fix(runtime-dom): enable set form attr to null on form-elements (#2840)

packages/runtime-dom/__tests__/patchProps.spec.ts

@@ -145,5 +145,8 @@ describe('runtime-dom: props patching', () => {
     // non existant element
     expect(el.form).toBe(null)
     expect(el.getAttribute('form')).toBe('foo')
+    // remove attribute
+    patchProp(el, 'form', 'foo', null)
+    expect(el.getAttribute('form')).toBe(null)
   })
 })

packages/runtime-dom/src/patchProp.ts

@@ -3,7 +3,13 @@ import { patchStyle } from './modules/style'
 import { patchAttr } from './modules/attrs'
 import { patchDOMProp } from './modules/props'
 import { patchEvent } from './modules/events'
-import { isOn, isString, isFunction, isModelListener } from '@vue/shared'
+import {
+  isOn,
+  isString,
+  isFunction,
+  isModelListener,
+  isFormTag
+} from '@vue/shared'
 import { RendererOptions } from '@vue/runtime-core'
 
 const nativeOnRE = /^on[a-z]/
@@ -93,9 +99,9 @@ function shouldSetAsProp(
     return false
   }
 
-  // #1787 form as an attribute must be a string, while it accepts an Element as
-  // a prop
-  if (key === 'form' && typeof value === 'string') {
+  // #1787, #2840 the form property is readonly and can only be set as an
+  // attribute using a string value
+  if (key === 'form' && isFormTag(el.tagName)) {
     return false
   }
 

packages/shared/src/domTagConfig.ts

@@ -30,6 +30,11 @@ const SVG_TAGS =
 const VOID_TAGS =
   'area,base,br,col,embed,hr,img,input,link,meta,param,source,track,wbr'
 
+const FORM_TAGS =
+  'button,datalist,fieldset,input,keygen,label,legend,meter,optgroup,option,' +
+  'output,progress,select,textarea'
+
 export const isHTMLTag = /*#__PURE__*/ makeMap(HTML_TAGS)
 export const isSVGTag = /*#__PURE__*/ makeMap(SVG_TAGS)
 export const isVoidTag = /*#__PURE__*/ makeMap(VOID_TAGS)
+export const isFormTag = /*#__PURE__*/ makeMap(FORM_TAGS, true)