[flang] Warn on invalid argument to SQRT() (#168607)

When folding SQRT(), notice invalid argument exceptions and optionally
warn about them.

Author: klausler

flang/lib/Evaluate/fold-real.cpp

@@ -425,8 +425,14 @@ Expr<Type<TypeCategory::Real, KIND>> FoldIntrinsicFunction(
             [](const Scalar<T> &x) -> Scalar<T> { return x.SPACING(); }));
   } else if (name == "sqrt") {
     return FoldElementalIntrinsic<T, T>(context, std::move(funcRef),
-        ScalarFunc<T, T>(
-            [](const Scalar<T> &x) -> Scalar<T> { return x.SQRT().value; }));
+        ScalarFunc<T, T>([&context](const Scalar<T> &x) -> Scalar<T> {
+          ValueWithRealFlags<Scalar<T>> result{x.SQRT()};
+          if (result.flags.test(RealFlag::InvalidArgument)) {
+            context.Warn(common::UsageWarning::FoldingValueChecks,
+                "Invalid argument to SQRT()"_warn_en_US);
+          }
+          return result.value;
+        }));
   } else if (name == "sum") {
     return FoldSum<T>(context, std::move(funcRef));
   } else if (name == "tiny") {

flang/test/Evaluate/folding03.f90

@@ -83,6 +83,8 @@ module real_tests
   real(4), parameter :: r4_pinf = 1._4/0._4
   !WARN: warning: division by zero [-Wfolding-exception]
   real(4), parameter :: r4_ninf = -1._4/0._4
+  !WARN: warning: Invalid argument to SQRT() [-Wfolding-value-checks]
+  real(4), parameter :: r4_sqrtneg = sqrt(-1._4)
 
   logical, parameter :: test_r4_nan_parentheses1 = .NOT.(((r4_nan)).EQ.r4_nan)
   logical, parameter :: test_r4_nan_parentheses2 = .NOT.(((r4_nan)).LT.r4_nan)
@@ -155,6 +157,8 @@ module real_tests
   TEST_ISNAN(r4_nan_add5)
   real(4), parameter :: r4_nan_add6 = r4_nan + r4_nan
   TEST_ISNAN(r4_nan_add6)
+  real(4), parameter :: r4_nan_sqrt = sqrt(r4_nan)
+  TEST_ISNAN(r4_nan_sqrt)
 
   !WARN: warning: overflow on multiplication [-Wfolding-exception]
   logical, parameter :: test_inf_r4_mult1 = (1.5_4*r4_pmax).eq.(r4_pinf)