[clang] `std::bit_cast` of a string literal is not a constant expression

[eisenwave]

The following code should succeed, but fails to compile:

#include <bit>

// error: constexpr variable 'x' must be initialized by a constant expression
constexpr auto x = std::bit_cast<int>("xyz");

error: constexpr variable 'x' must be initialized by a constant expression
constexpr auto x = std::bit_cast<int>("xyz");
               ^   ~~~~~~~~~~~~~~~~~~~~~~~~~
note: subexpression not valid in a constant expression
      return __builtin_bit_cast(_To, __from);
             ^
note: in call to 'bit_cast("xyz")'
constexpr auto x = std::bit_cast<int>("xyz");
                   ^
1 error generated.

Putting the string literal into a char[4] first solves the problem, but it should work either way.

See Compiler Explorer

[tbaederr]

The lvalue-to-rvalue conversion here:

llvm-project/clang/lib/AST/ExprConstant.cpp

Line 7348 in 5f1ba3a

if (!handleLValueToRValueConversion(

fails.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[tbaederr]

Okay, looks like this is a known limitation:

llvm-project/clang/lib/AST/ExprConstant.cpp

Lines 4315 to 4321 in 7505f2c

	if (LVal.Designator.Entries.empty()) {
	// Fail for now for LValue to RValue conversion of an array.
	// (This shouldn't show up in C/C++, but it could be triggered by a
	// weird EvaluateAsRValue call from a tool.)
	Info.FFDiag(Conv);
	return false;
	}