[LLVMbugs] [Bug 23006] New: Conditional expression accepted despite ambiguous common type: derived-to-base and base-to-derived UDC

[bugzilla-daemon at llvm.org]

https://llvm.org/bugs/show_bug.cgi?id=23006

            Bug ID: 23006
           Summary: Conditional expression accepted despite ambiguous
                    common type: derived-to-base and base-to-derived UDC
           Product: clang
           Version: trunk
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++11
          Assignee: unassignedclangbugs at nondot.org
          Reporter: hstong at ca.ibm.com
                CC: dgregor at apple.com, llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Conditional expressions where the second and third operands have different
types are ill-formed if conversion can be done both ways.

Clang fails to reject the conditional expression in the case below where one
conversion is derived-to-base with slicing and the other is a user-defined
conversion.

In the case below, this manifests as an error because the wrong partial
specialization is chosen.

### SOURCE (<stdin>):
struct A;

struct B {
   operator A &&();
} bObj;

struct A : B { };

enum E { Gnat };
A &&fooA(E);

bool b;

template <typename T, typename U = char [sizeof 0]> struct Hack { };
template <typename T> struct Hack<T, char [sizeof ((b ? bObj : fooA(T())),
0)]>;

void bar() {
   A &&t1 = bObj;
   B t2 = fooA(Gnat);
   Hack<E> hack;
}


### COMPILER INVOCATION:
clang++ -cc1 -std=c++11 -x c++ -


### ACTUAL OUTPUT:
<stdin>:20:12: error: implicit instantiation of undefined template 'Hack<E,
char [4]>'
   Hack<E> hack;
           ^
<stdin>:15:30: note: template is declared here
template <typename T> struct Hack<T, char [sizeof ((b ? bObj : fooA(T())),
0)]>;
                             ^
1 error generated.


### EXPECTED OUTPUT:
(Successful compile).


### COMPILER VERSION INFO:
clang version 3.7.0 (trunk 232951)
Target: x86_64-unknown-linux-gnu
Thread model: posix
Found candidate GCC installation:
/usr/local/gcc-4.8.2/lib/gcc/x86_64-unknown-linux-gnu/4.8.2
Selected GCC installation:
/usr/local/gcc-4.8.2/lib/gcc/x86_64-unknown-linux-gnu/4.8.2
Candidate multilib: .;@m64
Selected multilib: .;@m64

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