[clang] [Clang] Do not warn on UTF-16 -> UTF-32 conversions. (PR #163927)

[Shafik Yaghmour via cfe-commits]

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@@ -12309,13 +12309,20 @@ static void DiagnoseMixedUnicodeImplicitConversion(Sema &S, const Type *Source,
                                                    SourceLocation CC) {
   assert(Source->isUnicodeCharacterType() && Target->isUnicodeCharacterType() &&
          Source != Target);
+
+  // Lone surrogates have a distinct representation in UTF-32.
+  // Converting between UTF-16 and UTF-32 codepoints seems very widespread,
+  // so don't warn on such conversion.
+  if (Source->isChar16Type() && Target->isChar32Type())
+    return;
+
   Expr::EvalResult Result;
   if (E->EvaluateAsInt(Result, S.getASTContext(), Expr::SE_AllowSideEffects,
                        S.isConstantEvaluatedContext())) {
     llvm::APSInt Value(32);
     Value = Result.Val.getInt();
     bool IsASCII = Value <= 0x7F;
-    bool IsBMP = Value <= 0xD7FF || (Value >= 0xE000 && Value <= 0xFFFF);
+    bool IsBMP = Value <= 0xDFFF || (Value >= 0xE000 && Value <= 0xFFFF);
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shafik wrote:

This change is not really explain in the summary or at least I don't get it. I guess this cover the char32 to char16 `D800` test below. 

https://github.com/llvm/llvm-project/pull/163927