Lambda evaluated for constant template parameter of function inside templated generic lambda incorrectly complains about local variables needing captures

[MitalAshok]

This is a reduced example https://godbolt.org/z/WzshaKvxW:

// -std=c++20 -fsyntax-only
template<int N>
void f();

template<class>
struct X {
    using type = decltype([](auto) {
        f<[]{
            int result = 0;
            return result;
        }()>();
    }(0));
};

X<void> l;

<source>:9:20: error: variable 'result' cannot be implicitly captured in a lambda with no capture-default specified
    9 |             return result;
      |                    ^
<source>:7:13: note: while substituting into a lambda expression here
    7 |         f<[]{
      |             ^
<source>:6:27: note: in instantiation of function template specialization 'X<void>::(anonymous class)::operator()<int>' requested here
    6 |     using type = decltype([](auto) {
      |                           ^
<source>:14:9: note: in instantiation of template class 'X<void>' requested here
   14 | X<void> l;
      |         ^
<source>:8:17: note: 'result' declared here
    8 |             int result = 0;
      |                 ^
<source>:7:11: note: lambda expression begins here
    7 |         f<[]{
      |           ^
<source>:7:12: note: capture 'result' by value
    7 |         f<[]{
      |            ^
      |            result
<source>:7:12: note: capture 'result' by reference
    7 |         f<[]{
      |            ^
      |            &result
<source>:7:12: note: default capture by value
    7 |         f<[]{
      |            ^
      |            =
<source>:7:12: note: default capture by reference
    7 |         f<[]{
      |            ^
      |            &

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Mital Ashok (MitalAshok)

This is a reduced example <https://godbolt.org/z/WzshaKvxW>:

// -std=c++20 -fsyntax-only
template<int N>
void f();

template<class>
struct X {
    using type = decltype([](auto) {
        f<[]{
            int result = 0;
            return result;
        }()>();
    }(0));
};

X<void> l;

<source>:9:20: error: variable 'result' cannot be implicitly captured in a lambda with no capture-default specified
    9 |             return result;
      |                    ^
<source>:7:13: note: while substituting into a lambda expression here
    7 |         f<[]{
      |             ^
<source>:6:27: note: in instantiation of function template specialization 'X<void>::(anonymous class)::operator()<int>' requested here
    6 |     using type = decltype([](auto) {
      |                           ^
<source>:14:9: note: in instantiation of template class 'X<void>' requested here
   14 | X<void> l;
      |         ^
<source>:8:17: note: 'result' declared here
    8 |             int result = 0;
      |                 ^
<source>:7:11: note: lambda expression begins here
    7 |         f<[]{
      |           ^
<source>:7:12: note: capture 'result' by value
    7 |         f<[]{
      |            ^
      |            result
<source>:7:12: note: capture 'result' by reference
    7 |         f<[]{
      |            ^
      |            &result
<source>:7:12: note: default capture by value
    7 |         f<[]{
      |            ^
      |            =
<source>:7:12: note: default capture by reference
    7 |         f<[]{
      |            ^
      |            &

[shafik]

CC @cor3ntin

[Backl1ght]

I think we should return true here

llvm-project/clang/lib/Sema/SemaExpr.cpp

Line 19399 in 035f811

FunctionScopesIndex == MaxFunctionScopesIndex && VarDC == DC)

, but we won't currently due to mismatch of VarDC and DC. It looks like DC is instantiated from VarDC.