cgen: fix selector code to use interface method table on closure when needed (#18736)

vlib/v/gen/c/cgen.v

@@ -3579,9 +3579,6 @@ fn (mut g Gen) selector_expr(node ast.SelectorExpr) {
 		opt_base_typ := g.base_type(node.expr_type)
 		g.write('(*(${opt_base_typ}*)')
 	}
-	if sym.kind in [.interface_, .sum_type] {
-		g.write('(*(')
-	}
 	if sym.kind == .array_fixed {
 		if node.field_name != 'len' {
 			g.error('field_name should be `len`', node.pos)
@@ -3601,6 +3598,9 @@ fn (mut g Gen) selector_expr(node ast.SelectorExpr) {
 	if f := g.table.find_field_with_embeds(sym, node.field_name) {
 		field_sym := g.table.sym(f.typ)
 		field_typ = f.typ
+		if sym.kind in [.interface_, .sum_type] {
+			g.write('(*(')
+		}
 		if field_sym.kind in [.sum_type, .interface_] {
 			if !prevent_sum_type_unwrapping_once {
 				// check first if field is sum type because scope searching is expensive
@@ -3675,10 +3675,16 @@ fn (mut g Gen) selector_expr(node ast.SelectorExpr) {
 					method_name = g.generic_fn_name(rec_sym.info.concrete_types, m.name)
 				}
 			}
+			if rec_sym.info is ast.Interface {
+				left_cc_type := g.cc_type(g.table.unaliased_type(receiver.typ), false)
+				left_type_name := util.no_dots(left_cc_type)
+				sb.write_string('${c_name(left_type_name)}_name_table[a0->_typ]._method_${method_name}(')
+			} else {
 				sb.write_string('${expr_styp}_${method_name}(')
 				if !receiver.typ.is_ptr() {
 					sb.write_string('*')
 				}
+			}
 			for i in 0 .. m.params.len {
 				if i != 0 {
 					sb.write_string(', ')
@@ -3705,6 +3711,10 @@ fn (mut g Gen) selector_expr(node ast.SelectorExpr) {
 			g.write(')')
 			return
 		}
+	} else {
+		if sym.kind in [.interface_, .sum_type] {
+			g.write('(*(')
+		}
 	}
 	// var?.field_opt
 	field_is_opt := (node.expr is ast.Ident && node.expr.is_auto_heap()

vlib/v/tests/interface_closure_test.v

@@ -0,0 +1,30 @@
+interface ITest {
+mut:
+	caller(a Test) !
+}
+
+struct Test {
+}
+
+struct Test2 {
+}
+
+fn (t2 Test2) with_reader(func fn (a Test) !) ! {
+	return func(Test{})
+}
+
+fn (t Test) caller(a Test) ! {
+	println('ok')
+}
+
+fn get() ITest {
+	return Test{}
+}
+
+fn test_main() {
+	mut a := get()
+
+	b := Test2{}
+	b.with_reader(a.caller)!
+	assert true
+}