[PATCH] D55955: Properly diagnose [[nodiscard]] on the body of a range-based for loop

[Aaron Ballman via Phabricator via cfe-commits]

aaron.ballman marked 2 inline comments as done.
aaron.ballman added inline comments.


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Comment at: include/clang/Sema/Sema.h:5337
   ExprResult ActOnFinishFullExpr(Expr *Expr, SourceLocation CC,
-                                 bool DiscardedValue = false,
+                                 bool WarnOnDiscardedValue = false,
                                  bool IsConstexpr = false);
----------------
rsmith wrote:
> Quuxplusone wrote:
> > aaron.ballman wrote:
> > > rsmith wrote:
> > > > Why "WarnOn"? Shouldn't this flag simply indicate whether the expression is a discarded-value expression?
> > > It probably can; but then it feels like the logic is backwards from the suggested changes as I understood them. If it's a discarded value expression, then the value being unused should *not* be diagnosed because the expression only exists for its side effects (not its value computations), correct?
> > Peanut gallery says: There are at least three things that need to be computed somewhere: (1) Is this expression's value discarded? (2) Is this expression the result of a `[[nodiscard]]` function? (3) Is the diagnostic enabled? It is unclear to me who's responsible for computing which of these things. I.e., it is unclear to me whether `WarnOnDiscardedValue=true` means "Hey `ActOnFinishFullExpr`, please give a warning //because// this value is being discarded" (conjunction of 1,2, and maybe 3) or "Hey `ActOnFinishFullExpr`, please give a warning //if// this value is being discarded" (conjunction of 2 and maybe 3).
> > 
> > I also think it is needlessly confusing that `ActOnFinishFullExpr` gives `WarnOnDiscardedValue` a defaulted value of `false` but `ActOnExprStmt` gives `WarnOnDiscardedValue` a defaulted value of `true`. Defaulted values (especially of boolean type) are horrible, but context-dependent defaulted values are even worse.
> I don't think it makes sense for `ActOnFinishFullExpr` to have a default argument for `DiscardedValue`, because there's really no reason to assume one way or the other -- the values of some full-expressions are used, and the values of others are not. A default of `false` certainly seems wrong.
> 
> For `ActOnExprStmt`, the default argument makes sense to me: the expression in an expression-statement is by definition a discarded-value expression (http://eel.is/c++draft/stmt.stmt#stmt.expr-1.sentence-2) -- it's only the weird special case for a final expression-statement in an statement-expression that bucks the trend here.
> 
> > If it's a discarded value expression, then the value being unused should *not* be diagnosed because the expression only exists for its side effects (not its value computations), correct?
> 
> No. If it's a discarded-value expression, that means the value of the full-expression is not being used, so it should be diagnosed. If it's not a discarded-value expression, then the value of the full-expression is used for something (eg, it's a condition or an array bound or a template argument) and so we should not warn. Indeed, the wording for `[[nodiscard]]` suggests to warn (only) on potentially-evaluated discarded-value expressions.
> 
> Discarded-value expressions are things like expression-statements, the left-hand-side of a comma operator, and the operands of casts to void. (Note in the cast-to-void case is explicitly called out by the `[[nodiscard]]` wording as a discarded-value expression that should not warn: http://eel.is/c++draft/dcl.attr.nodiscard#2.sentence-2)
Thank you for the explanation @rsmith, my mental model was exactly backwards. :-D I was thrown off by http://eel.is/c++draft/expr.prop#expr.context-2.sentence-1 because I read it as saying the discarded nature of the results are *desired* rather than problematic. e.g., some statements only appear for their side effects, do not warn about such statements because the context makes it obvious that this is expected. I should have looked it it in the context of the nodiscard attribute wording rather than in isolation.


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  https://reviews.llvm.org/D55955/new/

https://reviews.llvm.org/D55955