add fallback option to plugins

This will allow us to use the fallback plugin if there are 2 colliding
plugins given a certain arbitrary value that's indistinguishable in
either plugin.

Normally we would then require a type cast, but now we can use the
fallback. This is useful so that in the future adding more colliding
plugins doesn't result in working code being broken because it requires
a typehint all of a sudden.

E.g.:
```js
<!-- Before -->
<div class="bg-[200px_100px]"></div><!-- Warning -->

<!-- Fix: -->
<div class="bg-[length:200px_100px]"></div>
<div class="bg-[position:200px_100px]"></div>
```

```js
<!-- After (no fix required) -->
<div class="bg-[200px_100px]"></div><!-- Generates styles for the background-size -->
```

src/corePlugins.js

@@ -1482,6 +1482,7 @@ export let corePlugins = {
 
   backgroundSize: createUtilityPlugin('backgroundSize', [['bg', ['background-size']]], {
     type: ['lookup', 'length', 'percentage'],
+    fallback: true,
   }),
 
   backgroundAttachment: ({ addUtilities }) => {

src/lib/generateRules.js

@@ -541,62 +541,80 @@ function* resolveMatches(candidate, context, original = candidate) {
     }
 
     if (isArbitraryValue(modifier)) {
-      // When generated arbitrary values are ambiguous, we can't know
-      // which to pick so don't generate any utilities for them
       if (matches.length > 1) {
-        let typesPerPlugin = matches.map((match) => new Set([...(typesByMatches.get(match) ?? [])]))
-
-        // Remove duplicates, so that we can detect proper unique types for each plugin.
-        for (let pluginTypes of typesPerPlugin) {
-          for (let type of pluginTypes) {
-            let removeFromOwnGroup = false
+        let fallback = matches
+          .slice()
+          .reverse()
+          .find((match) => {
+            return match.every(([{ options }, node]) => {
+              return options.fallback && isParsableNode(node)
+            })
+          })
 
-            for (let otherGroup of typesPerPlugin) {
-              if (pluginTypes === otherGroup) continue
+        if (fallback) {
+          console.dir(fallback, { depth: null })
+          matches = [fallback]
+        }
 
-              if (otherGroup.has(type)) {
-                otherGroup.delete(type)
-                removeFromOwnGroup = true
+        // When generated arbitrary values are ambiguous, we can't know
+        // which to pick so don't generate any utilities for them
+        else {
+          let typesPerPlugin = matches.map(
+            (match) => new Set([...(typesByMatches.get(match) ?? [])])
+          )
+
+          // Remove duplicates, so that we can detect proper unique types for each plugin.
+          for (let pluginTypes of typesPerPlugin) {
+            for (let type of pluginTypes) {
+              let removeFromOwnGroup = false
+
+              for (let otherGroup of typesPerPlugin) {
+                if (pluginTypes === otherGroup) continue
+
+                if (otherGroup.has(type)) {
+                  otherGroup.delete(type)
+                  removeFromOwnGroup = true
+                }
               }
-            }
 
-            if (removeFromOwnGroup) pluginTypes.delete(type)
+              if (removeFromOwnGroup) pluginTypes.delete(type)
+            }
           }
-        }
 
-        let messages = []
-
-        for (let [idx, group] of typesPerPlugin.entries()) {
-          for (let type of group) {
-            let rules = matches[idx]
-              .map(([, rule]) => rule)
-              .flat()
-              .map((rule) =>
-                rule
-                  .toString()
-                  .split('\n')
-                  .slice(1, -1) // Remove selector and closing '}'
-                  .map((line) => line.trim())
-                  .map((x) => `      ${x}`) // Re-indent
-                  .join('\n')
+          let messages = []
+
+          for (let [idx, group] of typesPerPlugin.entries()) {
+            for (let type of group) {
+              let rules = matches[idx]
+                .map(([, rule]) => rule)
+                .flat()
+                .map((rule) =>
+                  rule
+                    .toString()
+                    .split('\n')
+                    .slice(1, -1) // Remove selector and closing '}'
+                    .map((line) => line.trim())
+                    .map((x) => `      ${x}`) // Re-indent
+                    .join('\n')
+                )
+                .join('\n\n')
+
+              messages.push(
+                `  Use \`${candidate.replace('[', `[${type}:`)}\` for \`${rules.trim()}\``
               )
-              .join('\n\n')
-
-            messages.push(
-              `  Use \`${candidate.replace('[', `[${type}:`)}\` for \`${rules.trim()}\``
-            )
-            break
+              break
+            }
           }
-        }
 
-        log.warn([
-          `The class \`${candidate}\` is ambiguous and matches multiple utilities.`,
-          ...messages,
-          `If this is content and not a class, replace it with \`${candidate
-            .replace('[', '[')
-            .replace(']', ']')}\` to silence this warning.`,
-        ])
-        continue
+          log.warn([
+            `The class \`${candidate}\` is ambiguous and matches multiple utilities.`,
+            ...messages,
+            `If this is content and not a class, replace it with \`${candidate
+              .replace('[', '[')
+              .replace(']', ']')}\` to silence this warning.`,
+          ])
+          continue
+        }
       }
 
       matches = matches.map((list) => list.filter((match) => isParsableNode(match[1])))

src/util/createUtilityPlugin.js

@@ -3,7 +3,7 @@ import transformThemeValue from './transformThemeValue'
 export default function createUtilityPlugin(
   themeKey,
   utilityVariations = [[themeKey, [themeKey]]],
-  { filterDefault = false, ...options } = {}
+  { filterDefault = false, fallback = false, ...options } = {}
 ) {
   let transformValue = transformThemeValue(themeKey)
   return function ({ matchUtilities, theme }) {
@@ -24,6 +24,7 @@ export default function createUtilityPlugin(
           })
         }, {}),
         {
+          fallback,
           ...options,
           values: filterDefault
             ? Object.fromEntries(

tests/arbitrary-values.test.js

@@ -262,11 +262,35 @@ it('should not convert escaped underscores with spaces', () => {
   })
 })
 
-it('should warn and not generate if arbitrary values are ambiguous', () => {
-  // If we don't protect against this, then `bg-[200px_100px]` would both
-  // generate the background-size as well as the background-position utilities.
+it('should pick the fallback plugin when arbitrary values collide', () => {
   let config = {
-    content: [{ raw: html`<div class="bg-[200px_100px]"></div>` }],
+    content: [{ raw: html`<div class="bg-[200px_100px] md:bg-[100px_200px]"></div>` }],
+  }
+
+  return run('@tailwind utilities', config).then((result) => {
+    return expect(result.css).toMatchFormattedCss(css`
+      .bg-\[200px_100px\] {
+        background-size: 200px 100px;
+      }
+
+      @media (min-width: 768px) {
+        .md\:bg-\[100px_200px\] {
+          background-size: 100px 200px;
+        }
+      }
+    `)
+  })
+})
+
+it('should warn and not generate if arbitrary values are ambiguous (without fallback)', () => {
+  let config = {
+    content: [{ raw: html`<div class="foo-[200px_100px]"></div>` }],
+    plugins: [
+      function ({ matchUtilities }) {
+        matchUtilities({ foo: (value) => ({ value }) }, { type: ['position'] })
+        matchUtilities({ foo: (value) => ({ value }) }, { type: ['length'] })
+      },
+    ],
   }
 
   return run('@tailwind utilities', config).then((result) => {