Add tests for mismatched type_const

Author: camelid

tests/crashes/mgca/type_const-only-in-trait.rs

@@ -0,0 +1,23 @@
+//@ known-bug: #132980
+// Move this test to tests/ui/const-generics/mgca/type_const-only-in-trait.rs
+// once fixed.
+
+#![expect(incomplete_features)]
+#![feature(associated_const_equality, min_generic_const_args)]
+
+trait GoodTr {
+    #[type_const]
+    const NUM: usize;
+}
+
+struct BadS;
+
+impl GoodTr for BadS {
+    const NUM: usize = 42;
+}
+
+fn accept_good_tr<const N: usize, T: GoodTr<NUM = { N }>>(_x: &T) {}
+
+fn main() {
+    accept_good_tr(&BadS);
+}

tests/ui/const-generics/mgca/type_const-only-in-impl.rs

@@ -0,0 +1,20 @@
+#![expect(incomplete_features)]
+#![feature(associated_const_equality, min_generic_const_args)]
+
+trait BadTr {
+    const NUM: usize;
+}
+
+struct GoodS;
+
+impl BadTr for GoodS {
+    #[type_const]
+    const NUM: usize = 84;
+}
+
+fn accept_bad_tr<const N: usize, T: BadTr<NUM = { N }>>(_x: &T) {}
+//~^ ERROR use of trait associated const without `#[type_const]`
+
+fn main() {
+    accept_bad_tr::<84, _>(&GoodS);
+}

tests/ui/const-generics/mgca/type_const-only-in-impl.stderr

@@ -0,0 +1,10 @@
+error: use of trait associated const without `#[type_const]`
+  --> $DIR/type_const-only-in-impl.rs:15:43
+   |
+LL | fn accept_bad_tr<const N: usize, T: BadTr<NUM = { N }>>(_x: &T) {}
+   |                                           ^^^^^^^^^^^
+   |
+   = note: the declaration in the trait must be marked with `#[type_const]`
+
+error: aborting due to 1 previous error
+