cmd/go: maintain toolchain for godebug default in go mod tidy

src/cmd/go/internal/modload/init.go

@@ -1525,13 +1525,39 @@ func rootsFromModFile(ld *Loader, m module.Version, modFile *modfile.File, addTo
 	}
 	goVersion := gover.FromGoMod(modFile)
 	var toolchain string
-	if addToolchainRoot && modFile.Toolchain != nil {
-		toolchain = modFile.Toolchain.Name
+	if addToolchainRoot {
+		toolchain = toolchainForModFile(modFile)
 	}
 	roots = appendGoAndToolchainRoots(roots, goVersion, toolchain, direct)
 	return roots, direct
 }
 
+func toolchainForModFile(modFile *modfile.File) string {
+	toolchain := ""
+	if modFile.Toolchain != nil {
+		toolchain = modFile.Toolchain.Name
+	}
+
+	required := godebugDefaultToolchain(modFile.Godebug)
+	if required == "" || toolchain == "default" {
+		return toolchain
+	}
+	if toolchain == "" || gover.Compare(gover.FromToolchain(toolchain), gover.FromToolchain(required)) < 0 {
+		return required
+	}
+	return toolchain
+}
+
+func godebugDefaultToolchain(godebugs []*modfile.Godebug) string {
+	toolchain := ""
+	for _, g := range godebugs {
+		if g.Key == "default" && strings.HasPrefix(g.Value, "go") {
+			toolchain = g.Value
+		}
+	}
+	return toolchain
+}
+
 func appendGoAndToolchainRoots(roots []module.Version, goVersion, toolchain string, direct map[string]bool) []module.Version {
 	// Add explicit go and toolchain versions, inferring as needed.
 	roots = append(roots, module.Version{Path: "go", Version: goVersion})

src/cmd/go/testdata/script/mod_tidy_godebug_toolchain.txt

@@ -0,0 +1,13 @@
+env TESTGO_VERSION=go1.26.0
+env TESTGO_VERSION_SWITCH=switch
+env GOTOOLCHAIN=auto
+
+go mod init m
+go get go@1.25.0
+go mod edit -godebug default=go1.26.0
+go mod tidy
+grep 'toolchain go1.26.0' go.mod
+
+env TESTGO_VERSION=go1.25.0
+go list -m
+stdout '^m$'