fix: treat increment and decrement ops as non-repeatable

Fixes decaffeinate#878

src/stages/main/index.js

@@ -27,6 +27,7 @@ import FunctionApplicationPatcher from './patchers/FunctionApplicationPatcher';
 import FunctionPatcher from './patchers/FunctionPatcher';
 import HeregexPatcher from './patchers/HeregexPatcher';
 import IdentifierPatcher from './patchers/IdentifierPatcher';
+import IncrementDecrementPatcher from './patchers/IncrementDecrementPatcher';
 import InOpPatcher from './patchers/InOpPatcher';
 import InstanceofOpPatcher from './patchers/InstanceofOpPatcher';
 import JavaScriptPatcher from './patchers/JavaScriptPatcher';
@@ -87,10 +88,6 @@ export default class MainStage extends TransformCoffeeScriptStage {
       case 'Float':
       case 'Null':
       case 'Undefined':
-      case 'PostIncrementOp':
-      case 'PostDecrementOp':
-      case 'PreIncrementOp':
-      case 'PreDecrementOp':
       case 'Quasi':
         return PassthroughPatcher;
 
@@ -120,6 +117,12 @@ export default class MainStage extends TransformCoffeeScriptStage {
       case 'GTEOp':
         return EqualityPatcher;
 
+      case 'PostIncrementOp':
+      case 'PostDecrementOp':
+      case 'PreIncrementOp':
+      case 'PreDecrementOp':
+        return IncrementDecrementPatcher;
+
       case 'ObjectInitialiserMember':
         return ObjectInitialiserMemberPatcher;
 

src/stages/main/patchers/IncrementDecrementPatcher.js

@@ -0,0 +1,23 @@
+import NodePatcher from '../../../patchers/NodePatcher';
+import type { PatcherContext } from '../../../patchers/types';
+
+export default class IncrementDecrementPatcher extends NodePatcher {
+  expression: NodePatcher;
+
+  constructor(patcherContext: PatcherContext, expression: NodePatcher) {
+    super(patcherContext);
+    this.expression = expression;
+  }
+
+  initialize() {
+    this.expression.setRequiresExpression();
+  }
+
+  patchAsExpression() {
+    this.expression.patch();
+  }
+
+  isRepeatable() {
+    return false;
+  }
+}

test/unary_operator_test.js

@@ -1,4 +1,5 @@
 import check from './support/check';
+import validate from './support/validate';
 
 describe('unary operators', () => {
   it('passes unary minus through', () => {
@@ -113,4 +114,12 @@ describe('unary operators', () => {
       }
     `);
   });
+
+  it('treats the increment operator as non-repeatable', () => {
+    validate(`
+      n = 1
+      (n++)?.toString()
+      o = n
+    `, 2);
+  });
 });