[Bug c++/16963] New: Ambiguity? Seems implausible

[Bug c++/16963] New: Ambiguity? Seems implausible

[igodard at pacbell dot net]

struct C{};
struct D{};
template<typename T>
struct Foo {
    void Bar(T t) {}
    };
struct R : public Foo<C>, public Foo<D> {};

int main() {
    C c;
    D d;
    R r;

    r.Bar(c);
    r.Bar(d);
    return 0;
    }


gets you:

~/ootbc/members/src$ c++ foo.cc
foo.cc: In function `int main()':
foo.cc:14: error: request for member `Bar' is ambiguous
foo.cc:5: error: candidates are: void Foo<T>::Bar(T) [with T = D]
foo.cc:5: error:                 void Foo<T>::Bar(T) [with T = C]
foo.cc:15: error: request for member `Bar' is ambiguous
foo.cc:5: error: candidates are: void Foo<T>::Bar(T) [with T = D]
foo.cc:5: error:                 void Foo<T>::Bar(T) [with T = C]

-- 
           Summary: Ambiguity? Seems implausible
           Product: gcc
           Version: 3.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: igodard at pacbell dot net
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16963

[Bug c++/16963] Ambiguity? Seems implausible

[igodard at pacbell dot net]

------- Additional Comments From igodard at pacbell dot net  2004-08-10 14:22 -------
Simpler testcase (no templates):
struct C{};
struct D{};
struct X {
    void Bar(C t) {}
    };
struct Y {
    void Bar(D t) {}
    };

struct R : public X, public Y {};
int main() {
    C c;
    D d;
    R r;

    r.Bar(c);
    r.Bar(d);
    return 0;
    }


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16963

[Bug c++/16963] Ambiguity? Seems implausible

[bangerth at dealii dot org]

------- Additional Comments From bangerth at dealii dot org  2004-08-10 14:23 -------
icc rejects the code with the same error, though I must admit that I am 
at present at a loss as to why.  
 
A slightly simplified example is this: 
--------------------- 
template <typename T> 
struct Foo { 
    int Bar(T); 
}; 
 
struct R : public Foo<int>, public Foo<char> {}; 
 
int i = R().Bar(1); 
--------------------- 
 
g/x> /home/bangerth/bin/gcc-3.5-pre/bin/c++ -c x.cc 
x.cc:8: error: request for member `Bar' is ambiguous 
x.cc:3: error: candidates are: int Foo<T>::Bar(T) [with T = char] 
x.cc:3: error:                 int Foo<T>::Bar(T) [with T = int] 
 
It is true that name lookup happens before argument conversion, but 
I can't at present explain why not both functions are found at the 
same time and then overload resolution happens. 
 
W. 

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16963

[Bug c++/16963] Ambiguity? Seems implausible

[reichelt at gcc dot gnu dot org]

------- Additional Comments From reichelt at gcc dot gnu dot org  2004-08-10 18:15 -------
GCC and icc are right.
[10.2]/2 states:

The following steps define the result of name lookup in class scope.
First, every declaration for the name in the class and in each of its
base class sub-objects is considered. [...] If the resulting set of
declarations are not all from sub-objects of the same type [...]
there is an ambiguity and the program is ill-formed. [...]

Overload resolution takes place later, see [10.2]/4.


-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |reichelt at gcc dot gnu dot
                   |                            |org
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16963