[Clang] Destructor definition is not compiled for local class in generic lambda initializing a variable template

[MagentaTreehouse]

template <class>
constexpr auto f{[] (auto) {
    struct OnScopeExit { ~OnScopeExit() {} }
    pending;
}};

int main() {
    f<int>(0);
}

See https://compiler-explorer.com/z/6vneqf93z.

Warning and linker error since Clang 15:

<source>:3:26: warning: inline function '(anonymous class)::operator()(int)::OnScopeExit::~OnScopeExit' is not defined [-Wundefined-inline]
    3 |     struct OnScopeExit { ~OnScopeExit() {} }
      |                          ^
<source>:4:5: note: used here
    4 |     pending;
      |     ^
1 warning generated.
ASM generation compiler returned: 0
<source>:3:26: warning: inline function '(anonymous class)::operator()(int)::OnScopeExit::~OnScopeExit' is not defined [-Wundefined-inline]
    3 |     struct OnScopeExit { ~OnScopeExit() {} }
      |                          ^
<source>:4:5: note: used here
    4 |     pending;
      |     ^
1 warning generated.
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/15.0.1/../../../../x86_64-linux-gnu/bin/ld: /tmp/example-eeceaa.o: in function `auto f<int>::{lambda(auto:1)#1}::operator()<int>(int) const':
<source>:5:(.text._ZNK1fIiEMUlT_E_clIiEEDaS1_[_ZNK1fIiEMUlT_E_clIiEEDaS1_]+0x14): undefined reference to `f<int>::{lambda(auto:1)#1}::operator()<int>(int) const::OnScopeExit::~OnScopeExit()'
clang++: error: linker command failed with exit code 1 (use -v to see invocation)

[llvmbot]

@llvm/issue-subscribers-clang-codegen

Author: None (MagentaTreehouse)

```c++ template <class> constexpr auto f{[] (auto) { struct OnScopeExit { ~OnScopeExit() {} } pending; }};

int main() {
f<int>(0);
}

See https://compiler-explorer.com/z/6vneqf93z.

Warning and linker error since Clang 15:
```console
<source>:3:26: warning: inline function '(anonymous class)::operator()(int)::OnScopeExit::~OnScopeExit' is not defined [-Wundefined-inline]
    3 |     struct OnScopeExit { ~OnScopeExit() {} }
      |                          ^
<source>:4:5: note: used here
    4 |     pending;
      |     ^
1 warning generated.
ASM generation compiler returned: 0
<source>:3:26: warning: inline function '(anonymous class)::operator()(int)::OnScopeExit::~OnScopeExit' is not defined [-Wundefined-inline]
    3 |     struct OnScopeExit { ~OnScopeExit() {} }
      |                          ^
<source>:4:5: note: used here
    4 |     pending;
      |     ^
1 warning generated.
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/15.0.1/../../../../x86_64-linux-gnu/bin/ld: /tmp/example-eeceaa.o: in function `auto f&lt;int&gt;::{lambda(auto:1)#<!-- -->1}::operator()&lt;int&gt;(int) const':
&lt;source&gt;:5:(.text._ZNK1fIiEMUlT_E_clIiEEDaS1_[_ZNK1fIiEMUlT_E_clIiEEDaS1_]+0x14): undefined reference to `f&lt;int&gt;::{lambda(auto:1)#<!-- -->1}::operator()&lt;int&gt;(int) const::OnScopeExit::~OnScopeExit()'
clang++: error: linker command failed with exit code 1 (use -v to see invocation)

[shafik]

I was going to say it looks like this but this one is fixed in trunk: #115731

CC @cor3ntin @erichkeane