fix: keep a space when densing "-" before a negative operand

denseOperators removed the spaces around a "-" operator even when the
operand that follows it starts with another "-" (a unary minus or a
negative number literal). That glues the two together into "--", which
re-parses as a line comment and silently drops the rest of the line:

  format("SELECT 1 - -1 AS x", { denseOperators: true })
  => "SELECT\n  1--1 AS x"   // "--1 AS x" is now a comment

Guard against this in the layout: when a token that starts with "-" would
be placed directly after one that ends with "-", keep a single space so the
"--" comment marker can never form. Formatting is idempotent again.

Author: sarathfrancis90

src/formatter/Layout.ts

@@ -57,11 +57,22 @@ export default class Layout {
           this.items.push(WS.SINGLE_INDENT);
           break;
         default:
+          // Don't glue a token starting with "-" directly onto one ending with
+          // "-": that forms "--", which re-parses as a line comment and
+          // swallows the rest of the line (e.g. densing "a - -b" into "a--b").
+          if (typeof item === 'string' && item.startsWith('-') && this.lastTokenEndsWith('-')) {
+            this.items.push(WS.SPACE);
+          }
           this.items.push(item);
       }
     }
   }
 
+  private lastTokenEndsWith(suffix: string): boolean {
+    const lastItem = last(this.items);
+    return typeof lastItem === 'string' && lastItem.endsWith(suffix);
+  }
+
   private trimHorizontalWhitespace() {
     while (isHorizontalWhitespace(last(this.items))) {
       this.items.pop();

test/features/operators.ts

@@ -40,6 +40,17 @@ export default function supportsOperators(
     });
   });
 
+  it('does not glue a - in front of a negative operand into a line comment in dense mode', () => {
+    // "a - -b" / "1 - -1" must not be densed into "a--b" / "1--1": the "--"
+    // would re-parse as a line comment and silently swallow the rest of the line.
+    ['SELECT a - -b', 'SELECT 1 - -1'].forEach(sql => {
+      const result = format(sql, { denseOperators: true });
+      expect(result).not.toContain('--');
+      // ...and formatting stays idempotent.
+      expect(format(result, { denseOperators: true })).toBe(result);
+    });
+  });
+
   (cfg.logicalOperators || ['AND', 'OR']).forEach(op => {
     it(`supports ${op} operator`, () => {
       const result = format(`SELECT true ${op} false AS foo;`);