1 files changed, 68 insertions, 0 deletions
diff --git a/tests/ui/consts/const-float-bits-reject-conv.rs b/tests/ui/consts/const-float-bits-reject-conv.rs
new file mode 100644
index 00000000000..c77e99abbf6
--- /dev/null
+++ b/tests/ui/consts/const-float-bits-reject-conv.rs
@@ -0,0 +1,68 @@
+// compile-flags: -Zmir-opt-level=0
+// error-pattern: cannot use f32::to_bits on a NaN
+#![feature(const_float_bits_conv)]
+#![feature(const_float_classify)]
+
+// Don't promote
+const fn nop<T>(x: T) -> T { x }
+
+macro_rules! const_assert {
+    ($a:expr) => {
+        {
+            const _: () = assert!($a);
+            assert!(nop($a));
+        }
+    };
+    ($a:expr, $b:expr) => {
+        {
+            const _: () = assert!($a == $b);
+            assert_eq!(nop($a), nop($b));
+        }
+    };
+}
+
+fn f32() {
+    // Check that NaNs roundtrip their bits regardless of signalingness
+    // 0xA is 0b1010; 0x5 is 0b0101 -- so these two together clobbers all the mantissa bits
+    // ...actually, let's just check that these break. :D
+    const MASKED_NAN1: u32 = f32::NAN.to_bits() ^ 0x002A_AAAA;
+    //~^ inside
+    const MASKED_NAN2: u32 = f32::NAN.to_bits() ^ 0x0055_5555;
+    //~^ inside
+
+    // The rest of the code is dead because the constants already fail to evaluate.
+
+    const_assert!(f32::from_bits(MASKED_NAN1).is_nan());
+    const_assert!(f32::from_bits(MASKED_NAN1).is_nan());
+
+    // LLVM does not guarantee that loads and stores of NaNs preserve their exact bit pattern.
+    // In practice, this seems to only cause a problem on x86, since the most widely used calling
+    // convention mandates that floating point values are returned on the x87 FPU stack. See #73328.
+    // However, during CTFE we still preserve bit patterns (though that is not a guarantee).
+    const_assert!(f32::from_bits(MASKED_NAN1).to_bits(), MASKED_NAN1);
+    const_assert!(f32::from_bits(MASKED_NAN2).to_bits(), MASKED_NAN2);
+}
+
+fn f64() {
+    // Check that NaNs roundtrip their bits regardless of signalingness
+    // 0xA is 0b1010; 0x5 is 0b0101 -- so these two together clobbers all the mantissa bits
+    // ...actually, let's just check that these break. :D
+    const MASKED_NAN1: u64 = f64::NAN.to_bits() ^ 0x000A_AAAA_AAAA_AAAA;
+    //~^ inside
+    const MASKED_NAN2: u64 = f64::NAN.to_bits() ^ 0x0005_5555_5555_5555;
+    //~^ inside
+
+    // The rest of the code is dead because the constants already fail to evaluate.
+
+    const_assert!(f64::from_bits(MASKED_NAN1).is_nan());
+    const_assert!(f64::from_bits(MASKED_NAN1).is_nan());
+
+    // See comment above.
+    const_assert!(f64::from_bits(MASKED_NAN1).to_bits(), MASKED_NAN1);
+    const_assert!(f64::from_bits(MASKED_NAN2).to_bits(), MASKED_NAN2);
+}
+
+fn main() {
+    f32();
+    f64();
+}