cgen: correct function definitions for callbacks in imported modules (fix #25700) (#25719)

Author: dy-tea

vlib/v/gen/c/cgen.v

@@ -1980,7 +1980,7 @@ pub fn (mut g Gen) write_fn_typesymbol_declaration(sym ast.TypeSymbol) {
 			''
 		}
 		ret_typ :=
-			if !func.return_type.has_flag(.option) && g.table.sym(func.return_type).kind == .array_fixed { '_v_' } else { '' } +
+			if !func.return_type.has_flag(.option) && !func.return_type.has_flag(.result) && g.table.sym(func.return_type).kind == .array_fixed { '_v_' } else { '' } +
 			g.styp(func.return_type)
 		g.type_definitions.write_string('typedef ${ret_typ} (${msvc_call_conv}*${fn_name})(')
 		for i, param in func.params {

vlib/v/gen/c/fn.v

@@ -791,9 +791,9 @@ fn (mut g Gen) fn_decl_params(params []ast.Param, scope &ast.Scope, is_variadic
 			info := param_type_sym.info as ast.FnType
 			func := info.func
 			if !g.inside_c_extern {
-				g.write('${g.styp(func.return_type)} (*${caname})(')
+				g.write('${g.ret_styp(func.return_type)} (*${caname})(')
 			}
-			g.definitions.write_string('${g.styp(func.return_type)} (*${caname})(')
+			g.definitions.write_string('${g.ret_styp(func.return_type)} (*${caname})(')
 			g.fn_decl_params(func.params, unsafe { nil }, func.is_variadic, func.is_c_variadic)
 			if !g.inside_c_extern {
 				g.write(')')

vlib/v/tests/modules/callback_consumer_module/callback_consumer_test.v

@@ -0,0 +1,7 @@
+import mod1
+import mod2
+
+fn test_main() {
+	mod1.function()!
+	mod2.function()
+}

vlib/v/tests/modules/callback_consumer_module/main.v

@@ -0,0 +1,8 @@
+module main
+
+import mod1
+import mod2
+
+fn main() {
+	dump('main')
+}

vlib/v/tests/modules/callback_consumer_module/mod1/faulty.v

@@ -0,0 +1,15 @@
+module mod1
+
+pub fn function() ! {
+	callback_consumer(callback_fn)!
+}
+
+type CallbackType = fn () ![4]u8
+
+fn callback_consumer(callback CallbackType) ! {
+	callback()!
+}
+
+fn callback_fn() ![4]u8 {
+	return [u8(1), 2, 3, 255]!
+}

vlib/v/tests/modules/callback_consumer_module/mod2/faulty.v

@@ -0,0 +1,15 @@
+module mod2
+
+pub fn function() {
+	callback_consumer(callback_fn)
+}
+
+type CallbackType = fn () [4]u8
+
+fn callback_consumer(callback CallbackType) {
+	callback()
+}
+
+fn callback_fn() [4]u8 {
+	return [u8(1), 2, 3, 255]!
+}