Fix #18086: suppress NuGet restore stdout under FSI --quiet

docs/release-notes/.FSharp.Compiler.Service/11.0.100.md

@@ -57,6 +57,7 @@
 * Fix parser recovery, name resolution, and code completion for unfinished enum patterns ([PR #19708](https://github.com/dotnet/fsharp/pull/19708))
 * Parser: fix unexpected diagnostics in debug builds, improve error messages ([PR #19730](https://github.com/dotnet/fsharp/pull/19730))
 * Fix signature conformance: overloaded member with unit parameter `M(())` now matches sig `member M: unit -> unit`. ([Issue #19596](https://github.com/dotnet/fsharp/issues/19596), [PR #19615](https://github.com/dotnet/fsharp/pull/19615))
+* Fix `--quiet` not suppressing NuGet restore output on stdout in F# Interactive ([Issue #18086](https://github.com/dotnet/fsharp/issues/18086))
 
 ### Added
 

src/Compiler/Interactive/fsi.fs

@@ -2819,8 +2819,15 @@ type internal FsiDynamicCompiler
 
                             if result.Success then
 
+                                // Issue #18086: under --quiet (tcConfigB.noFeedback), NuGet/MSBuild
+                                // restore chatter and warnings must not pollute stdout. Route what
+                                // would have gone to stdout to stderr instead so the information is
+                                // still discoverable for users who redirect stderr.
+                                let stdOutSink: System.IO.TextWriter =
+                                    if tcConfigB.noFeedback then Console.Error else Console.Out
+
                                 for line in result.StdOut do
-                                    Console.Out.WriteLine(line)
+                                    stdOutSink.WriteLine(line)
 
                                 for line in result.StdError do
                                     Console.Error.WriteLine(line)

tests/FSharp.Compiler.ComponentTests/CompilerOptions/fsi/FsiCliTests.fs

@@ -67,3 +67,47 @@ module FsiCliTests =
         let result = runFsiProcess [option]
         Assert.NotEqual(0, result.ExitCode)
         Assert.Contains(expectedError, result.StdErr)
+
+    // ============================================================================
+    // Issue #18086: --quiet must suppress NuGet restore stdout chatter
+    // ============================================================================
+
+    let private writeTempScript (content: string) : string =
+        let path = System.IO.Path.Combine(System.IO.Path.GetTempPath(), $"fsi_quiet_{System.Guid.NewGuid():N}.fsx")
+        System.IO.File.WriteAllText(path, content)
+        path
+
+    let private runFsiScript (extraArgs: string list) (scriptBody: string) =
+        let scriptPath = writeTempScript scriptBody
+        try
+            let result = runFsiProcess (extraArgs @ [scriptPath])
+            result
+        finally
+            try System.IO.File.Delete(scriptPath) with _ -> ()
+
+    [<Fact>]
+    let ``FSI quiet mode suppresses NuGet restore output from stdout`` () =
+        let script = """
+#r "nuget: Newtonsoft.Json, 13.0.3"
+printfn "RESULT_MARKER_18086"
+"""
+        let result = runFsiScript ["--quiet"] script
+        Assert.Contains("RESULT_MARKER_18086", result.StdOut)
+        Assert.DoesNotContain("Determining projects to restore", result.StdOut)
+        Assert.DoesNotContain("Restored ", result.StdOut)
+        Assert.DoesNotContain("NU1", result.StdOut)
+
+    [<Fact>]
+    let ``FSI default (non-quiet) mode still evaluates script and prints user output`` () =
+        let script = """
+#r "nuget: Newtonsoft.Json, 13.0.3"
+printfn "RESULT_MARKER_18086_DEFAULT"
+"""
+        let result = runFsiScript [] script
+        Assert.Contains("RESULT_MARKER_18086_DEFAULT", result.StdOut)
+
+    [<Fact>]
+    let ``FSI quiet mode still prints user printfn output to stdout`` () =
+        let script = """printfn "hello from quiet script" """
+        let result = runFsiScript ["--quiet"] script
+        Assert.Contains("hello from quiet script", result.StdOut)