[cfe-commits] [PATCH] Fix ParenListExpr with null type

[Manuel Klimek]

Changed to always require a type in ParenListExpr and tried to fill in the
right types (sanity checking welcome :)

On Fri, Jun 17, 2011 at 9:15 AM, Douglas Gregor <dgregor at apple.com> wrote:

>
> On Jun 17, 2011, at 8:11 AM, Douglas Gregor wrote:
>
>
> On Jun 16, 2011, at 11:47 PM, Manuel Klimek wrote:
>
> On Thu, Jun 16, 2011 at 11:36 PM, Douglas Gregor <dgregor at apple.com>wrote:
>
>>
>> On Jun 15, 2011, at 10:11 AM, Manuel Klimek wrote:
>>
>> On Tue, Jun 14, 2011 at 8:21 PM, Douglas Gregor <dgregor at apple.com>wrote:
>>
>>>
>>> On Jun 14, 2011, at 6:06 PM, Manuel Klimek wrote:
>>>
>>> > The attached patch
>>> > - fixes the introduction of null types for ParenListExpr's that end up
>>> > in the AST for explicit initializers by making the constructor of
>>> > ParenListExpr take a type (as suggested by dgregor on irc)
>>> > - gets rid of some code I assume is dead (tested by running the tests
>>> > and by running it over all of our internal C++ code without hitting
>>> > any of those asserts - and by my inability to come up with an example
>>> > that hits that code path (which admittedly doesn't mean a lot))
>>> > - tested that the other in-use cases that create ParenListExpr's
>>> > without a type don't lead to them being in the AST in the end
>>>
>>> Index: lib/Sema/SemaDeclCXX.cpp
>>> diff --git a/lib/Sema/SemaDeclCXX.cpp b/lib/Sema/SemaDeclCXX.cpp
>>> index
>>> ce99efbd0bd2020f702da71fc87a80d0b86759a8..a95ef69085526291eb7532aeb9788f68bab235f6
>>> 100644
>>> --- a/lib/Sema/SemaDeclCXX.cpp
>>> +++ b/lib/Sema/SemaDeclCXX.cpp
>>> @@ -1617,7 +1617,7 @@ Sema::BuildMemberInitializer(ValueDecl *Member,
>>> Expr **Args,
>>>     // Can't check initialization for a member of dependent type or when
>>>     // any of the arguments are type-dependent expressions.
>>>     Init = new (Context) ParenListExpr(Context, LParenLoc, Args, NumArgs,
>>> -                                       RParenLoc);
>>> +                                       RParenLoc, QualType());
>>>
>>>     // Erase any temporaries within this evaluation context; we're not
>>>     // going to track them in the AST, since we'll be rebuilding the
>>>
>>> Why not just use Member->getType() as the type of the ParenListExpr, so
>>> it never has a NULL type?
>>>
>>
>> Because I wasn't able to get it to make a difference, in which case I
>> default to not changing it. If you think the invariant should be that the
>> ParenListExpr always has a type, I'm happy to change this. I'll have a hard
>> time writing a test for it, though ... ;)
>>
>>
>> I just figured that if you're going to fix the problem in one place, it
>> would be better to just make it an AST invariant.
>>
>>
>>> Index: lib/Sema/SemaInit.cpp
>>> diff --git a/lib/Sema/SemaInit.cpp b/lib/Sema/SemaInit.cpp
>>> index
>>> a33f5d0b2f3ef530c689a2ddc28ebaef3074635e..f0d39893a449e62902f21160f10bd6a400353789
>>> 100644
>>> --- a/lib/Sema/SemaInit.cpp
>>> +++ b/lib/Sema/SemaInit.cpp
>>> @@ -3684,19 +3684,9 @@ InitializationSequence::Perform(Sema &S,
>>>
>>>       }
>>>     }
>>> -
>>> -    if (Kind.getKind() == InitializationKind::IK_Copy ||
>>> Kind.isExplicitCast())
>>> -      return ExprResult(Args.release()[0]);
>>> -
>>> -    if (Args.size() == 0)
>>> -      return S.Owned((Expr *)0);
>>> -
>>> -    unsigned NumArgs = Args.size();
>>> -    return S.Owned(new (S.Context) ParenListExpr(S.Context,
>>> -                                                 SourceLocation(),
>>> -                                                 (Expr
>>> **)Args.release(),
>>> -                                                 NumArgs,
>>> -                                                 SourceLocation()));
>>> +    assert(Kind.getKind() == InitializationKind::IK_Copy ||
>>> +           Kind.isExplicitCast());
>>> +    return ExprResult(Args.release()[0]);
>>>   }
>>>
>>>   // No steps means no initialization.
>>>
>>> Hrm, interesting. This is only dead because its potential callers seem to
>>> avoid building InitializationSequences in dependent cases. Please update the
>>> assert to always check that Args.size() ==1, and if *that* doesn't trigger
>>> any failures, this change is okay. We can always resurrect the code if the
>>> callers change.
>>>
>>
>> I'm happy to do this, but I don't understand the underlying assumption yet
>> - even if Args.size() != 1, if the assert that I added is true, the code
>> will behave exactly the same way as before (before we would early return if
>> it is true). So I'm not sure what the extra assert would change (if it
>> breaks, and that makes the code wrong, I'd say the code was wrong before...)
>>
>>
>> The previous code would return *all* of the arguments in Args (NumArgs of
>> them) as a ParenListExpr, indicating direct initialization.
>>
>> The new code only returns the first of the arguments, so if we're going to
>> make that change, we need to assert that there is exactly one argument.
>>
>
>
> Now I feel like I'm really missing something here...
> In the previous code:
>
>     if (Kind.getKind() == InitializationKind::IK_Copy ||
>> Kind.isExplicitCast())
>>      return ExprResult(Args.release()[0]);
>
>
> ... reads to me as:
> if (condition)
>   return statement;
> <... some code that was never executed ...>
> which I (at least hope that I did) transformed to:
> assert(condition);
> return statement;
> <... delete code that was never executed ...>
> which I would think is equivalent, given that the assert always holds true.
>
> What am I missing?
>
>
> The code that is currently never executed is still nonetheless correct, and
> a minor tweak the any of the callers' handling of initialization involving
> type-dependent expressions could make this code relevant again. If we take
> your simplification, and one of those tweaks happen, we'll silently
> transform ASTs into something semantically different and cause ourselves
> some serious debugging pain.
>
> So if you want to simplify this code, I'm asking you to add appropriate
> assertions to make sure that we catch the cases where this code will be
> broken. In particular, the code itself is designed to handle NumArgs != 1,
> but your simplification assumes NumArgs == 1 without checking.
>
>
> Apparently, I'm just misreading patches right now. This change is fine;
> sorry for the confusion.
>
> - Doug
>
>
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