Extended Description

The warning -Wunused-lambda-capture may indicate that a capture is unused if the body of the lambda contains an if constexpr construct and the capture is only used in the branch which was not instantiated.

Although Clang is technically correct that the capture is not used, the warning is unhelpful, since the capture cannot simply be removed without breaking cases that instantiate the the other branch.

If this is hard to fix "correctly", then Clang should suppress this warning for lambdas that contain an if-constexpr block.

Example:

template <bool b>
int foo(int a) {
  auto f = [&a]() {
    if constexpr(b) {
      return a;
    } else {
      return 0;
    }
  };
  return f();
}

void bar() {
  foo<false>(123);  // generates unused-capture warning in instantiation
  foo<true>(123);   // no warning
}