be less strict on cached bits bitwidth

zokrates_core/src/flatten/mod.rs

@@ -1958,12 +1958,21 @@ impl<'ast, T: Field> Flattener<'ast, T> {
             // constant
 
             let from = std::cmp::max(from, to);
-            match self.bits_cache.entry(e.field.clone().unwrap()) {
+            let res = match self.bits_cache.entry(e.field.clone().unwrap()) {
                 Entry::Occupied(entry) => {
                     let res: Vec<_> = entry.get().clone();
-                    // if we already know a decomposition, it has to be of the size of the target bitwidth
-                    assert_eq!(res.len(), to);
-                    res
+                    // if we already know a decomposition, its number of elements has to be smaller or equal to `to`
+                    assert!(res.len() <= to);
+
+                    // we then pad it with zeroes on the left (big endian) to return `to` bits
+                    if res.len() == to {
+                        res
+                    } else {
+                        (0..to - res.len())
+                            .map(|_| FlatExpression::Number(T::zero()))
+                            .chain(res)
+                            .collect()
+                    }
                 }
                 Entry::Vacant(_) => {
                     let bits = (0..from).map(|_| self.use_sym()).collect::<Vec<_>>();
@@ -2006,7 +2015,11 @@ impl<'ast, T: Field> Flattener<'ast, T> {
 
                     bits
                 }
-            }
+            };
+
+            assert_eq!(res.len(), to);
+
+            res
         })
     }
 

zokrates_core_test/tests/tests/range_check/assert_lt_u8.json

@@ -0,0 +1,55 @@
+{
+    "entry_point": "./tests/tests/range_check/assert_lt_u8.zok",
+    "max_constraint_count": 9,
+    "curves": ["Bn128"],
+    "tests": [
+        {
+            "input": {
+                "values": ["0x00"]
+            },
+            "output": {
+                "Ok": {
+                    "values": []
+                }
+            }
+        },
+        {
+            "input": {
+                "values": ["0x01"]
+            },
+            "output": {
+                "Ok": {
+                    "values": []
+                }
+            }
+        },
+        {
+            "input": {
+                "values": ["0x02"]
+            },
+            "output": {
+                "Err": {
+                    "UnsatisfiedConstraint": {
+                        "error": {
+                            "SourceAssertion": "Assertion failed at ./tests/tests/range_check/assert_lt_u8.zok:2:5"
+                        }
+                    }
+                }
+            }
+        },
+        {
+            "input": {
+                "values": ["0x0f"]
+            },
+            "output": {
+                "Err": {
+                    "UnsatisfiedConstraint": {
+                        "error": {
+                            "SourceAssertion": "Assertion failed at ./tests/tests/range_check/assert_lt_u8.zok:2:5"
+                        }
+                    }
+                }
+            }
+        }
+    ]
+}

zokrates_core_test/tests/tests/range_check/assert_lt_u8.zok

@@ -0,0 +1,3 @@
+def main(field x):
+    assert(x < 2)
+    return