Ensure underscore in theme() are also preserved

CHANGELOG.md

@@ -9,7 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Changed
 
-- Don't convert underscores in the first argument to `var()` to spaces ([#14776](https://github.com/tailwindlabs/tailwindcss/pull/14776))
+- Don't convert underscores in the first argument to `var()` and `theme()` to spaces ([#14776](https://github.com/tailwindlabs/tailwindcss/pull/14776), [#14781](https://github.com/tailwindlabs/tailwindcss/pull/14781))
 
 ### Fixed
 

packages/tailwindcss/src/candidate.test.ts

@@ -1105,6 +1105,31 @@ it('should not replace `_` in the first argument to `var()`', () => {
     `)
 })
 
+it('should not replace `_` in the first argument to `theme()`', () => {
+  let utilities = new Utilities()
+  utilities.functional('ml', () => [])
+
+  expect(run('ml-[theme(--spacing-1_5,_theme(--spacing-2_5,_1rem))]', { utilities }))
+    .toMatchInlineSnapshot(`
+      [
+        {
+          "important": false,
+          "kind": "functional",
+          "modifier": null,
+          "negative": false,
+          "raw": "ml-[theme(--spacing-1_5,_theme(--spacing-2_5,_1rem))]",
+          "root": "ml",
+          "value": {
+            "dataType": null,
+            "kind": "arbitrary",
+            "value": "theme(--spacing-1_5, theme(--spacing-2_5, 1rem))",
+          },
+          "variants": [],
+        },
+      ]
+    `)
+})
+
 it('should parse arbitrary properties', () => {
   expect(run('[color:red]')).toMatchInlineSnapshot(`
     [

packages/tailwindcss/src/index.test.ts

@@ -116,7 +116,7 @@ describe('compiling CSS', () => {
     ).toMatchSnapshot()
   })
 
-  test('unescapes underscores to spaces inside arbitrary values except for `url()` and first argument of `var()`', async () => {
+  test('unescapes underscores to spaces inside arbitrary values except for `url()` and first argument of `var()` and `theme()`', async () => {
     expect(
       await compileCss(
         css`
@@ -126,14 +126,22 @@ describe('compiling CSS', () => {
           }
           @tailwind utilities;
         `,
-        ['bg-[no-repeat_url(./my_file.jpg)', 'ml-[var(--spacing-1_5,_var(--spacing-2_5,_1rem))]'],
+        [
+          'bg-[no-repeat_url(./my_file.jpg)',
+          'ml-[var(--spacing-1_5,_var(--spacing-2_5,_1rem))]',
+          'ml-[theme(--spacing-1_5,theme(--spacing-2_5,_1rem)))]',
+        ],
       ),
     ).toMatchInlineSnapshot(`
       ":root {
         --spacing-1_5: 1.5rem;
         --spacing-2_5: 2.5rem;
       }
 
+      .ml-\\[theme\\(--spacing-1_5\\,theme\\(--spacing-2_5\\,_1rem\\)\\)\\)\\] {
+        margin-left: 1.5rem;
+      }
+
       .ml-\\[var\\(--spacing-1_5\\,_var\\(--spacing-2_5\\,_1rem\\)\\)\\] {
         margin-left: var(--spacing-1_5, var(--spacing-2_5, 1rem));
       }

packages/tailwindcss/src/utils/decode-arbitrary-value.test.ts

@@ -29,6 +29,14 @@ describe('decoding arbitrary values', () => {
     )
   })
 
+  it('should not replace underscores in the first argument of theme()', () => {
+    expect(decodeArbitraryValue('theme(--spacing-1_5)')).toBe('theme(--spacing-1_5)')
+    expect(decodeArbitraryValue('theme(--spacing-1_5,_1rem)')).toBe('theme(--spacing-1_5, 1rem)')
+    expect(decodeArbitraryValue('theme(--spacing-1_5,_theme(--spacing-2_5,_1rem))')).toBe(
+      'theme(--spacing-1_5, theme(--spacing-2_5, 1rem))',
+    )
+  })
+
   it('should leave var(…) as is', () => {
     expect(decodeArbitraryValue('var(--foo)')).toBe('var(--foo)')
     expect(decodeArbitraryValue('var(--headings-h1-size)')).toBe('var(--headings-h1-size)')

packages/tailwindcss/src/utils/decode-arbitrary-value.ts

@@ -55,7 +55,12 @@ function recursivelyDecodeArbitraryValues(ast: ValueParser.ValueAstNode[]) {
           break
         }
 
-        if (node.value === 'var' || node.value.endsWith('_var')) {
+        if (
+          node.value === 'var' ||
+          node.value.endsWith('_var') ||
+          node.value === 'theme' ||
+          node.value.endsWith('_theme')
+        ) {
           // Don't decode underscores in the first argument of var() but do
           // decode the function name
           node.value = convertUnderscoresToWhitespace(node.value)
@@ -78,11 +83,11 @@ function recursivelyDecodeArbitraryValues(ast: ValueParser.ValueAstNode[]) {
         break
       }
       default:
-        never()
+        never(node)
     }
   }
 }
 
-function never(): never {
-  throw new Error('This should never happen')
+function never(value: never): never {
+  throw new Error(`Unexpected value: ${value}`)
 }