[llvm-bugs] [Bug 39108] New: constexpr std::string_view global no longer compiles

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https://bugs.llvm.org/show_bug.cgi?id=39108

            Bug ID: 39108
           Summary: constexpr std::string_view global no longer compiles
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: release blocker
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: jorg.brown at gmail.com
                CC: dgregor at apple.com, llvm-bugs at lists.llvm.org

clang 7.0 accepted global variables of type std::string_view, even if declared
constexpr.

The current trunk of clang (clang version 8.0.0 (trunk 343155) ) does not.

For that matter, std::string::traits_type::length also does not.

Repro: this compiles under 7.0 but not 8.0.0 / trunk 343155:

#include <string>

constexpr size_t one = std::string::traits_type::length("1");

size_t return_1() { return one; }

Giving this error:

<source>:3:18: error: constexpr variable 'one' must be initialized by a
constant expression
constexpr size_t one = std::string::traits_type::length("1");
                 ^     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

<source>:3:24: note: non-constexpr function 'length' cannot be used in a
constant expression
constexpr size_t one = std::string::traits_type::length("1");
                       ^

/opt/compiler-explorer/gcc-7.2.0/lib/gcc/x86_64-linux-gnu/7.2.0/../../../../include/c++/7.2.0/bits/char_traits.h:268:7:
note: declared here
      length(const char_type* __s)
      ^

Obligatory godbolt link: https://gcc.godbolt.org/z/yOmaJr

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