fix: Panic while canonicalizing erroneous projection type

crates/hir-ty/src/infer.rs

@@ -1436,7 +1436,8 @@ impl<'a> InferenceContext<'a> {
         let remaining = unresolved.map(|it| path.segments()[it..].len()).filter(|it| it > &0);
         let ty = match ty.kind(Interner) {
             TyKind::Alias(AliasTy::Projection(proj_ty)) => {
-                self.db.normalize_projection(proj_ty.clone(), self.table.trait_env.clone())
+                let ty = self.table.normalize_projection_ty(proj_ty.clone());
+                self.table.resolve_ty_shallow(&ty)
             }
             _ => ty,
         };

crates/hir-ty/src/tests/regression.rs

@@ -2141,3 +2141,24 @@ fn test() {
 }"#,
     );
 }
+
+#[test]
+fn issue_17866() {
+    check_infer(
+        r#"
+trait T {
+    type A;
+}
+
+type Foo = <S as T>::A;
+
+fn main() {
+    Foo {};
+}
+"#,
+        expect![[r#"
+            60..75 '{     Foo {}; }': ()
+            66..72 'Foo {}': {unknown}
+        "#]],
+    );
+}

crates/hir-ty/src/traits.rs

@@ -14,13 +14,13 @@ use hir_def::{
 };
 use hir_expand::name::Name;
 use intern::sym;
-use stdx::panic_context;
+use stdx::{never, panic_context};
 use triomphe::Arc;
 
 use crate::{
     db::HirDatabase, infer::unify::InferenceTable, utils::UnevaluatedConstEvaluatorFolder, AliasEq,
     AliasTy, Canonical, DomainGoal, Goal, Guidance, InEnvironment, Interner, ProjectionTy,
-    ProjectionTyExt, Solution, TraitRefExt, Ty, TyKind, WhereClause,
+    ProjectionTyExt, Solution, TraitRefExt, Ty, TyKind, TypeFlags, WhereClause,
 };
 
 /// This controls how much 'time' we give the Chalk solver before giving up.
@@ -90,6 +90,16 @@ pub(crate) fn normalize_projection_query(
     projection: ProjectionTy,
     env: Arc<TraitEnvironment>,
 ) -> Ty {
+    if projection.substitution.iter(Interner).any(|arg| {
+        arg.ty(Interner)
+            .is_some_and(|ty| ty.data(Interner).flags.intersects(TypeFlags::HAS_TY_INFER))
+    }) {
+        never!(
+            "Invoking `normalize_projection_query` with a projection type containing inference var"
+        );
+        return TyKind::Error.intern(Interner);
+    }
+
     let mut table = InferenceTable::new(db, env);
     let ty = table.normalize_projection_ty(projection);
     table.resolve_completely(ty)