go-to-definition doesn not work when "class template argument dedection" is used

[lh123]

template<typename T>
struct Test {
    Test(T t);
};

int main() {
    Test a(5);
//  ^^^^ go-to-def not works
}

[HighCommander4]

The type specifier here has a BuiltinTypeLoc with an empty source range, so it doesn't get selected, only the enclosing VarDecl does, which targets itself.

[HighCommander4]

The type specifier here has a BuiltinTypeLoc with an empty source range

Sorry, this was happening because my testcase wasn't being parsed in C++17 mode. The "BuiltinTypeLoc with an empty source range" was the error type.

In C++17 mode, we get a DeducedTemplateSpecializationTypeLoc, and it does get selected, so hopefully this will be a simple fix to TargetFinder.

[HighCommander4]

I tried to fix this in TargetFinder, but ran into PR42914, which appears to affect DeducedTemplateSpecializationTypeLoc as well.

We could, however, work around this if we're fine with always navigating to the primary template (as opposed to using the deduced template specialization, which could potentially come from a partial or explicit specialization) for now.

[HighCommander4]

It looks like PR42914 is not an easy fix, so I suggest we go ahead with the workaround.

Proposed patch.