[clang-cl] Cannot mangle decltype when used in an unevaluated context

[MaxEW707]

https://godbolt.org/z/cGsG8T4G7

template<typename T, typename = void>
struct Widget;

template <class T>
struct Widget<T, decltype(T::test2([](){}))> 
{
};

struct Baz
{
    template<typename F>
    static constexpr decltype(auto) test2(F&& f) {}
};

void test()
{
    Widget<Baz> w;
}

Similar issue as #115990 which is fixed here #117845.

test2 must be instantiated since it has a deduced return type. The instantiation of the lambda is within the context of the uninstantiated Widget template. CodeGen has no way to know that a template argument is defined inside an uninstantiated template. So the function get put into DeferredDecls which is keyed off of the mangled name of the function so we need to mangle the decltype.

[llvmbot]

@llvm/issue-subscribers-clang-codegen

Author: Max Winkler (MaxEW707)

https://godbolt.org/z/cGsG8T4G7 ``` template<typename T, typename = void> struct Widget;

template <class T>
struct Widget<T, decltype(T::test2({}))>
{
};

struct Baz
{
template<typename F>
static constexpr decltype(auto) test2(F&& f) {}
};

void test()
{
Widget<Baz> w;
}

Similar issue as https://github.com/llvm/llvm-project/issues/115990 which is fixed here https://github.com/llvm/llvm-project/pull/117845.

`test2` must be instantiated since it has a deduced return type. The instantiation of the lambda is within the context of the uninstantiated Widget template. CodeGen has no way to know that a template argument is defined inside an uninstantiated template. So the functions get put into `DeferredDecls` which is keyed off of the mangled name of the function so we need to mangle the `decltype`.
</details>

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Max Winkler (MaxEW707)

https://godbolt.org/z/cGsG8T4G7 ``` template<typename T, typename = void> struct Widget;

template <class T>
struct Widget<T, decltype(T::test2({}))>
{
};

struct Baz
{
template<typename F>
static constexpr decltype(auto) test2(F&& f) {}
};

void test()
{
Widget<Baz> w;
}

Similar issue as https://github.com/llvm/llvm-project/issues/115990 which is fixed here https://github.com/llvm/llvm-project/pull/117845.

`test2` must be instantiated since it has a deduced return type. The instantiation of the lambda is within the context of the uninstantiated Widget template. CodeGen has no way to know that a template argument is defined inside an uninstantiated template. So the function get put into `DeferredDecls` which is keyed off of the mangled name of the function so we need to mangle the `decltype`.
</details>