[PATCH] D144285: [Clang] Implement CWG2518 - static_assert(false)

[Corentin Jabot via Phabricator via cfe-commits]

cor3ntin added inline comments.


================
Comment at: clang/lib/Sema/SemaDeclCXX.cpp:16824-16841
       if (InnerCond && isa<ConceptSpecializationExpr>(InnerCond)) {
         // Drill down into concept specialization expressions to see why they
         // weren't satisfied.
         Diag(StaticAssertLoc, diag::err_static_assert_failed)
           << !AssertMessage << Msg.str() << AssertExpr->getSourceRange();
         ConstraintSatisfaction Satisfaction;
         if (!CheckConstraintSatisfaction(InnerCond, Satisfaction))
----------------
aaron.ballman wrote:
> cor3ntin wrote:
> > rsmith wrote:
> > > I wonder if it's worth adding a custom diagnostic (eg, "this template cannot be instantiated: %0") for the case where we're in template instantiation and the expression is the bool literal `false`.
> > I'm not sure i see the motivation. Why would we want to special case `false`? The expression could also be an always false, never dependent expression
> Richard may have different ideas in mind, but the motivation to me is code like: 
> ```
> template <typename Ty>
> struct S {
>   static_assert(false, "you have to use one of the valid specializations, not the primary template");
> };
> 
> template <>
> struct S<int> {
> };
> 
> template <>
> struct S<float> {    
> };
> 
> int main() {
>   S<int> s1;
>   S<float> s2;
>   S<double> s3;
> }
> ```
> Rather than telling the user the static_assert failed because false is not true, having a custom diagnostic might read better for users. GCC doesn't produce a custom diagnostic -- the behavior isn't terrible, but the "false evaluates to false" note is effectively just noise, too: https://godbolt.org/z/456bzWG7c
OH. That makes sense now,  thanks. I think I agree.
Interestingly, in gcc immediate calls are really immediate :) https://godbolt.org/z/b3vrzf4sj 


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  https://reviews.llvm.org/D144285/new/

https://reviews.llvm.org/D144285