katzdm wrote:
Looking further into this, I think this is a narrow problem that arises when
selecting a function from an overload set which includes a specialization of a
conversion function template.
Consider a class:
```cpp
struct S {
template <typename> void fn();
template <typename T> operator T();
};
```
When looking up `&S::fn<int>`, the associated `LookupResult` will be a
singleton containing the `FunctionTemplateDecl` for `fn`, _not_ the function
template specialization `fn<int>`. The `AddressOfFunctionResolver` in
`SemaOverload.cpp` will then call
`Sema::ResolveSingleFunctionTemplateSpecialization`, which will use template
argument deduction machinery to instantiate `fn<int>`.
Looking up `&S::operator int`, however, is something stranger: Because there
are no apparent template-arguments, the specialization is resolved [during
lookup](https://github.com/llvm/llvm-project/blob/main/clang/lib/Sema/SemaLookup.cpp#L1174-L1207)
instead of afterwards. This is necessary because the information needed to
select the specialization is encoded in the unqualified-id itself, rather than
in names and expressions that are parsed thereafter. From what I've been able
to tell, this situation seems unique to conversion-function-templates, and I
haven't found other cases where the `LookupResult` directly contains a function
template specialization.
>From here, deducing the placeholder type for `auto r = &S::operator int;` uses
>[`Sema::resolveAddressOfSingleOverloadCandidate`](https://github.com/llvm/llvm-project/blob/5b8479bc28a8641f02be3d64f87770b9e0b1a427/clang/lib/Sema/SemaOverload.cpp#L13208-L13291)
> to try to narrow to a single candidate. In the presence of constraints, this
>calls
>[`Sema::getMoreConstrainedFunction`](https://github.com/llvm/llvm-project/blob/5b8479bc28a8641f02be3d64f87770b9e0b1a427/clang/lib/Sema/SemaOverload.cpp#L13285)
> to select a "best viable function."
The implementation of `Sema::getMoreConstrainedFunction`
[already](https://github.com/llvm/llvm-project/blob/5b8479bc28a8641f02be3d64f87770b9e0b1a427/clang/lib/Sema/SemaTemplateDeduction.cpp#L5828-L5832)
attempts to map instantiated member-functions back to their "pattern
declarations" (which will have type `TK_FunctionTemplate`) before calling
`Sema::IsAtLeastAsConstrained`. This handles cases like:
```cpp
template <typename>
struct S {
void fn();
};
```
but not our conversion-function-template. Since I believe that specializations
of conversion-function-templates are the only specializations that can appear
in the `LookupResult`, I suggest a narrow carve-out to handle this case, and an
assertion to alert us if there are any other such cases which we aren't aware
of. All tests seem to pass.
https://github.com/llvm/llvm-project/pull/98671
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