fix: fix an ICE happening when we call a closure result from if/else

crates/nargo_cli/tests/test_data/higher_order_functions/src/main.nr

@@ -36,6 +36,21 @@ fn main() -> pub Field {
     x += 1;
     assert(closure_capturing_mutable(1) == 5);
 
+    // Fixing an ICE, where rewriting the closures
+    // during monomorphization didn't correspond
+    // to an internal `if` type
+    // found by @jfecher:
+    // https://github.com/noir-lang/noir/pull/1959#issuecomment-1658992989
+    let x2: u32 = 32;
+
+    let closure_if_else = if x2 > 2 {
+        || x2
+    } else {
+        || x2 + 2342
+    };
+
+    assert(closure_if_else() == 32);
+
     let ret = twice(add1, 3);
 
     test_array_functions();

crates/noirc_frontend/src/monomorphization/mod.rs

@@ -690,7 +690,18 @@ impl<'interner> Monomorphizer<'interner> {
                 let args = vecmap(args, Self::convert_type);
                 let ret = Box::new(Self::convert_type(ret));
                 let env = Box::new(Self::convert_type(env));
-                ast::Type::Function(args, ret, env)
+                match (*env).clone() {
+                    ast::Type::Unit => ast::Type::Function(args, ret, env),
+                    ast::Type::Tuple(elements) => ast::Type::Tuple(vec![
+                        elements[0].clone(),
+                        ast::Type::Function(args, ret, env),
+                    ]),
+                    _ => {
+                        unreachable!(
+                            "internal Type::Function env should be either a Unit or a Tuple, not {env}"
+                        )
+                    }
+                }
             }
 
             HirType::MutableReference(element) => {