Properly resolve theme('someKey.DEFAULT') when only --some-key-* keys exist

packages/tailwindcss/src/compat/plugin-functions.ts

@@ -115,7 +115,7 @@ function readFromCss(theme: Theme, path: string[]) {
   }
 
   // We have to turn the map into object-like structure for v3 compatibility
-  let obj = {}
+  let obj: Record<string, unknown> = {}
   let useNestedObjects = false // paths.some((path) => nestedKeys.has(path))
 
   for (let [key, value] of map) {
@@ -134,20 +134,19 @@ function readFromCss(theme: Theme, path: string[]) {
     set(obj, path, value)
   }
 
-  if ('DEFAULT' in obj) {
-    // The request looked like `theme('animation.DEFAULT')` and was turned into
-    // a lookup for `--animation-*` and we should extract the value for the
-    // `DEFAULT` key from the list of possible values
-    if (path[path.length - 1] === 'DEFAULT') {
-      return obj.DEFAULT
-    }
+  // If the request looked like `theme('animation.DEFAULT')` it would have been
+  // turned into a lookup for `--animation-*` so we should extract the value for
+  // the `DEFAULT` key from the list of possible values. If there is no
+  // `DEFAULT` in the list, there is no match so return `null`.
+  if (path[path.length - 1] === 'DEFAULT') {
+    return obj?.DEFAULT ?? null
+  }
 
-    // The request looked like `theme('animation.spin')` and was turned into a
-    // lookup for `--animation-spin-*` which had only one entry which means it
-    // should be returned directly
-    if (Object.keys(obj).length === 1) {
-      return obj.DEFAULT
-    }
+  // The request looked like `theme('animation.spin')` and was turned into a
+  // lookup for `--animation-spin-*` which had only one entry which means it
+  // should be returned directly.
+  if ('DEFAULT' in obj && Object.keys(obj).length === 1) {
+    return obj.DEFAULT
   }
 
   return obj

packages/tailwindcss/src/plugin-api.test.ts

@@ -804,6 +804,35 @@ describe('theme', async () => {
     expect(fn).toHaveBeenCalledWith('blue')
   })
 
+  test("`theme('*.DEFAULT')` resolves to `undefined` when all theme keys in that namespace have a suffix", async ({
+    expect,
+  }) => {
+    let input = css`
+      @tailwind utilities;
+      @plugin "my-plugin";
+      @theme {
+        --transition-timing-function-in: ease-in;
+        --transition-timing-function-out: ease-out;
+      }
+    `
+
+    let fn = vi.fn()
+
+    await compile(input, {
+      loadPlugin: async () => {
+        return plugin(({ theme }) => {
+          fn(theme('transitionTimingFunction.DEFAULT'))
+          fn(theme('transitionTimingFunction.in'))
+          fn(theme('transitionTimingFunction.out'))
+        })
+      },
+    })
+
+    expect(fn).toHaveBeenNthCalledWith(1, undefined)
+    expect(fn).toHaveBeenNthCalledWith(2, 'ease-in')
+    expect(fn).toHaveBeenNthCalledWith(3, 'ease-out')
+  })
+
   test('nested theme key lookups work even for flattened keys', async ({ expect }) => {
     let input = css`
       @tailwind utilities;