[llvm-bugs] [Bug 49520] New: Erroneous template deduction

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=49520

            Bug ID: 49520
           Summary: Erroneous template deduction
           Product: clang
           Version: unspecified
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: ColinMacLean at lbl.gov
                CC: blitzrakete at gmail.com, dgregor at apple.com,
                    erik.pilkington at gmail.com, llvm-bugs at lists.llvm.org,
                    richard-llvm at metafoo.co.uk

I discovered a bug in Clang's template argument deduction while working on
array support for a pointer wrapping class. This incorrect behavior can be
simplified into the following reproducer:

#include <type_traits>

template<typename T>
struct A {};

template<typename T>
struct A<T[]> : A<T> {};

template<typename T>
void foo(T* a, A<T>& b) {}

template<typename T>
void bar(T* a, A<std::type_identity_t<T>>& b) {}

void test()
{
    int * t1;
    A<int> t2;
    A<int[]> t3;
    foo(t1,t2);
    foo(t1,t3); // GCC: correctly errors on conflicting deductions for T: int
and int[]
                // Clang: Erroneously performs implicit cast, which should only
be allowed 
                //        after a successful template deduction
    bar(t1,t2);
    bar(t1,t3); // Compliant implicit cast for second argument.
}

In the case of the function foo(), the deduction should try to deduce to both
int and int[] simultaneously with the parameters t1 and t3 and fail due to
being different types. Godbolt shows T is deduced to int on Clang.
std::type_identity_t<T> should be necessary to establish a non-deduced context
if implicit conversion to A<int> is desired. This implicit conversion should
only happen after a template deduction is successful.

This bug affects all known Clang versions, tested from 3.0 to trunk with
Godbolt. It also affects all C++ standard options used from C++98 to C++2a.

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