[clang] [C23] Implement N3018: The constexpr specifier for object definitions (PR #73099)

[Aaron Ballman via cfe-commits]

================
@@ -414,20 +415,42 @@ NarrowingKind StandardConversionSequence::getNarrowingKind(
       if (Initializer->isValueDependent())
         return NK_Dependent_Narrowing;
 
-      if (Initializer->isCXX11ConstantExpr(Ctx, &ConstantValue)) {
+      Expr::EvalResult R;
+      if ((Ctx.getLangOpts().C23 && Initializer->EvaluateAsRValue(R, Ctx)) ||
+          Initializer->isCXX11ConstantExpr(Ctx, &ConstantValue)) {
         // Constant!
+        if (Ctx.getLangOpts().C23)
+          ConstantValue = R.Val;
         assert(ConstantValue.isFloat());
         llvm::APFloat FloatVal = ConstantValue.getFloat();
         // Convert the source value into the target type.
         bool ignored;
-        llvm::APFloat::opStatus ConvertStatus = FloatVal.convert(
-          Ctx.getFloatTypeSemantics(ToType),
-          llvm::APFloat::rmNearestTiesToEven, &ignored);
-        // If there was no overflow, the source value is within the range of
-        // values that can be represented.
-        if (ConvertStatus & llvm::APFloat::opOverflow) {
-          ConstantType = Initializer->getType();
-          return NK_Constant_Narrowing;
+        llvm::APFloat Converted = FloatVal;
+        llvm::APFloat::opStatus ConvertStatus =
+            Converted.convert(Ctx.getFloatTypeSemantics(ToType),
+                              llvm::APFloat::rmNearestTiesToEven, &ignored);
+        Converted.convert(Ctx.getFloatTypeSemantics(FromType),
+                          llvm::APFloat::rmNearestTiesToEven, &ignored);
+        if (Ctx.getLangOpts().C23) {
+          if (FloatVal.isNaN() && Converted.isNaN()) {
+            if (!FloatVal.isSignaling() && !Converted.isSignaling()) {
+              // Quiet NaNs are considered the same value, regardless of
+              // payloads.
+              return NK_Not_Narrowing;
+            }
+          }
----------------
AaronBallman wrote:

```suggestion
          if (FloatVal.isNaN() && Converted.isNaN() &&
             !FloatVal.isSignaling() && !Converted.isSignaling()) {
            // Quiet NaNs are considered the same value, regardless of
            // payloads.
            return NK_Not_Narrowing;
          }
```
Combining some logic, NFC

https://github.com/llvm/llvm-project/pull/73099