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

[Nick Desaulniers via llvm-commits]

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@@ -48,6 +54,8 @@ TEST(LlvmLibcFenvTest, GetExceptFlagAndSetExceptFlag) {
   LIBC_NAMESPACE::fputil::raise_except(FE_INVALID);
   fexcept_t eflags;
   LIBC_NAMESPACE::fegetexceptflag(&eflags, FE_ALL_EXCEPT);
+  ASSERT_EQ(LIBC_NAMESPACE::fetestexceptflag(&eflags, FE_ALL_EXCEPT),
+            FE_INVALID);
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nickdesaulniers wrote:

Can you add one more `fetestexceptflag` after `fputil::raise_except(FE_OVERFLOW | FE_INEXACT)` that verifies that those 2 are set?

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