[PATCH] D127487: [Sema] Fix assertion failure when instantiating requires expression

[Erich Keane via Phabricator via cfe-commits]

erichkeane added inline comments.


================
Comment at: clang/lib/Sema/SemaConcept.cpp:352
+      [this](const Expr *AtomicExpr) -> ExprResult {
+        // We only do this to immitate lvalue-to-rvalue conversion.
+        return PerformContextuallyConvertToBool(const_cast<Expr*>(AtomicExpr));
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ilya-biryukov wrote:
> erichkeane wrote:
> > Can you explain this more?  How does this work, and why don't we do that directly instead?
> That's entangled with `calculateConstraintSatisfaction`. I actually tried to do it directly, but before passing expressions to this function `calculateConstraintSatisfaction` calls `IgnoreParenImpCasts()`, which strips away the lvalue-to-rvalue conversion.
> And we need this conversion so that the evaluation that runs after this callback returns actually produces an r-value.
> 
> Note that the other call to `calculateConstraintSatisfaction` also calls `PerformContextuallyConvertToBool` after doing template substitution into the constraint expression.
> 
> I don't have full context on why it's the way it is, maybe there is a more fundamental change that helps with both cases.
Hmm... my understanding is we DO need these to be a boolean expression eventually, since we have to test them as a bool, so that is why the other attempts the converesion.  If you think of any generalizations of this, it would be appreciated, I'll think it through as well.


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Comment at: clang/lib/Sema/SemaTemplateInstantiate.cpp:2042
+        !SemaRef.CheckConstraintExpression(TransConstraint.get())) {
+      assert(Trap.hasErrorOccurred() && "CheckConstraintExpression failed, but "
+                                        "did not produce a SFINAE error");
----------------
ilya-biryukov wrote:
> erichkeane wrote:
> > This branch ends up being empty if asserts are off.  Also, it results in CheckConstraintExpression happening 2x, which ends up being more expensive after https://reviews.llvm.org/D126907
> > This branch ends up being empty if asserts are off.  Also, it results in CheckConstraintExpression happening 2x, which ends up being more expensive after https://reviews.llvm.org/D126907
> 
> Yeah, good point, I have update it.
> 
> I am not sure why would `CheckConstraintExpression` be called twice, could you elaborate? Note that we do not call `BuildNestedRequirement` anymore and use placement new directly to avoid extra template instantiations. Instead we call `CheckConstraintExpression` directly to account for any errors.
This check does not seem to cause a 'return' to the function, but then falls through to the version on 2052, right?  

`CheckConstraintExpression`/`CheckConstraintSatisfaction`(i think?) ends up now having to re-instantiate every time it is called, so any ability to cache results ends up being beneficial here.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D127487/new/

https://reviews.llvm.org/D127487