`constexpr string` from `constexpr string_view` is not always a constant expression

[fekir]

Consider following program, which compiles with gcc and msvc

#include <string_view>
#include <string>

template <typename T>
constexpr int foo() {
  constexpr auto test = std::string_view( "test" );

  static_assert(test.size() >1);
  static_assert(std::string(test).size() >1); // should not fail
  return 0;
}


int bar(){
    return foo<int>();
}

With clang it fails to compile with "error: static assertion expression is not an integral constant expression".

Strangely, adding a local char array and a static_assert, changes the behavior

This code still fails to compile

#include <string_view>
#include <string>

template <typename T>
constexpr int foo() {
  constexpr auto test = std::string_view( "test" );
  constexpr char test2[] = "test";

  static_assert(test.size() >1);
  static_assert(std::string(test).size() >1); // should still not fail
  static_assert(std::string(test2).size() >1);
  return 0;
}


int bar(){
    return foo<int>();
}

This one compiles:

#include <string_view>
#include <string>

template <typename T>
constexpr int foo() {
  constexpr auto test = std::string_view( "test" );
  constexpr char test2[] = "test";

  static_assert(test.size() >1);
  static_assert(std::string(test2).size() >1);
  static_assert(std::string(test).size() >1); // success
  return 0;
}


int bar(){
    return foo<int>();
}

live demo: https://godbolt.org/z/KK1WjajPq

[frederick-vs-ja]

See also #112894.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Federico Kircheis (fekir)

Consider following program, which compiles with gcc and msvc

#include <string_view>
#include <string>

template <typename T>
constexpr int foo() {
  constexpr auto test = std::string_view( "test" );

  static_assert(test.size() >1);
  static_assert(std::string(test).size() >1); // should not fail
  return 0;
}


int bar(){
    return foo<int>();
}

With clang it fails to compile with "error: static assertion expression is not an integral constant expression".

Strangely, adding a local char array and a static_assert, changes the behavior

This code still fails to compile

#include <string_view>
#include <string>

template <typename T>
constexpr int foo() {
  constexpr auto test = std::string_view( "test" );
  constexpr char test2[] = "test";

  static_assert(test.size() >1);
  static_assert(std::string(test).size() >1); // should still not fail
  static_assert(std::string(test2).size() >1);
  return 0;
}


int bar(){
    return foo<int>();
}

This one compiles:

#include <string_view>
#include <string>

template <typename T>
constexpr int foo() {
  constexpr auto test = std::string_view( "test" );
  constexpr char test2[] = "test";

  static_assert(test.size() >1);
  static_assert(std::string(test2).size() >1);
  static_assert(std::string(test).size() >1); // success
  return 0;
}


int bar(){
    return foo<int>();
}

live demo: https://godbolt.org/z/KK1WjajPq