Add regression test for 125877

tests/ui/traits/const-traits/const-trait-impl-parameter-mismatch.rs

@@ -0,0 +1,32 @@
+// This test demonstrates an ICE that may occur when we try to resolve the instance
+// of a impl that has different generics than the trait it's implementing. This ensures
+// we first check that the args are compatible before resolving the body, just like
+// we do in projection before substituting a GAT.
+//
+// Regression test for issue #125877.
+
+//@ compile-flags: -Znext-solver
+
+#![feature(const_trait_impl, effects)]
+//~^ ERROR feature has been removed
+
+#[const_trait]
+trait Main {
+    fn compute<T: ~const Aux>() -> u32;
+}
+
+impl const Main for () {
+    fn compute<'x>() -> u32 {
+        //~^ ERROR associated function `compute` has 0 type parameters but its trait declaration has 1 type parameter
+        0
+    }
+}
+
+#[const_trait]
+trait Aux {}
+
+impl const Aux for () {}
+
+fn main() {
+    const _: u32 = <()>::compute::<()>();
+}

tests/ui/traits/const-traits/const-trait-impl-parameter-mismatch.stderr

@@ -0,0 +1,21 @@
+error[E0557]: feature has been removed
+  --> $DIR/const-trait-impl-parameter-mismatch.rs:10:30
+   |
+LL | #![feature(const_trait_impl, effects)]
+   |                              ^^^^^^^ feature has been removed
+   |
+   = note: removed, redundant with `#![feature(const_trait_impl)]`
+
+error[E0049]: associated function `compute` has 0 type parameters but its trait declaration has 1 type parameter
+  --> $DIR/const-trait-impl-parameter-mismatch.rs:19:16
+   |
+LL |     fn compute<T: ~const Aux>() -> u32;
+   |                - expected 1 type parameter
+...
+LL |     fn compute<'x>() -> u32 {
+   |                ^^ found 0 type parameters
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0049, E0557.
+For more information about an error, try `rustc --explain E0049`.