aaron.ballman added inline comments.
================
Comment at: lib/AST/ASTImporter.cpp:6160
+ // condition-dependent.
+ bool CondIsTrue = false;
+ if (!E->isConditionDependent())
----------------
tmroeder wrote:
> a_sidorin wrote:
> > aaron.ballman wrote:
> > > tmroeder wrote:
> > > > aaron.ballman wrote:
> > > > > a_sidorin wrote:
> > > > > > aaron.ballman wrote:
> > > > > > > `bool CondIsTrue = E->isConditionDependent() ? false :
> > > > > > > E->isConditionTrue();`
> > > > > > `bool CondIsTrue = E->isConditionDependent() &&
> > > > > > E->isConditionTrue();`?
> > > > > I like that even better than my suggestion. :-)
> > > > Wait, this doesn't have the same truth table as my original code.
> > > >
> > > > let `CD = E->isConditionDependent()`
> > > > let `CT = E->isConditionTrue()`
> > > >
> > > > in
> > > >
> > > > ```
> > > > bool CondIsTrue = false;
> > > > if (!CD)
> > > > CondIsTrue = CT;
> > > > ```
> > > >
> > > > has the table for `CondIsTrue`:
> > > >
> > > > | `CD` | `CT` | `CondIsTrue` |
> > > > | T | T | F |
> > > > | T | F | F |
> > > > | F | T | T |
> > > > | F | F | F |
> > > > i.e., if CD is true, then CondIsTrue is always false. Otherwise, it's
> > > > the value of CT.
> > > >
> > > > The suggested line has the truth table
> > > >
> > > > | `CD` | `CT` | `CondIsTrue` |
> > > > | T | T | T |
> > > > | T | F | F |
> > > > | F | T | F |
> > > > | F | F | F |
> > > >
> > > > That is, the effect of CD is swapped.
> > > >
> > > > Aaron's suggestion matches my original table. I based my code on
> > > > include/clang/AST/Expr.h line 4032, which asserts
> > > > !isConditionDependent() in the implementation of isConditionTrue.
> > > >
> > > > I realized this after I "fixed" my comment to match the implementation
> > > > change. Am I missing something? Or is the assertion in Expr.h wrong? I
> > > > think this should be
> > > >
> > > > ```
> > > > bool CondIsTrue = !E->isConditionDependent() && E->isConditionTrue();
> > > > ```
> > > >
> > > > I've changed my code to that and reverted the comment change.
> > > Good catch -- I think my eyes just missed the change in logic. Perhaps we
> > > should add a test case that exercises this?
> > Wow, that's a nice catch. Sorry for the misleading.
> I started to look for a way to test this, then realized that while it's
> possible to test the code itself, it's impossible to make a ChooseExpr with
> isConditionDependent() that returns true for real code.
>
> TL;DR: ChooseExpr is a C-only construct, and isConditionDependent is a
> C++-only condition; it's always false in C.
>
> Details:
>
> ChooseExpr only represents __builtin_choose_expr which is only valid in C,
> not C++. That means ChooseExpr::isConditionDependent() will always be false.
>
> The definition is
>
> ```
> bool isConditionDependent() const {
> return getCond()->isTypeDependent() || getCond() ->isValueDependent();
> }
> ```
>
> However Expr::isTypeDependent() (see Expr.h line 158) is a purely C++
> property to do with templates ([temp.dep.expr]): it is true if the type of
> the expression could change from one template instantiation to another.
>
> Similarly Expr::isValueDependent() (see Expr.h line 144) is a purely C++
> property to do with templates ([temp.dep.expr]): it is true for
> value-dependent types in templates.
>
> Both will always be false in all instantiations of ChooseExpr, due to the
> language difference.
>
> So, I think ChooseExpr can be refactored to remove isConditionDependent and
> change its constructor to remove TypeDependent and ValueDependent.
>
> If it's OK with you, I'll do that in a followup patch.
> ChooseExpr only represents __builtin_choose_expr which is only valid in C,
> not C++.
We allow it in C++, though: https://godbolt.org/z/_f1DPV
Repository:
rC Clang
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D58292/new/
https://reviews.llvm.org/D58292
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