TC: Check that we haven't inferred a nonsensical type for if conditions

src/lib/type_check.ml

@@ -3599,6 +3599,7 @@ and infer_exp env (E_aux (exp_aux, (l, uannot)) as exp) =
       (* Try to infer the type of the condition - in some cases it may be a constant `true`
          or `false`, e.g. `xlen == 32`. If that fails check it is a bool without inference. *)
       let cond' = try irule infer_exp env cond with Type_error _ -> crule check_exp env cond bool_typ in
+      subtyp (exp_loc cond) env (typ_of cond') bool_typ;
 
       (* Constraints to apply when reasoning about the branch types. The condition must be
          true when evaluating the type of the `then` branch, and false for `else`. *)

test/typecheck/fail/non_bool_if.expect

@@ -0,0 +1,5 @@
+Type error:
+fail/non_bool_if.sail:2.11-17:
+2 |let _ = if "test" then 1 else 2
+  |           ^----^
+  | Type mismatch between bool('ex1#) and string

test/typecheck/fail/non_bool_if.sail

@@ -0,0 +1,2 @@
+
+let _ = if "test" then 1 else 2