impossible preds const pattern

Author: BoxyUwU

tests/ui/match/impossible_preds_const_pattern.rs

@@ -0,0 +1,58 @@
+#![feature(generic_const_items)]
+#![expect(incomplete_features)]
+
+trait Unimplemented<'a> {}
+
+trait Trait<T>
+where
+    for<'a> T: Unimplemented<'a>,
+{
+    const ASSOC: usize;
+}
+
+impl<T> Trait<T> for ()
+where
+    for<'a> T: Unimplemented<'a>,
+{
+    const ASSOC: usize = 0;
+}
+
+trait Global {
+    const ASSOC: usize
+    where
+        for<'a> (): Unimplemented<'a>;
+}
+
+impl Global for () {
+    const ASSOC: usize = 0
+    //~^ ERROR: evaluation of constant value failed
+    where
+        for<'a> (): Unimplemented<'a>;
+}
+
+fn works(x: usize)
+where
+    for<'a> (): Unimplemented<'a>,
+{
+    // In order for match exhaustiveness to determine this match is OK, CTFE must
+    // be able to evalaute `<() as Trait>::ASSOC` which depends on a trivially
+    // false bound for well formedness.
+    match x {
+        <() as Trait<()>>::ASSOC => todo!(),
+        1.. => todo!(),
+    }
+}
+
+fn errors(x: usize)
+where
+    for<'a> (): Unimplemented<'a>,
+{
+    // This errors due to the MIR for `ASSOC` being empty due to the
+    // globally false bound.
+    match x {
+        <() as Global>::ASSOC => todo!(),
+        1.. => todo!(),
+    }
+}
+
+fn main() {}

tests/ui/match/impossible_preds_const_pattern.stderr

@@ -0,0 +1,12 @@
+error[E0080]: evaluation of constant value failed
+  --> $DIR/impossible_preds_const_pattern.rs:27:5
+   |
+LL | /     const ASSOC: usize = 0
+LL | |
+LL | |     where
+LL | |         for<'a> (): Unimplemented<'a>;
+   | |______________________________________^ entering unreachable code
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0080`.