[cfe-commits] [PATCH] Remove PotentiallyPotentiallyEvaluated

[John McCall]

On Jan 18, 2012, at 5:28 PM, Eli Friedman wrote:
> On Wed, Jan 18, 2012 at 5:22 PM, John McCall <rjmccall at apple.com> wrote:
>> On Jan 18, 2012, at 4:53 PM, Eli Friedman wrote:
>>> This patch passes all the regression tests except for one,
>>> CXX/expr/expr.prim/expr.prim.general/p12-0x.cpp, which I haven't
>>> figured out how to fix. (The issue is that given a DeclRefExpr
>>> pointing at a FieldDecl, there isn't any easy way to distinguish
>>> whether it is part of member pointer formation.)
>> 
>> A DeclRefExpr pointing at a FieldDecl should always be a part
>> of member pointer formation.  Any other use would be a MemberExpr.
> 
> Code:
> struct A { int a; };
> int y = sizeof A::a;

Aha!  That this is well-formed is news to me.  Very interesting.

Can you expand on the issue?  It sounds like these are generally
valid in UE contexts now;  I would assume that there's just some
random code that's not aware of that.

John.

P.S.  Doug and I talked about this rule and thought it might be
unsound, but it appears that it's actually just another important
client of your re-analysis.  The idea is that this code is ill-formed:

  struct V { virtual void foo(); };
  V &foo(int);
  struct A { int x; };
  void test() { (void) typeid(foo(A::x)); }

But it's not possible to notice that it's ill-formed except by walking back
over the operand to typeid and retroactively complaining about the use
of A::x in a PE context.

Memo to Doug:  our example doesn't quite work.  The rule is that it's
a member expression if 'this' is available and it's either PE *or* the
context is a (non-strict) subclass of the declaring class of the field.
In any context where it would be potentially valid as a member
expression, it is one.  The problem is in other contexts, like the test
above, where there's an "ambiguity" between being ill-formed
and being an abstract member reference.  Clearly this should be
resolved in favor of being an abstract member reference, with
this extra check being required after deciding that something is
really PE.  Up to you whether this rises to the level of a drafting error.

John.