[Bug c++/70737] Invalid C++ code compiles when using explicit template conversions

[matteo at mitalia dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70737

Matteo Italia <matteo at mitalia dot net> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |matteo at mitalia dot net

--- Comment #2 from Matteo Italia <matteo at mitalia dot net> ---
Templates don't seem to have a relevant role here, I found that this simpler
test case exhibits the same problem:

    struct A {
        explicit operator bool();
        operator int();
        operator double();
    };

    bool foo(A& a) {
        return a;
    }

    bool bar(A& a) {
        return bool(a);
    }

As in Andrea's example, `foo`'s code is not valid, since `operator bool` is
disabled (it's marked as explicit), and it's ambiguous whether `operator int`
or `operator double` should be invoked.

clang correctly refuses to compile this snippet (https://godbolt.org/g/lZa2Qx),
while all gcc>=4.6 (and <=6.1) I tried on gcc.godbolt.org compile `foo` to a
no-op function (`rep ret` on x86), thus leading to garbage being returned:

    foo(A&):
            rep ret
    bar(A&):
            jmp     A::operator bool()

(https://godbolt.org/g/xmrMGN)
(here -O3 optimizes the tail call and removes some stack setup/cleanup noise,
but the problem is present at all optimization levels)

So, the core of the bug seems to be that gcc discards completely a conversion
that becomes ambiguous once the "obvious" candidate is discarded for being
marked as `explicit`.

The gcc 4 series at least issued a dubious "warning: control reaches end of
non-void function [-Wreturn-type]", from gcc 5 onwards even this warning
disappeared (although, this may save you in this artificial example, in my
original code gcc dropped a conditional assignment to an already-initialized
variable, so the issue was completely silent).