Fix desugar step for pizza operator

crates/compiler/can/src/operator.rs

@@ -609,20 +609,27 @@ fn desugar_bin_ops<'a>(
     let mut op_stack: Vec<Loc<BinOp>> = Vec::with_capacity_in(lefts.len(), arena);
 
     for (loc_expr, loc_op) in lefts {
-        arg_stack.push(desugar_expr(arena, loc_expr));
+        arg_stack.push(desugar_expr_keep_parens(arena, loc_expr));
         match run_binop_step(arena, whole_region, &mut arg_stack, &mut op_stack, *loc_op) {
             Err(problem) => return problem,
             Ok(()) => continue,
         }
     }
 
-    let mut expr = desugar_expr(arena, right);
+    let mut expr = desugar_expr_keep_parens(arena, right);
 
     for (left, loc_op) in arg_stack.into_iter().zip(op_stack.into_iter()).rev() {
         expr = arena.alloc(new_op_call_expr(arena, left, loc_op, expr));
     }
 
-    expr
+    desugar_expr(arena, expr)
+}
+
+fn desugar_expr_keep_parens<'a>(arena: &'a Bump, expr: &'a Loc<Expr<'a>>) -> &'a Loc<Expr<'a>> {
+    match expr.value {
+        ParensAround(_) => expr,
+        _ => desugar_expr(arena, expr),
+    }
 }
 
 enum Step<'a> {

crates/reporting/tests/test_reporting.rs

@@ -13841,4 +13841,44 @@ In roc, functions are always written as a lambda, like{}
     Tip: It looks like it takes too many arguments. I'm seeing 1 extra.
     "###
     );
+
+    test_report!(
+        pizza_parens_right,
+        indoc!(
+            r#"
+            2 |> (Num.sub 3)
+            "#
+        ),
+        @r###"
+        ── TOO FEW ARGS ────────────────────────────────────────── /code/proj/Main.roc ─
+
+        The `sub` function expects 2 arguments, but it got only 1:
+        
+        4│      2 |> (Num.sub 3)
+                      ^^^^^^^
+        
+        Roc does not allow functions to be partially applied. Use a closure to
+        make partial application explicit.
+        "###
+    );
+
+    test_report!(
+        pizza_parens_middle,
+        indoc!(
+            r#"
+            2 |> (Num.sub 3) |> Num.sub 3
+            "#
+        ),
+        @r###"
+        ── TOO FEW ARGS ────────────────────────────────────────── /code/proj/Main.roc ─
+
+        The `sub` function expects 2 arguments, but it got only 1:
+        
+        4│      2 |> (Num.sub 3) |> Num.sub 3
+                      ^^^^^^^
+        
+        Roc does not allow functions to be partially applied. Use a closure to
+        make partial application explicit.
+        "###
+    );
 }