[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

           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");

flang/test/Evaluate/errors01.f90

     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