Rollup merge of #108115 - eggyal:unmet_trait_alias_bound, r=compiler-errors

Do not ICE on unmet trait alias bounds

Rework of #108093 following feedback on that PR.

Fixes #108072

r? `@compiler-errors`

compiler/rustc_hir_typeck/src/fn_ctxt/adjust_fulfillment_errors.rs

@@ -477,12 +477,17 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         // This is the "trait" (meaning, the predicate "proved" by this `impl`) which provides the `Self` type we care about.
         // For the purposes of this function, we hope that it is a `struct` type, and that our current `expr` is a literal of
         // that struct type.
-        let impl_trait_self_ref: Option<ty::TraitRef<'tcx>> =
-            self.tcx.impl_trait_ref(obligation.impl_def_id).map(|impl_def| impl_def.skip_binder());
-
-        let Some(impl_trait_self_ref) = impl_trait_self_ref else {
+        let impl_trait_self_ref = if self.tcx.is_trait_alias(obligation.impl_def_id) {
+            self.tcx.mk_trait_ref(
+                obligation.impl_def_id,
+                ty::InternalSubsts::identity_for_item(self.tcx, obligation.impl_def_id),
+            )
+        } else {
+            self.tcx
+                .impl_trait_ref(obligation.impl_def_id)
+                .map(|impl_def| impl_def.skip_binder())
                 // It is possible that this is absent. In this case, we make no progress.
-            return Err(expr);
+                .ok_or(expr)?
         };
 
         // We only really care about the `Self` type itself, which we extract from the ref.

tests/ui/traits/alias/issue-108072-unmet-trait-alias-bound.rs

@@ -0,0 +1,11 @@
+// Regression test for #108072: do not ICE upon unmet trait alias constraint
+
+#![feature(trait_alias)]
+
+trait IteratorAlias = Iterator;
+
+fn f(_: impl IteratorAlias) {}
+
+fn main() {
+    f(()) //~ `()` is not an iterator
+}

tests/ui/traits/alias/issue-108072-unmet-trait-alias-bound.stderr

@@ -0,0 +1,19 @@
+error[E0277]: `()` is not an iterator
+  --> $DIR/issue-108072-unmet-trait-alias-bound.rs:10:7
+   |
+LL |     f(())
+   |     - ^^ `()` is not an iterator
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = help: the trait `Iterator` is not implemented for `()`
+   = note: required for `()` to implement `IteratorAlias`
+note: required by a bound in `f`
+  --> $DIR/issue-108072-unmet-trait-alias-bound.rs:7:14
+   |
+LL | fn f(_: impl IteratorAlias) {}
+   |              ^^^^^^^^^^^^^ required by this bound in `f`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.