[PATCH] Implement throw in conditional operator

[Richard Smith]

On Wed, Jun 11, 2014 at 1:25 PM, Marius Wachtler <undingen at gmail.com> wrote:

> Hi
>
> I added a testcase for the function you mention but I also added const to
> the return type because otherwise gcc won't compile it ("error: invalid
> initialization of non-const reference of type ‘int&’ from an rvalue of type
> ‘int’") - clang compiles both.
>

GCC doesn't implement one of the relevant DRs; you don't need to make the
testcase work with it.

The generated code from clang for the function looks correct and works as I
> would expect it.
> Compiling it with g++ 4.8.2 and -O2 will instead produce and object which
> will return an address to a temporary in the function and not a reference
> to the global val.
>
> I'm not sure if clangs behavior is correct or if it should also only
> accept a const reference? If the error is somewhere else I would appreciate
> some guidance to where to look around and what the expected behavior is.
>

See http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1550;
Clang's behavior is right, this is just a bug with IR generation.

The C++11 non final standard I have says (5.16-2):
> "If either the second or the third operand has type (possibly cv-qualified)
> void, then the lvalue-to-rvalue (4.1), array-to-pointer (4.2), and
> function-to-pointer (4.3) standard conversions are performed on the second
> and third operands, and one of the following shall hold:
> — The second or the third operand (but not both) is a throw-expression
> (15.1); the result is of the type
> of the other and is a prvalue.
> — Both the second and the third operands have type void; the result is of
> type void and is a prvalue. [ Note: This includes the case where both
> operands are throw-expressions. — end note ]
> "
> So this means gcc is correct?
>
> Thank you very much
> - Marius
>
>
>
>
>
> On Wed, Jun 11, 2014 at 1:05 AM, Richard Smith <richard at metafoo.co.uk>
> wrote:
>
>> Please also include a test that the produced lvalue actually has the
>> right value. Something like:
>>
>> int &test7(bool cond) {
>>   return cond ? throw 1 : val;
>> }
>>
>> ... and check that the value returned is @_Z3val
>>
>> On Tue, Jun 10, 2014 at 3:01 PM, Marius Wachtler <undingen at gmail.com>
>> wrote:
>>
>>> Hi
>>> Attached you can find an updates patch.
>>>
>>>
>>>
>>>
>>> On Tue, Jun 10, 2014 at 11:50 PM, David Majnemer <
>>> david.majnemer at gmail.com> wrote:
>>>
>>>> +
>>>>> +  case Expr::CXXThrowExprClass: {
>>>>> +    const auto* throwExpr = cast<CXXThrowExpr>(E);
>>>>>
>>>>
>>>> Please stick the star on the RHS. Also, it is customary in LLVM and
>>>> clang to spell it like "ThrowExpr"
>>>>
>>>>
>>>>> +    EmitCXXThrowExpr(throwExpr);
>>>>> +    const QualType subExprType = throwExpr->getSubExpr()->getType();
>>>>>
>>>>
>>>> Likewise for "subExprType"
>>>>
>>>> +    llvm::Type *Ty =
>>>>> llvm::PointerType::getUnqual(ConvertType(subExprType));
>>>>> +    return MakeAddrLValue(llvm::UndefValue::get(Ty), subExprType);
>>>>>    }
>>>>
>>>>
>>>>
>>>> On Tue, Jun 10, 2014 at 5:19 PM, Marius Wachtler <undingen at gmail.com>
>>>> wrote:
>>>>
>>>>> Hello
>>>>>
>>>>> Attached you can find my first clang patch which should implement
>>>>> "cond ? throw 1 : val".
>>>>>
>>>>> Without this patch I get: "cannot compile this l-value expression yet"
>>>>>
>>>>> As this is my first time looking under the hood of clang I'm not sure
>>>>> if this is the right approach. I'm Looking forward to your feedback.
>>>>>
>>>>> - Marius Wachtler
>>>>>
>>>>> _______________________________________________
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>>>>> cfe-commits at cs.uiuc.edu
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>>>>>
>>>>>
>>>>
>>>
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>>
>
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