[clang] [Interp] assertion failure on lambda capture of lambda (-fexperimental-new-constant-interpreter)

[sethp]

Given:

void wrapper() {
  constexpr auto f = []() constexpr {
    return 0;
  };

  constexpr auto g = [f]() constexpr {
    return f();
  };
  static_assert(g() == f());
}

With -fexperimental-new-constant-interpreter clang crashes evaluating the declaration of g:

clang++: /root/llvm-project/clang/include/clang/AST/APValue.h:553: clang::APValue& clang::APValue::getStructField(unsigned int): Assertion `i < getStructNumFields() && "field index OOB"' failed.
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace, preprocessed source, and associated run script.
Stack dump:
0.	Program arguments: /opt/compiler-explorer/clang-assertions-trunk/bin/clang++ -gdwarf-4 -g -o /app/output.s -mllvm --x86-asm-syntax=intel -S --gcc-toolchain=/opt/compiler-explorer/gcc-snapshot -fcolor-diagnostics -fno-crash-diagnostics -fexperimental-new-constant-interpreter <source>
...

(see: https://godbolt.org/z/nz8v5sffc )

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (sethp)

Given:

void wrapper() {
  constexpr auto f = []() constexpr {
    return 0;
  };

  constexpr auto g = [f]() constexpr {
    return f();
  };
  static_assert(g() == f());
}

With -fexperimental-new-constant-interpreter clang crashes evaluating the declaration of g:

clang++: /root/llvm-project/clang/include/clang/AST/APValue.h:553: clang::APValue& clang::APValue::getStructField(unsigned int): Assertion `i < getStructNumFields() && "field index OOB"' failed.
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace, preprocessed source, and associated run script.
Stack dump:
0.	Program arguments: /opt/compiler-explorer/clang-assertions-trunk/bin/clang++ -gdwarf-4 -g -o /app/output.s -mllvm --x86-asm-syntax=intel -S --gcc-toolchain=/opt/compiler-explorer/gcc-snapshot -fcolor-diagnostics -fno-crash-diagnostics -fexperimental-new-constant-interpreter <source>
...

(see: https://godbolt.org/z/nz8v5sffc )

[sethp]

I ran into this as part of a test for #74775 (specifically: https://github.com/llvm/llvm-project/pull/74775/files#diff-94c69e2ff8b1d09c63fd7fbeda780d54700f26e7d58e3e3fdb4ee19914927ef6R380-R401 ) that I was playing around with in the new interpreter, and then I shrunk it down to the minimal example above.

Almost certainly there's a better way to express what I was going for (suggestions welcome, here or on the PR!), and this sample works fine:

void wrapper() {
  constexpr auto f = []() constexpr {
    return 0;
  };

  constexpr auto g = [](auto f) constexpr {
    return f();
  };
  static_assert(g(f) == f());
}

So probably pretty low priority, but I wanted to write this one up in case the clang frontend folks are interested (especially @tbaederr ?).