[clang] crash when meeting template friend function

[HerrCai0907]

template <class T> int Foo(T X);
template <int N, int B, int C> struct Thief {
  template <class T> friend int Foo(T X) { 
    return B; 
  }
};
template struct Thief<1,2,3>;
int main() {
  Foo(1.0);
}

https://godbolt.org/z/vEGdvhc3q

[HerrCai0907]

I have done some work for it. The issue is in
clang/lib/Sema/SemaTemplateInstantiateDecl.cpp Line 5065

    MultiLevelTemplateArgumentList TemplateArgs = getTemplateInstantiationArgs(
        Function, /*Final=*/false, nullptr, false, PatternDecl);

TemplateArgs in here is [[T]]. But it should be [[N, B, C], [T]]

[llvmbot]

@llvm/issue-subscribers-clang-codegen

[shafik]

Confirmed, feels like something that @erichkeane @alexander-shaposhnikov might know about

[erichkeane]

https://reviews.llvm.org/D146178 alters that a bit, but I could definitely believe we have another bug in there. The getTemplateInstantiationArgs function is still a 'work in progress' these days (its being pushed to do things it never had to do before, particularly with concepts), but it looks like this is friend related (which is a little surprising).

Perhaps we're just quitting too early on collecting args on inline-defined-friends like this,

[HerrCai0907]

but it looks like this is friend related.

Yeah, for inline friend, it does not consider the template in outsider RecordDecl.
But I don't find a solution to get it from FunctionDecl's context. I need a ClassTemplateSpecializationDecl to get the template arg list but I can only get a CXXRecord or ClassTemplateDecl.

@shafik @erichkeane Do you have some suggestion to fix it?

[erichkeane]

So you SHOULD have been able to get it from the lexicalDeclContext, but it appears that isn't set correctly. So you'll have to fix that FIRST.

I see that the FunctionDecl is created in SemaTemplateInstantiateDecl.cpp ~2138 and ~2160 seems to not be doing the right thing here. For some reason, Owner isn't the right value I think, so some debugging will be needed there.