fix: error related to npm defaulting when only bun and yarn exist

Author: Strengthless

README.md

@@ -161,7 +161,8 @@ team.
 
   By default, patch-package checks whether you use npm, yarn or bun based on
   which lockfile you have. If you have multiple lockfiles, it uses npm by
-  default. Set this option to override that default and always use yarn.
+  default (in cases where npm is not available, it will resort to yarn). Set
+  this option to override that default and always use yarn.
 
 - `--use-bun`
 

src/detectPackageManager.ts

@@ -45,6 +45,18 @@ deleting the conflicting lockfile if you don't need it
   )
 }
 
+function printSelectingDefaultYarnMessage() {
+  console.info(
+    `${chalk.bold(
+      "patch-package",
+    )}: you have both yarn.lock and bun.lockb lockfiles
+Defaulting to using ${chalk.bold("yarn")}
+You can override this setting by passing --use-bun, or
+deleting yarn.lock if you don't need it
+`,
+  )
+}
+
 function checkForYarnOverride(overridePackageManager: PackageManager | null) {
   if (overridePackageManager === "yarn") {
     printNoYarnLockfileError()
@@ -86,6 +98,11 @@ export const detectPackageManager = (
     if (overridePackageManager) {
       return overridePackageManager
     }
+    if (!packageLockExists && !shrinkWrapExists) {
+      // The only case where we don't want to default to npm is when we have both yarn and bun lockfiles.
+      printSelectingDefaultYarnMessage()
+      return "yarn"
+    }
     printSelectingDefaultMessage()
     return shrinkWrapExists ? "npm-shrinkwrap" : "npm"
   } else if (packageLockExists || shrinkWrapExists) {

src/index.ts

@@ -200,7 +200,8 @@ Usage:
 
         By default, patch-package checks whether you use npm, yarn or bun based on
         which lockfile you have. If you have multiple lockfiles, it uses npm by
-        default. Set this option to override that default and always use yarn.
+        default (in cases where npm is not available, it will resort to yarn). Set 
+        this option to override that default and always use yarn.
         
     ${chalk.bold("--use-bun")}