[LLVMbugs] [Bug 16243] New: this pointer isn't qualified within decltype() when used in member function declaration with late specified return type

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=16243

            Bug ID: 16243
           Summary: this pointer isn't qualified within decltype() when
                    used in member function declaration with late
                    specified return type
           Product: clang
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++11
          Assignee: unassignedclangbugs at nondot.org
          Reporter: mitchnull+llvm at gmail.com
                CC: dgregor at apple.com, llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Created attachment 10636
  --> http://llvm.org/bugs/attachment.cgi?id=10636&action=edit
test program showing the failure

The explicit 'this' pointer in member function declaration with late specified
return type using decltype is not const-qualified if the member function is
declared 'const' resulting in wrong return type.

Example:

template <typename T>
struct DecltypeConstThis {

    void f() {}
    T f() const { return T{}; }

    auto g() -> decltype(this->f()) { return f(); }
    auto g() const  ->  decltype(this->f()) { return f(); } // apparently
'this' is not const within the decltype() here

};

int main() {
    DecltypeConstThis<int> d;
    const DecltypeConstThis<int> &cd = d;

    d.g();
    cd.g(); // fails
    return 0;
}

compilation error:
[mitch at deneb src]$ clang++ -std=c++11 -o decltype-const-this
decltype-const-this.cpp
decltype-const-this.cpp:8:47: error: void function 'g' should not return a
value
      [-Wreturn-type]
    auto g() const  ->  decltype(this->f()) { return f(); } // apparently
'this' is...
                                              ^      ~~~
decltype-const-this.cpp:17:8: note: in instantiation of member function
      'DecltypeConstThis<int>::g' requested here
    cd.g(); // fails

The same code works as expected with implicit 'this':

template <typename T>
struct DecltypeConstThis {

    void f() {}
    T f() const { return T{}; }

    auto g() -> decltype(f()) { return f(); }
    auto g() const  ->  decltype(f()) { return f(); } //  decltype() correctly
picks 'f() const' here

};

int main() {
    DecltypeConstThis<int> d;
    const DecltypeConstThis<int> &cd = d;

    d.g();
    cd.g();
    return 0;
}

(ps: gcc-4.8 requires the explicit this-> here, filing a bug report that way,
too)

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