cgen, coroutines: fix go expr handle referencing undeclared thread__tN (#27379)

examples/coroutines/go_handle.v

@@ -0,0 +1,24 @@
+// vtest build: false // This should be built with: `v -use-coroutines go_handle.v`
+// Note: the Photon wrapper is not yet trivial enough to build/install on the CI.
+//
+// Demonstrates storing the handle returned by a `go` expression and waiting on
+// it. Because the Photon coroutine wrapper exposes no joinable handle, a `go`
+// whose handle is used is lowered to the regular `spawn` (pthread) path, so the
+// handle can be waited on just like `spawn`.
+module main
+
+struct App {
+mut:
+	worker thread
+}
+
+fn (a App) loop() {
+	println('hello from the worker coroutine')
+}
+
+fn main() {
+	mut a := App{}
+	a.worker = go a.loop()
+	a.worker.wait()
+	println('done')
+}

vlib/v/gen/c/spawn_and_go.v

@@ -15,8 +15,19 @@ fn (mut g Gen) spawn_and_go_expr(node ast.SpawnExpr, mode SpawnGoMode) {
 	if node.call_expr.should_be_skipped {
 		return
 	}
-	is_spawn := mode == .spawn_
-	is_go := mode == .go_
+	mut is_spawn := mode == .spawn_
+	mut is_go := mode == .go_
+	if is_go && node.is_expr {
+		// A `go expr` whose handle is used as a value (e.g. `h := go f()` or
+		// `obj.field = go f()`) needs a real, joinable thread handle. The photon
+		// coroutine wrapper (`photon_thread_create*`) returns void and exposes no
+		// such handle, so fall back to the regular `spawn` (pthread) path here,
+		// which declares and assigns the `thread_<tmp>` handle that is read back
+		// as the expression's value. Plain statement form `go f()` (no handle
+		// used) keeps using the photon work-pool path below.
+		is_spawn = true
+		is_go = false
+	}
 	if is_spawn {
 		g.writeln('/*spawn (thread) */')
 	} else {

vlib/v/markused/walker.v

@@ -1079,6 +1079,20 @@ fn (mut w Walker) expr(node_ ast.Expr) {
 		ast.GoExpr {
 			if node.is_expr {
 				w.fn_by_name('free')
+				// A `go expr` whose handle is used as a value is lowered to the
+				// `spawn` (pthread) codegen path (see spawn_and_go_expr), since the
+				// photon coroutine wrapper returns no joinable handle. Mark the same
+				// thread-handle type and pthread error helpers that `spawn` needs,
+				// otherwise -skip-unused prunes them and the generated C fails to link.
+				w.mark_by_type(w.table.find_or_register_thread(node.call_expr.return_type))
+				w.uses_spawn = true
+				if w.pref.os == .windows {
+					w.fn_by_name('panic_lasterr')
+					w.fn_by_name('winapi_lasterr_str')
+				} else {
+					w.fn_by_name('c_error_number_str')
+					w.fn_by_name('panic_error_number')
+				}
 			}
 			w.mark_by_type(node.call_expr.return_type)
 			w.expr(node.call_expr)