fix: Allow two TypeVariable::Constant(N) to unify even if their con…

…stants are not equal (#3225)

compiler/noirc_frontend/src/hir_def/types.rs

@@ -684,9 +684,16 @@ impl Type {
                             *binding.borrow_mut() = TypeBinding::Bound(clone);
                             Ok(())
                         }
-                        TypeVariableKind::Constant(_) | TypeVariableKind::IntegerOrField => {
-                            Err(UnificationError)
+                        // The lengths don't match, but neither are set in stone so we can
+                        // just set them both to NotConstant. See issue 2370
+                        TypeVariableKind::Constant(_) => {
+                            // *length != target_length
+                            drop(borrow);
+                            *var.borrow_mut() = TypeBinding::Bound(Type::NotConstant);
+                            *binding.borrow_mut() = TypeBinding::Bound(Type::NotConstant);
+                            Ok(())
                         }
+                        TypeVariableKind::IntegerOrField => Err(UnificationError),
                     },
                 }
             }

tooling/nargo_cli/tests/execution_success/slices/src/main.nr

@@ -49,6 +49,7 @@ fn main(x : Field, y : pub Field) {
     regression_2083();
     // The parameters to this function must come from witness values (inputs to main)
     regression_merge_slices(x, y);
+    regression_2370();
 }
 
 // Ensure that slices of struct/tuple values work.
@@ -301,3 +302,10 @@ fn merge_slices_remove_between_ifs(x: Field, y: Field) -> [Field] {
 
     slice
 }
+
+// Previously, we'd get a type error when trying to assign an array of a different size to
+// an existing array variable. Now, we infer the variable must be a slice.
+fn regression_2370() {
+    let mut slice = [];
+    slice = [1, 2, 3];
+}