when UBsan enabled

["eyalroz at technion dot ac.il" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97553

            Bug ID: 97553
           Summary: [missed optimization] constexprness not noticed when
                    UBsan enabled
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: eyalroz at technion dot ac.il
  Target Milestone: ---

(GodBolt example: https://godbolt.org/z/Kvan5c)

Consider the following code:

  #include <string_view>

  constexpr std::string_view f() { return "hello"; }

  static constexpr std::string_view g() {
      auto x { f() };
      return x.substr(1, 3);
  } 

  int foo() { return g().length(); }

if you compile it with flags `--std=c++17 -O3`, it results in a pleasant:

  foo():
          mov     eax, 3
          ret

but if you also enabled undefined-behavior sanitization, i.e. `--std=c++17
-fsanitize=undefined -O3`, then you get a much longer program with UB-related
instrumentation - which is never used.

I'm not sure if it's because some optimizations are disabled with UBsan, in
which case this might be a "misfeature", or whether they're enabled but the
optimization is just missed.