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

Tue Jun 21 11:32:09 PDT 2011

FYI, patch generally looks good.

I think in two places you're needlessly checking for a reference type: the
base type of a base initializer.

On Mon, Jun 20, 2011 at 10:07 AM, Manuel Klimek <klimek at google.com> wrote:

> 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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