[clang] [clang] Allow C-style casts in constexpr in MS compatible mode (PR #197005)

[via cfe-commits]

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@@ -18,6 +18,21 @@ using namespace clang::interp;
 
 State::~State() {}
 
+bool State::shouldRelaxDiag(const SourceLocation &Loc, diag::kind DiagId) {
+  if (!Ctx.getLangOpts().MSVCCompat)
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eleviant wrote:

> This is calling into the constant evaluator through StandardConversionSequence::getNarrowingKind()?

Yes

> I think specifically in the context of BuildConvertedConstantExpression calling getNarrowingKind(), it's okay to use relaxed mode, then throw away the diagnostic: we know we're going to be evaluating the expression again, so any diagnostic will get emitted later.

I've tried setting temporary pointer in ASTContext to set dummy `ExtendedDiag` pointer during `BuildConvertedConstantExpression`  (see latest update). Not sure, may be there there exists better way to accomplish this?

> More generally, I think we don't want to accept this as a constant expression in contexts where we haven't considered it; that could lead to weird results; that means we treat an expression that's not a constant express as a constant expression, and that could lead to rejecting a valid program if SFINAE gets involved.

For now we allow relaxed constant folding in these places only:
- VarDecl::evaluateValueImpl
- Sema::VerifyIntegerConstantExpression
- Sema::BuildConvertedConstantExpression / Sema::EvaluateConvertedConstantExpression



https://github.com/llvm/llvm-project/pull/197005