Add incremental tests.

tests/incremental/const-generics/change-const-param-gat.rs

@@ -0,0 +1,29 @@
+// revisions: rpass1 rpass2 rpass3
+// compile-flags: -Zincremental-ignore-spans
+#![feature(generic_associated_types)]
+
+// This test unsures that with_opt_const_param returns the
+// def_id of the N param in the Foo::Assoc GAT.
+
+trait Foo {
+    type Assoc<const N: usize>;
+    fn foo(
+        &self,
+    ) -> Self::Assoc<{ if cfg!(rpass2) { 3 } else { 2 } }>;
+}
+
+impl Foo for () {
+    type Assoc<const N: usize> = [(); N];
+    fn foo(
+        &self,
+    ) -> Self::Assoc<{ if cfg!(rpass2) { 3 } else { 2 } }> {
+        [(); { if cfg!(rpass2) { 3 } else { 2 } }]
+    }
+}
+
+fn main() {
+    assert_eq!(
+        ().foo(),
+        [(); { if cfg!(rpass2) { 3 } else { 2 } }]
+    );
+}