Clang won't compile a template specialization that gcc will

Question

Gcc compiles this fine, but Clang (trunk) refuses with the message:

<source>:7:8: error: class template partial specialization is not more specialized than the primary template [-Winvalid-partial-specialization]

https://godbolt.org/g/h8rsWC

template<class T, T x>
struct S{};

template<int& x>
struct S<int&, x> { };

Is this code correct or not?

Answer 1

This only manifests in -std=c++17 and later. The "more specialized" determination requires synthesizing a pair of function templates from the class templates, synthesizing unique types, values, and templates for the template parameters, and finally performing template argument deduction in both directions. A template is "more specialized" if deduction succeeds in one direction but not the other.

Here, Clang is deducing T from two sources and getting divergent results:

• from the explicitly specified int&, it deduces T := int&.
• from the non-type argument x, it deduces T := int per the usual deduction rules (which generally do not deduce a reference type). This ability to deduce from the type of a non-type template argument was added in C++17.

This makes the deduction fail in both directions, so the partial specialization fails the "more specialized" test.

This looks like a defect in the standard. The workaround is to wrap the T in the original template in a non-deduced context.