Leave the load scripts to the restore after paket update --no-install

RELEASE_NOTES.md

@@ -1,6 +1,7 @@
 #### Unreleased
 
 * `paket pack --symbols` no longer fails on a project with Compile entries without an `Include` or `Update` attribute, such as `<Compile Remove="..." />` or a Compile item definition in an `ItemDefinitionGroup`, and leaves out of the symbols package the files that a Remove names without wildcards - https://github.com/fsprojects/Paket/issues/4222
+* `paket update --no-install` no longer fails when a group has `generate_load_scripts: true` and the packages of the new paket.lock aren't all restored, as on a fresh checkout or for a package that the update adds: it leaves the load scripts to the next restore, which downloads the packages that they load - https://github.com/fsprojects/Paket/issues/2785
 
 #### 12.0.0-alpha005 - 2026-11-07
 

integrationtests/Paket.IntegrationTests/LoadingScriptGenerationTests.fs

@@ -268,6 +268,24 @@ let ``#3238 restore regenerates load scripts when .paket/load folder is missing`
 
     nHibernate35Expectations |> assertExpectations scenario ExpectationType.ShouldContain
 
+[<Test; Category("scriptgen")>]
+let ``#2785 update --no-install leaves the load scripts to the restore`` () =
+    // generate_load_scripts: true, and nothing restored yet
+    let scenario = "issue-2939"
+    let cleanup, output = paket "update --no-install" scenario
+    use __ = cleanup
+    StringAssert.Contains("The load scripts will be generated by the next paket restore", output)
+
+    let tempPath = scenarioTempPath scenario
+    Assert.IsTrue(File.Exists(Path.Combine(tempPath, "paket.lock")), "update should write paket.lock")
+    Assert.IsFalse(Directory.Exists(Path.Combine(tempPath, "packages", "Newtonsoft.Json")), "--no-install should not download the packages")
+    Assert.IsFalse((scriptRoot scenario).Exists, "--no-install should not generate the load scripts")
+
+    directPaket "restore" scenario |> ignore<string>
+
+    let newtonsoftScript = Path.Combine((scriptRoot scenario).FullName, "Newtonsoft.Json.fsx")
+    Assert.IsTrue(File.Exists newtonsoftScript, "restore should generate the load scripts")
+
 [<Test; Category("scriptgen dependencies")>]
 let ``issue 2156 netstandard`` () =
     let scenario = "issue-2156"

src/Paket.Core/Installation/UpdateProcess.fs

@@ -297,7 +297,12 @@ let SmartInstall(dependenciesFile:DependenciesFile, updateMode, options : Update
         |> Seq.tryHead
         |> Option.isSome
 
-    if shouldGenerateScripts then
+    // --no-install doesn't download the packages that the scripts load. When the lock file changes,
+    // the restore cache no longer matches it, so the next restore downloads them and generates the scripts (#2785)
+    if shouldGenerateScripts && options.NoInstall then
+        if hasChanged then
+            tracefn "The load scripts will be generated by the next paket restore, which downloads the packages."
+    elif shouldGenerateScripts then
         let groupsToGenerate =
           if options.Common.GenerateLoadScripts then [] else
           dependenciesFile.Groups