cgen, checker: fix array fixed return type for interface methods (#22320)

vlib/v/checker/interface.v

@@ -141,6 +141,16 @@ fn (mut c Checker) interface_decl(mut node ast.InterfaceDecl) {
 						}
 					}
 				}
+			} else if !method.return_type.has_option_or_result() {
+				ret_sym := c.table.sym(method.return_type)
+				if ret_sym.info is ast.ArrayFixed && !ret_sym.info.is_fn_ret {
+					c.cast_to_fixed_array_ret(method.return_type, ret_sym)
+				} else if ret_sym.info is ast.Alias {
+					parent_sym := c.table.sym(ret_sym.info.parent_type)
+					if parent_sym.info is ast.ArrayFixed && !parent_sym.info.is_fn_ret {
+						c.cast_to_fixed_array_ret(ret_sym.info.parent_type, parent_sym)
+					}
+				}
 			}
 			for j, param in method.params {
 				if j == 0 && is_js {

vlib/v/gen/c/cgen.v

@@ -5473,12 +5473,7 @@ fn (mut g Gen) return_stmt(node ast.Return) {
 	}
 	tmpvar := g.new_tmp_var()
 	g.defer_return_tmp_var = tmpvar
-	mut ret_typ := g.typ(g.unwrap_generic(fn_ret_type))
+	ret_typ := g.ret_typ(g.unwrap_generic(fn_ret_type))
-	if fn_ret_type.has_flag(.generic) && fn_return_is_fixed_array {
-		ret_typ = '_v_${ret_typ}'
-	} else if sym.kind == .alias && fn_return_is_fixed_array {
-		ret_typ = '_v_' + g.typ((sym.info as ast.Alias).parent_type)
-	}
 	if node.exprs.len == 1 {
 		// `return fn_call_opt()`
 		if (fn_return_is_option || fn_return_is_result) && node.exprs[0] is ast.CallExpr
@@ -7661,7 +7656,7 @@ fn (mut g Gen) interface_table() string {
 		for k, method_name in inter_methods {
 			method := isym.find_method_with_generic_parent(method_name) or { continue }
 			methodidx[method.name] = k
-			ret_styp := g.typ(method.return_type)
+			ret_styp := g.ret_typ(method.return_type)
 			methods_struct_def.write_string('\t${ret_styp} (*_method_${c_fn_name(method.name)})(void* _')
 			// the first param is the receiver, it's handled by `void*` above
 			for i in 1 .. method.params.len {
@@ -7847,7 +7842,7 @@ static inline __shared__${interface_name} ${shared_fn_name}(__shared__${cctype}*
 					method_call = '${cctype}_${name}'
 					// inline void Cat_speak_Interface_Animal_method_wrapper(Cat c) { return Cat_speak(*c); }
 					iwpostfix := '_Interface_${interface_name}_method_wrapper'
-					methods_wrapper.write_string('static inline ${g.typ(method.return_type)} ${cctype}_${name}${iwpostfix}(')
+					methods_wrapper.write_string('static inline ${g.ret_typ(method.return_type)} ${cctype}_${name}${iwpostfix}(')
 					params_start_pos := g.out.len
 					mut params := method.params.clone()
 					// hack to mutate typ
@@ -8036,6 +8031,23 @@ fn (mut g Gen) trace_last_lines(fbase string, params TraceLastLinesParams) {
 	println('`'.repeat(80))
 }
 
+// ret_type generates proper type name for return type context
+pub fn (mut g Gen) ret_typ(typ ast.Type) string {
+	mut ret_styp := g.typ(typ)
+	if !typ.has_option_or_result() {
+		ret_sym := g.table.sym(typ)
+		if ret_sym.info is ast.ArrayFixed && !ret_sym.info.is_fn_ret {
+			ret_styp = '_v_${ret_styp}'
+		} else if ret_sym.info is ast.Alias {
+			unalias_sym := g.table.sym(ret_sym.info.parent_type)
+			if unalias_sym.info is ast.ArrayFixed && !unalias_sym.info.is_fn_ret {
+				ret_styp = '_v_${g.typ(ret_sym.info.parent_type)}'
+			}
+		}
+	}
+	return ret_styp
+}
+
 pub fn (mut g Gen) get_array_depth(el_typ ast.Type) int {
 	typ := g.unwrap_generic(el_typ)
 	sym := g.table.final_sym(typ)

vlib/v/gen/c/fn.v

@@ -225,21 +225,7 @@ fn (mut g Gen) gen_fn_decl(node &ast.FnDecl, skip bool) {
 	}
 
 	mut name := g.c_fn_name(node)
-	mut type_name := g.typ(g.unwrap_generic(node.return_type))
+	type_name := g.ret_typ(g.unwrap_generic(node.return_type))
-
-	ret_sym := g.table.sym(g.unwrap_generic(node.return_type))
-	if node.return_type.has_flag(.generic) && !node.return_type.has_option_or_result()
-		&& ret_sym.kind == .array_fixed {
-		type_name = '_v_${type_name}'
-	} else if ret_sym.kind == .alias && !node.return_type.has_option_or_result() {
-		unalias_typ := g.table.unaliased_type(node.return_type)
-		unalias_sym := g.table.sym(unalias_typ)
-		if unalias_sym.kind == .array_fixed {
-			type_name = if !(unalias_sym.info as ast.ArrayFixed).is_fn_ret { '_v_' } else { '' } +
-				g.typ(unalias_typ)
-		}
-	}
-
 	if g.pref.obfuscate && g.cur_mod.name == 'main' && name.starts_with('main__') && !node.is_main
 		&& node.name != 'str' {
 		mut key := node.name

vlib/v/tests/fns/interface_methods_fixed_arr_test.v

@@ -0,0 +1,21 @@
+module main
+
+pub type Mat4 = [16]f32
+
+interface IGameObject {
+	world_transform() Mat4
+}
+
+struct Foo implements IGameObject {
+}
+
+fn (f Foo) world_transform() Mat4 {
+	return Mat4{}
+}
+
+fn test_main() {
+	t := Foo{}
+	a := t.world_transform()
+	b := Mat4{}
+	assert a == b
+}