[clang] [Clang] Added explanation why a is constructible evaluated to false. (PR #143309)

[Corentin Jabot via cfe-commits]

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@@ -2159,6 +2167,26 @@ static void DiagnoseNonTriviallyCopyableReason(Sema &SemaRef,
   }
 }
 
+static void DiagnoseNonConstructibleReason(Sema &SemaRef, SourceLocation Loc,
+                                           QualType T) {
+  SemaRef.Diag(Loc, diag::note_unsatisfied_trait)
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cor3ntin wrote:

I think what would be useful here is to try to run overload resolution again on all the arguments,
and show the errors - https://compiler-explorer.com/z/aMG1nEoz1

You can look at how `static bool EvaluateBooleanTypeTrait` works (in the same file).

I think that by calling `InitializationSequence::Diagnose` we would get super useful diagnostics. 

We don't have a good way to turn the errors that would be produced into notes, though @AaronBallman @erichkeane @Sirraide  - but maybe emitting errors is good enough as a first approach

https://github.com/llvm/llvm-project/pull/143309