[cfe-dev] Incorrect result from DeclRefExpr::refersToEnclosingVariableCapture?

[Tom Honermann via cfe-dev]

On 10/21/2015 9:16 PM, Richard Smith via cfe-dev wrote:
> On Wed, Oct 21, 2015 at 4:08 PM, James Dennett via cfe-dev
> <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>> wrote:
>
>     On Wed, Oct 21, 2015 at 11:14 AM, Tim Prince via cfe-dev
>     <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>> wrote:
>
>         Hi everyone,
>
>         We've been working on upgrading our frontend from clang 3.4 to
>         3.7 and have run
>         into an issue where
>         'DeclRefExpr::refersToEnclosingVariableOrCapture' seems to be
>         returning the incorrect result. Here's a test case:
>
>            int fn1() {
>              const int i = 0;
>              return [] { return i; }();
>            }
>
>         Calling 'refersToEnclosingVariableOrCapture' on the
>         'DeclRefExpr' for 'i' within
>         the lambda body yields false. In clang 3.4, the equivalent
>         'refersToCapturedVariable'
>         method yields true. It looks this behavior was probably
>         introduced by r224323,
>         which changed 'Sema::BuildDeclRefExpr' to determine this via
>         'tryCaptureVariable'.
>
>
>     That's an interesting case.  `i` is a compile-time constant
>     expression there, so there's
>     no ODR-use of the variable, hence it's not captured.  I don't know
>     what that should
>     imply for refersToEnclosingVariableOrCapture; the specification is a
>     touch vague.
>
>
> What it really means is "this DeclRefExpr is referring to a capture of
> this variable rather than to the variable itself". The "enclosing
> variable" part is for the "captured statement" feature, which models
> captures somewhat differently.

Ok, so refersToEnclosingVariableOrCapture() is not a drop in substitute 
for the former refersToEnclosingLocal().

>         So with that background, I have a few questions:
>
>         - Is this code even valid? It seems like it shouldn't be since
>         there's no
>            capture-default or capture-list, but both clang and GCC
>         accept it.
>
>
>     I believe it is, as only the value of the constant expression is used.
>
>         - Assuming the code is valid, is this a bug in clang?
>
>
>     I don't know.
>
>
> It is; we don't yet implement the new "or e is a discarded-value
> expression" wording in [basic.def.odr]/2.

Richard, can you elaborate?  It isn't clear to me what you think the bug 
is.  Clang 3.7 does accept the code as presented.  According to your 
description of refersToEnclosingVariableOrCapture(), the function 
correctly returns false for this case (since the reference is to a 
variable that is not captured).  The variable reference doesn't seem to 
me to be a discarded-value expression since the value is used for the 
function return value.

It would be useful to have a predicate for "this DeclRefExpr refers to a 
local variable of an enclosing scope".

Tom.