[flang] Warn about overflow from folding complex ABS()

[klausler]

Summary

Emit a warning when the result of folding a call to
ABS() with a complex argument results in an overflow.

[PeteSteinfeld]

All builds and tests correctly and looks good.

flang/lib/Evaluate/fold-real.cpp

if (name == "acos" || name == "acosh" || name == "asin" || name == "asinh" ||
     }
   } else if (name == "abs") { // incl. zabs & cdabs
     // Argument can be complex or real
     if (auto *x{UnwrapExpr<Expr<SomeReal>>(args[0])}) {
       return FoldElementalIntrinsic<T, T>(
           context, std::move(funcRef), &Scalar<T>::ABS);
     } else if (auto *z{UnwrapExpr<Expr<SomeComplex>>(args[0])}) {
       return FoldElementalIntrinsic<T, ComplexT>(context, std::move(funcRef),
-          ScalarFunc<T, ComplexT>([](const Scalar<ComplexT> &z) -> Scalar<T> {
-            return z.ABS().value;
+          ScalarFunc<T, ComplexT>([&name, &context](
+                                      const Scalar<ComplexT> &z) -> Scalar<T> {
+            ValueWithRealFlags<Scalar<T>> y{z.ABS()};
+            if (y.flags.test(RealFlag::Overflow)) {
+              context.messages().Say(
+                  "complex ABS intrinsic folding overflow"_warn_en_US, name);
+            }
+            return y.value;
           }));
     } else {
       common::die(" unexpected argument type inside abs");
     }
   } else if (name == "aimag") {
     if (auto *zExpr{UnwrapExpr<Expr<ComplexT>>(args[0])}) {
       return Fold(context, Expr<T>{ComplexComponent{true, std::move(*zExpr)}});
     }

flang/test/Evaluate/errors01.f90

subroutine s9
     !CHECK: error: DIM=4 argument to SPREAD must be between 1 and 3
     integer, parameter :: bad3 = spread(matrix, 4, 1)
   end subroutine
   subroutine warnings
     real, parameter :: ok1 = scale(0.0, 99999) ! 0.0
     real, parameter :: ok2 = scale(1.0, -99999) ! 0.0
     !CHECK: SCALE intrinsic folding overflow
     real, parameter :: bad1 = scale(1.0, 99999)
+    !CHECK: complex ABS intrinsic folding overflow
+    real, parameter :: bad2 = abs(cmplx(huge(0.),huge(0.)))
   end subroutine
 end module