Reference collapsing works wrong

[adromanov]

For the following piece of code:

template <class T>
void foo(T && t)
{ }

struct Test {};

int main()
{
    Test test;
    foo(test);
    return 0;
}

output of an instantiation of foo is the following:

/* First instantiated from: insights.cpp:10 */
#ifdef INSIGHTS_USE_TEMPLATE
template<>
void foo<Test &>(Test && t)
{
}
#endif

Argument type should be Test & but not Test && because of reference collapsing.

[andreasfertig]

Hello,

thank for reporting that. The collapsing is sometimes a problem with the goal to get compiling code. I need to have a deep look into this case and check to not break others.

[adromanov]

I've made some investigation on the following snippet:

template <class T>
void foo(T && t)
{ }

int main()
{
    int test = 0; 
    foo(test);
    return 0;
}

When insights processes a template instantiation it eventually calls

static const std::string GetAsCPPStyleString(const QualType& t);

Actually, at this point t->getTypeClass() is Type::LValueReference and t->isLValueReferenceType() is true.
The reason that t.getAsString(..) returns int && is how clang implements

// File is llvm/tools/clang/lib/AST/TypePrinter.cpp 
void TypePrinter::print(const Type *T, Qualifiers Quals, raw_ostream &OS, StringRef PlaceHolder);

Simplified call graph looks like:

TypePrinter::print()
├─printBefore()
│ └─printLValueReferenceBefore()
│   ├─printBefore() // <- getPointeeTypeAsWritten = SubstTemplateTypeParmType
│   │ └─printSubstTemplateTypeParmBefore()
│   │   └─printBefore() // <- getReplacementType = LValueRefeferenceType
│   │     └─printLValueReferenceBefore()
│   │       ├─printBefore() // <- getPointeeTypeAsWritten = BuiltinType                                                                                    
│   │       │ └─printBuiltinBefore()
│   │       │   └─OS << T->getName();
│   │       └─OS << "&";
│   └─OS << "&";
├─...
└─printAfter()
  └─printLValueReferenceAfter()
    └─printAfter() // <- getPointeeTypeAsWritten = SubstTemplateTypeParmType
      └─printSubstTemplateTypeParmAfter()
        └─printAfter() // <- getReplacementType = LValueRefeferenceType
          └─printLValueReferenceAfter()
            └─printAfter() // <- getPointeeTypeAsWritten = BuiltinType
              └─printBuiltinAfter()
                └─// do nothing

So, clang itself produces 2 ampersands for lvalues. Looks like a bug in clang for me, but 'm not sure.
Anyway, I guess this issue could be solved at insights level with some workaround.

P.S.: tested on llvm-6.0.1

[andreasfertig]

Hello adromanov,

thanks for nailing it down. It sure looks like a bug in clang. And I agree, it can be workaround in insights. I already have an idea, but this issue did draw my attention more to templates. Currently I'm looking into more than just this issue. There are a couple of other things which are incorrect. As soon as I have all together I will push a (temporary) fix.
Do you know how to open a bug report to clang? Because, I would prefer them to fix it.

[adromanov]

Do you know how to open a bug report to clang? Because, I would prefer them to fix it.

I'll try to report in a couple of days.

[adromanov]

That workaround doesn't work with:

template <class F>
void foo(F && f)
{ }

using F = int && (int &&);                                                                                                                                                                                         

int && f(int && i)
{
    return static_cast<int &&>(i);
}

int main()
{
    F & rf = f;
    foo(rf);
    return 0;
}

Top level && should be replaced with &, not the first one. This is tricky when we have only string representation of a type.

[adromanov]

Reported https://bugs.llvm.org/show_bug.cgi?id=38363

[andreasfertig]

Thanks! I'm curious what they say. I will revert the not so well working patch.

[adromanov]

Please, remove double ampersand hack from static const std::string GetAsCPPStyleString(const QualType& t):

1. it doesn't work properly
2. With new LLVM 7.0.0 there is no need in it - everything works fine, & is printed instead of &&

Thanks!

[andreasfertig]

Hello adromanov,

I just reverted the hack. However, I could not confirm that it is fixed in my clang 7.0.0 version. That could be as it is a couple days if not weeks old. Also it will not show up in the web-frontend as Travis CI does not support the new llvm 7 repository yet. At least not easily.

[adromanov]

I've built a fresh 7.0.0 yesterday (tags/RELEASE_700/final) - it works for me.

[andreasfertig]

Ok now I see it as well. The next problem is static linking such that I can use it for the web frontend.
I leave this issue open until I have figured it out.

Thanks for keeping an eye on that issue!

[andreasfertig]

Hello adromanov,

I finally updated to Clang 7.0. As far as I can see this issue is be solved now.