Restore diagnostics when no inherent const candidates found

compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

@@ -1522,7 +1522,18 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         // as potential fallback candidates (#142006). To temporarily mask that issue, let's not
         // select at all if there are no early inherent candidates.
         if candidates.is_empty() {
-            return Ok(None);
+            return match assoc_tag {
+                ty::AssocTag::Type => Ok(None),
+                ty::AssocTag::Const => Err(self.report_unresolved_inherent_assoc_item(
+                    name,
+                    self_ty,
+                    candidates,
+                    vec![],
+                    span,
+                    assoc_tag,
+                )),
+                ty::AssocTag::Fn => unreachable!(),
+            };
         }
 
         let (applicable_candidates, fulfillment_errors) =

tests/ui/associated-inherent-types/iat-const-resolution.rs

@@ -0,0 +1,18 @@
+//! regression test for #148949, #149809
+#![expect(incomplete_features)]
+#![feature(min_generic_const_args)]
+#![feature(inherent_associated_types)]
+struct Foo<'a> {
+    x: &'a (),
+}
+
+impl<'a> Foo<'a> {
+    fn bar(_: [u8; Foo::X]) {}
+    //~^ ERROR: associated constant `X` not found for `Foo<'_>` in the current scope
+    // #[type_const]
+    // const Y: usize = 10;
+    // fn foo(_: [u8; Foo::Y]) {}
+    // associated constant `Y` not found for `Foo<'_>` in the current scope
+}
+
+fn main() {}

tests/ui/associated-inherent-types/iat-const-resolution.stderr

@@ -0,0 +1,15 @@
+error[E0220]: associated constant `X` not found for `Foo<'_>` in the current scope
+  --> $DIR/iat-const-resolution.rs:10:25
+   |
+LL | struct Foo<'a> {
+   | -------------- associated constant `X` not found for this struct
+...
+LL |     fn bar(_: [u8; Foo::X]) {}
+   |                         ^ associated item not found in `Foo<'_>`
+   |
+   = note: the associated constant was found for
+
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0220`.