[clang] For-range with structured bindings during constant evaluation

[katzdm]

As far as I can tell, the following should be well-formed:

consteval auto f() -> int {
    struct Pair { int first; int second; };

    Pair arr[] = {{1, 1}};
    for (auto const& [key, value] : arr) {
        [=] { [[maybe_unused]] int s = key; }();
    }
    return 0;
}

int p = f();

https://compiler-explorer.com/z/5cGfrMTd7

GCC and MSVC accept; Clang and EDG reject, but EDG has acknowledged their behavior as a bug.

I did some preliminary digging, and the issue looks to occur while computing the lvalue-to-rvalue conversion of the DeclRefExpr that names key in the initializer of s; the findCompleteObject routine seems to be tripping over the structured binding and returns an empty CompleteObject here.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Daniel M. Katz (katzdm)

As far as I can tell, the following should be well-formed:

consteval auto f() -> int {
    struct Pair { int first; int second; };

    Pair arr[] = {{1, 1}};
    for (auto const& [key, value] : arr) {
        [=] { [[maybe_unused]] int s = key; }();
    }
    return 0;
}

int p = f();

https://compiler-explorer.com/z/5cGfrMTd7

GCC and MSVC accept; Clang and EDG reject, but EDG has acknowledged their behavior as a bug.

I did some preliminary digging, and the issue looks to occur while computing the lvalue-to-rvalue conversion of the DeclRefExpr that names key in the initializer of s; the findCompleteObject routine seems to be tripping over the structured binding and returns an empty CompleteObject here.

[shafik]

Looks kind of like: #145776

If we remove the inner lambda the error goes away.

[efriedma-quic]

Slightly simplified:

namespace GH145956 {
  consteval int f() {
    struct Pair { int first; int second; };
    Pair p = {1, 2};
    auto const& [key, value] = p;
    return [=] { return key; }();
  }
  static_assert(f() == 1);
}

The issue is just that the constant evaluator doesn't use the value captured by the lambda; it instead treats the binding as constexpr-unknown, then errors out trying to read the value. (The equivalent code for normal variables is at the beginning of LValueExprEvaluator::VisitVarDecl.)