[flang] Don't warn about a NaN argument to ISNAN()

[klausler]

Summary

Don't emit a warning when a compile-time constant argument to
ISNAN() or IEEE_IS_NAN() is a NaN.

[PeteSteinfeld]

All builds and tests correctly and looks good.

flang/lib/Evaluate/fold-logical.cpp

if (args[0] && args[1]) {
       auto t1{args[1]->GetType()};
       if (t0 && t1) {
         if (auto result{t0->ExtendsTypeOf(*t1)}) {
           return Expr<T>{*result};
         }
       }
     }
   } else if (name == "isnan" || name == "__builtin_ieee_is_nan") {
-    using DefaultReal = Type<TypeCategory::Real, 4>;
     // Only replace the type of the function if we can do the fold
     if (args[0] && args[0]->UnwrapExpr() &&
         IsActuallyConstant(*args[0]->UnwrapExpr())) {
+      auto restorer{context.messages().DiscardMessages()};
+      using DefaultReal = Type<TypeCategory::Real, 4>;
       return FoldElementalIntrinsic<T, DefaultReal>(context, std::move(funcRef),
           ScalarFunc<T, DefaultReal>([](const Scalar<DefaultReal> &x) {
             return Scalar<T>{x.IsNotANumber()};
           }));
     }
   } else if (name == "__builtin_ieee_is_negative") {
     auto restorer{context.messages().DiscardMessages()};
     using DefaultReal = Type<TypeCategory::Real, 4>;

flang/test/Evaluate/errors01.f90

subroutine warnings
     !CHECK: complex ABS intrinsic folding overflow
     real, parameter :: bad2 = abs(cmplx(huge(0.),huge(0.)))
     !CHECK: warning: DIM intrinsic folding overflow
     real, parameter :: bad3 = dim(huge(1.),-.5*huge(1.))
     !CHECK: warning: DIM intrinsic folding overflow
     integer, parameter :: bad4 = dim(huge(1),-1)
     !CHECK: warning: HYPOT intrinsic folding overflow
     real, parameter :: bad5 = hypot(huge(0.), huge(0.))
+    !CHECK: warning: SUM() of INTEGER(4) data overflowed
+    integer, parameter :: bad6 = sum([huge(1),huge(1)])
+    !CHECK: warning: SUM() of REAL(4) data overflowed
+    real, parameter :: bad7 = sum([huge(1.),huge(1.)])
+    !CHECK: warning: SUM() of COMPLEX(4) data overflowed
+    complex, parameter :: bad8 = sum([(huge(1.),0.),(huge(1.),0.)])
+    !CHECK: warning: PRODUCT() of INTEGER(4) data overflowed
+    integer, parameter :: bad9 = product([huge(1),huge(1)])
+    !CHECK: warning: PRODUCT() of REAL(4) data overflowed
+    real, parameter :: bad10 = product([huge(1.),huge(1.)])
+    !CHECK: warning: PRODUCT() of COMPLEX(4) data overflowed
+    complex, parameter :: bad11 = product([(huge(1.),0.),(huge(1.),0.)])
     !CHECK: warning: overflow on REAL(8) to REAL(4) conversion
     x = 1.D40
+    !CHECK-NOT: warning: invalid argument
+    if (.not. isnan(real(z'ffffffffffffffff',8))) stop
   end subroutine
 end module