Impossible preds const arg test

Author: BoxyUwU

tests/ui/const-generics/mgca/impossible_preds_on_anon_const.rs

@@ -0,0 +1,50 @@
+#![feature(generic_const_items, min_generic_const_args)]
+#![expect(incomplete_features)]
+
+// Tests behaviour of attempting to evaluate const arguments whose wellformedness
+// depends on impossible to satisfy predicates, that are satisfied from another
+// trivially false clause in the environment.
+//
+// Additionally tests how (or if) this behaviour differs depending on whether the
+// constants where trivially false where clause is global or not.
+
+trait Unimplemented<'a> {}
+
+trait Trait {
+    const NON_GLOBAL<T>: usize
+    where
+        for<'a> T: Unimplemented<'a>;
+
+    const GLOBAL: usize
+    where
+        for<'a> (): Unimplemented<'a>;
+}
+
+impl Trait for u8 {
+    const NON_GLOBAL<T>: usize = 1
+    where
+        for<'a> T: Unimplemented<'a>;
+
+    const GLOBAL: usize = 1
+    //~^ ERROR: evaluation of constant value failed
+    where
+        for<'a> (): Unimplemented<'a>;
+}
+
+struct Foo<const N: usize>;
+
+fn non_global()
+where
+    for<'a> (): Unimplemented<'a>,
+{
+    let _: Foo<1> = Foo::<{ <u8 as Trait>::NON_GLOBAL::<()> }>;
+}
+
+fn global()
+where
+    for<'a> (): Unimplemented<'a>,
+{
+    let _: Foo<1> = Foo::<{ <u8 as Trait>::GLOBAL }>;
+}
+
+fn main() {}

tests/ui/const-generics/mgca/impossible_preds_on_anon_const.stderr

@@ -0,0 +1,12 @@
+error[E0080]: evaluation of constant value failed
+  --> $DIR/impossible_preds_on_anon_const.rs:28:5
+   |
+LL | /     const GLOBAL: usize = 1
+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`.