[libc] [llvm] [libc][c23][fenv] Implement fetestexceptflag (PR #87828)

[Nick Desaulniers via llvm-commits]

================
@@ -39,19 +40,33 @@ TEST(LlvmLibcFenvTest, GetExceptFlagAndSetExceptFlag) {
     ASSERT_EQ(LIBC_NAMESPACE::fesetexceptflag(&eflags, FE_ALL_EXCEPT), 0);
     ASSERT_NE(LIBC_NAMESPACE::fputil::test_except(FE_ALL_EXCEPT) & e, 0);
 
+    // Exception flags are exactly the flags corresponding to the previously
+    // raised exception.
+    ASSERT_EQ(LIBC_NAMESPACE::fetestexceptflag(&eflags, FE_ALL_EXCEPT),
+              LIBC_NAMESPACE::fputil::test_except(FE_ALL_EXCEPT));
+
     // Cleanup. We clear all excepts as raising excepts like FE_OVERFLOW
     // can also raise FE_INEXACT.
     LIBC_NAMESPACE::fputil::clear_except(FE_ALL_EXCEPT);
   }
 
-  // Next, we will raise one exception and save the flags.
+  // Next, we will raise one exception, save the flag and clear all exceptions.
   LIBC_NAMESPACE::fputil::raise_except(FE_INVALID);
-  fexcept_t eflags;
-  LIBC_NAMESPACE::fegetexceptflag(&eflags, FE_ALL_EXCEPT);
-  // Clear all exceptions and raise two other exceptions.
+  fexcept_t invalid_flag;
+  LIBC_NAMESPACE::fegetexceptflag(&invalid_flag, FE_ALL_EXCEPT);
+  ASSERT_EQ(LIBC_NAMESPACE::fetestexceptflag(&invalid_flag, FE_ALL_EXCEPT),
+            FE_INVALID);
   LIBC_NAMESPACE::fputil::clear_except(FE_ALL_EXCEPT);
+
+  // Raise two other exceptions and verify that they are set.
   LIBC_NAMESPACE::fputil::raise_except(FE_OVERFLOW | FE_INEXACT);
+  fexcept_t overflow_and_inexact_flag;
+  LIBC_NAMESPACE::fegetexceptflag(&overflow_and_inexact_flag, FE_ALL_EXCEPT);
+  ASSERT_EQ(LIBC_NAMESPACE::fetestexceptflag(&overflow_and_inexact_flag,
+                                             FE_ALL_EXCEPT),
+            FE_OVERFLOW | FE_INEXACT);
+
----------------
nickdesaulniers wrote:

Mind adding one more assert:
```suggestion
  ASSERT_EQ(LIBC_NAMESPACE::fetestexceptflag(&overflow_and_inexact_flag,
                                             FE_OVERFLOW | FE_INEXACT),
            FE_OVERFLOW | FE_INEXACT);

```
I think that should work?

https://github.com/llvm/llvm-project/pull/87828