Lazily evaluate values in the extend section

In a way this combines PRs tailwindlabs#655 and tailwindlabs#774.

Author: CvX

__tests__/resolveConfig.test.js

@@ -757,3 +757,64 @@ test('the theme function can use a default value if the key is missing', () => {
     },
   })
 })
+
+test('theme values in the extend section are lazily evaluated', () => {
+  const userConfig = {
+    theme: {
+      colors: {
+        red: 'red',
+        green: 'green',
+        blue: 'blue',
+      },
+      extend: {
+        borderColor: theme => ({
+          default: theme('colors.red'),
+        }),
+      },
+    },
+  }
+
+  const defaultConfig = {
+    prefix: '-',
+    important: false,
+    separator: ':',
+    theme: {
+      colors: {
+        cyan: 'cyan',
+        magenta: 'magenta',
+        yellow: 'yellow',
+      },
+      borderColor: theme => ({
+        default: theme('colors.yellow', 'currentColor'),
+        ...theme('colors'),
+      }),
+    },
+    variants: {
+      borderColor: ['responsive', 'hover', 'focus'],
+    },
+  }
+
+  const result = resolveConfig([userConfig, defaultConfig])
+
+  expect(result).toEqual({
+    prefix: '-',
+    important: false,
+    separator: ':',
+    theme: {
+      colors: {
+        red: 'red',
+        green: 'green',
+        blue: 'blue',
+      },
+      borderColor: {
+        default: 'red',
+        red: 'red',
+        green: 'green',
+        blue: 'blue',
+      },
+    },
+    variants: {
+      borderColor: ['responsive', 'hover', 'focus'],
+    },
+  })
+})

src/util/resolveConfig.js

@@ -17,15 +17,17 @@ function resolveFunctionKeys(object) {
 
 function mergeExtensions({ extend, ...theme }) {
   return mergeWith({}, theme, extend, (_, extensions, key) => {
-    return isFunction(theme[key])
-      ? mergedTheme => ({
-          ...theme[key](mergedTheme),
-          ...extensions,
-        })
-      : {
-          ...theme[key],
-          ...extensions,
-        }
+    if (isFunction(theme[key]) || (extend && isFunction(extend[key]))) {
+      return mergedTheme => ({
+        ...(isFunction(theme[key]) ? theme[key](mergedTheme) : theme[key]),
+        ...(extend && isFunction(extend[key]) ? extend[key](mergedTheme) : extensions),
+      })
+    } else {
+      return {
+        ...theme[key],
+        ...extensions,
+      }
+    }
   })
 }