v-bind merge strategy modifier

active-rfcs/0000-v-bind-merge-strategy-modifier.md

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+- Start Date: 2020-09-28
+- Target Major Version: 3.x
+- Reference Issues: [2059](https://github.com/vuejs/vue-next/issues/2059)
+- Implementation PR: (leave this empty)
+
+# Summary
+
+Revert `v-bind` merge behaviour change to be the same as in Vue 2.
+Add `.replace` modifier for `v-bind` to change default merge strategy.
+
+# Basic example
+
+```html
+<div foo="bar" v-bind="{ foo: 'baz' }"></div>
+<!--would render to-->
+<div foo="bar"></div>
+```
+
+```html
+<div foo="bar" v-bind.replace="{ foo: 'baz' }"></div>
+<!--would render to-->
+<div foo="baz"></div>
+```
+
+# Motivation
+
+In Vue 2 `v-bind` would not replace already declared attributes or props on the element\component.
+In Vue 3 `v-bind` that default merge strategy has been changed.
+Now it _will_ replace anything that has been passed to `v-bind`.
+Moreover, this behaviour depends on the `v-bind` placement order.
+If you place `v-bind` directive **before** your attributes – it **will not** replace them and vice versa.
+
+This change has some serious downsides:
+
+* Reliance on attribute order is not an expected behaviour.
+  We've learned that HTML and CSS never relies on attribute order and this change goes into conflict with that knowledge.
+
+* Teams might have code standards that define specific order for
+  attributes\directives\listeners for better readability and consistency.
+  This change might be in conflict with these standards and introduce inconsistency into an already consistent codebase.
+
+* Migration from Vue 2 will have a more complicated path.
+  You'll have to check your whole codebase for `v-bind` placement order unless a sophisticated codemod is released.
+
+In order to fix these issues this RFC focuses on making these 2 changes:
+
+1. Revert `v-bind` merge behaviour to the one used in Vue 2.
+2. Add a `.replace` modifier to `v-bind` to manually control merge behaviour.
+
+## Why revert current behaviour?
+
+Since Vue 3 is not yet widely adopted it is still possible to revert this change with minimal impact on current Vue 3 users.
+At the time of writing this RFC the complete migration path from Vue 2 to Vue 3 is not completely ready,
+and with this change in place we could skip another migration step. 
+
+# Detailed design
+
+`v-bind` directive placement order no longer affects its merge strategy. Instead, this strategy has to be changed explicitly.
+
+By default `v-bind` will not replace attributes or props that have been already declared.
+To change that `v-bind` should support a `.replace` modifier that will instruct compiler
+to replace everything that's passed to a `v-bind` directive.
+
+Given this template:
+
+```html
+<div foo="bar" v-bind="$attrs"></div>
+<div foo="bar" v-bind.replace="$attrs"></div>
+```
+
+It should roughly compile to:
+
+```js
+[
+  h('div', {
+    ...this.$attrs,
+    foo: 'bar',  
+  }),
+  h('div', {
+    foo: 'bar',
+    ...this.$attrs
+  })
+]
+```
+
+# Drawbacks
+
+Users who already relay on that behaviour would have to go through a migration step.
+
+# Alternatives
+
+
+# Adoption strategy
+
+It is to be discussed whether this change should be released as a minor update or not.
+
+# Unresolved questions
+
+