[clang] [clang] Improve bit-field in ref NTTP diagnostic (PR #71077)

[via cfe-commits]

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@@ -5992,6 +5992,15 @@ static ExprResult BuildConvertedConstantExpression(Sema &S, Expr *From,
                                   /*InOverloadResolution=*/false,
                                   /*AllowObjCWritebackConversion=*/false,
                                   /*AllowExplicit=*/false);
+
+  // TryCopyInitialization returns incorrect info for attempts to bind reference
+  // to bit-field due to C++ [over.ics.ref]p4, so check it here.
+  if (From->refersToBitField() && T.getTypePtr()->isReferenceType()) {
+    return S.Diag(From->getBeginLoc(),
+                  diag::err_reference_bind_to_bitfield_in_cce)
+           << From->getSourceRange();
+  }
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cor3ntin wrote:

I think the wording that makes it not a constant expression is

> A [converted constant expression](https://eel.is/c++draft/expr.const#def:expression,converted_constant) of type T is an expression, implicitly converted to type T, where the converted expression is a constant expression [...] where the reference binding (if any) binds directly. Bitfields don't bind directly.
It might be worth saying that in the comment


https://github.com/llvm/llvm-project/pull/71077