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

[Arthur O'Dwyer via Phabricator via cfe-commits]

Quuxplusone 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);
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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.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D55955/new/

https://reviews.llvm.org/D55955