cmd/go: disable workspace mode in GOPATH mode

Workspace mode is specifically for working with modules;
it doesn't make sense in GOPATH mode.

This also fixes a panic in (*modload.MainModuleSet).GoVersion
when go.work is present in GOPATH mode.

For #58767.

Change-Id: Ic6924352afb486fecc18e009e6b517f078e81094
Reviewed-on: https://go-review.googlesource.com/c/go/+/471600
Auto-Submit: Bryan Mills <[email protected]>
Run-TryBot: Bryan Mills <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: David Chase <[email protected]>

Author: Bryan C. Mills

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

@@ -388,8 +388,8 @@ func Init() {
 			base.Fatalf("go: -modfile cannot be used with commands that ignore the current module")
 		}
 		modRoots = nil
-	} else if inWorkspaceMode() {
-		// We're in workspace mode.
+	} else if workFilePath != "" {
+		// We're in workspace mode, which implies module mode.
 	} else {
 		if modRoot := findModuleRoot(base.Cwd()); modRoot == "" {
 			if cfg.ModFile != "" {
@@ -496,6 +496,9 @@ func inWorkspaceMode() bool {
 	if !initialized {
 		panic("inWorkspaceMode called before modload.Init called")
 	}
+	if !Enabled() {
+		return false
+	}
 	return workFilePath != ""
 }
 

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

@@ -0,0 +1,13 @@
+# Regression test for https://go.dev/issue/58767:
+# with an empty go.work file in GOPATH mode, calls to load.defaultGODEBUG for a
+# package named "main" panicked in modload.MainModules.GoVersion.
+
+env GO111MODULE=off
+cd example
+go list example/m
+
+-- example/go.work --
+go 1.21
+-- example/m/main.go --
+package main
+func main() {}