Rollup merge of #81911 - BoxyUwU:constgenericgaticefix, r=nikomatsakis

GAT/const_generics: Allow with_opt_const_param to return GAT param def_id

Fixes #75415
Fixes #79666
cc ```@lcnr```

I've absolutely no idea who to r?  for this...

3 files changed, 71 insertions, 0 deletions

diff --git a/src/test/ui/generic-associated-types/const-generics-gat-in-trait-return-type-1.rs b/src/test/ui/generic-associated-types/const-generics-gat-in-trait-return-type-1.rs
new file mode 100644
index 00000000000..ab33ef6f244
--- /dev/null
+++ b/src/test/ui/generic-associated-types/const-generics-gat-in-trait-return-type-1.rs
@@ -0,0 +1,22 @@
+// run-pass
+#![feature(generic_associated_types)]
+#![allow(incomplete_features)]
+
+// 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<3>;
+}
+
+impl Foo for () {
+    type Assoc<const N: usize> = [(); N];
+    fn foo(&self) -> Self::Assoc<3> {
+        [(); 3]
+    }
+}
+
+fn main() {
+    assert_eq!(().foo(), [(); 3]);
+}
diff --git a/src/test/ui/generic-associated-types/const-generics-gat-in-trait-return-type-2.rs b/src/test/ui/generic-associated-types/const-generics-gat-in-trait-return-type-2.rs
new file mode 100644
index 00000000000..ba9a82ae721
--- /dev/null
+++ b/src/test/ui/generic-associated-types/const-generics-gat-in-trait-return-type-2.rs
@@ -0,0 +1,22 @@
+// run-pass
+#![feature(generic_associated_types)]
+#![allow(incomplete_features)]
+
+// 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<const N: usize>(&self) -> Self::Assoc<N>;
+}
+
+impl Foo for () {
+    type Assoc<const N: usize> = [(); N];
+    fn foo<const N: usize>(&self) -> Self::Assoc<N> {
+        [(); N]
+    }
+}
+
+fn main() {
+    assert_eq!(().foo::<10>(), [(); 10]);
+}
diff --git a/src/test/ui/generic-associated-types/const-generics-gat-in-trait-return-type-3.rs b/src/test/ui/generic-associated-types/const-generics-gat-in-trait-return-type-3.rs
new file mode 100644
index 00000000000..9da5334056a
--- /dev/null
+++ b/src/test/ui/generic-associated-types/const-generics-gat-in-trait-return-type-3.rs
@@ -0,0 +1,27 @@
+// run-pass
+#![feature(generic_associated_types)]
+#![allow(incomplete_features)]
+
+// This test unsures that with_opt_const_param returns the
+// def_id of the N param in the Bar::Assoc GAT.
+
+trait Bar {
+    type Assoc<const N: usize>;
+}
+trait Foo: Bar {
+    fn foo(&self) -> Self::Assoc<3>;
+}
+
+impl Bar for () {
+    type Assoc<const N: usize> = [(); N];
+}
+
+impl Foo for () {
+    fn foo(&self) -> Self::Assoc<3> {
+        [(); 3]
+    }
+}
+
+fn main() {
+    assert_eq!(().foo(), [(); 3]);
+}