checker, cgen: fix generic method variable (#18509)

vlib/v/checker/checker.v

@@ -1484,7 +1484,7 @@ fn (mut c Checker) selector_expr(mut node ast.SelectorExpr) ast.Type {
 		}
 		return field.typ
 	}
-	if mut method := c.table.find_method(sym, field_name) {
+	if mut method := sym.find_method_with_generic_parent(field_name) {
 		if c.expected_type != 0 && c.expected_type != ast.none_type {
 			fn_type := ast.new_type(c.table.find_or_register_fn_type(method, false, true))
 			// if the expected type includes the receiver, don't hide it behind a closure

vlib/v/gen/c/cgen.v

@@ -3619,7 +3619,7 @@ fn (mut g Gen) selector_expr(node ast.SelectorExpr) {
 				}
 			}
 		}
-	} else if m := g.table.find_method(sym, node.field_name) {
+	} else if m := sym.find_method_with_generic_parent(node.field_name) {
 		mut has_embeds := false
 		if sym.info in [ast.Struct, ast.Aggregate] {
 			if node.from_embed_types.len > 0 {
@@ -3651,7 +3651,14 @@ fn (mut g Gen) selector_expr(node ast.SelectorExpr) {
 			} else {
 				sb.write_string('\t')
 			}
-			sb.write_string('${expr_styp}_${m.name}(')
+			mut method_name := m.name
+			rec_sym := g.table.sym(receiver.typ)
+			if rec_sym.info is ast.Struct {
+				if rec_sym.info.concrete_types.len > 0 {
+					method_name = g.generic_fn_name(rec_sym.info.concrete_types, m.name)
+				}
+			}
+			sb.write_string('${expr_styp}_${method_name}(')
 			if !receiver.typ.is_ptr() {
 				sb.write_string('*')
 			}

vlib/v/tests/generics_method_variable_test.v

@@ -0,0 +1,21 @@
+type Func = fn (string) string
+
+struct Struct[T] {
+	a T
+mut:
+	func Func
+}
+
+fn (st Struct[T]) foo[T](s string) string {
+	println('${st.a} - ${s}')
+	return '${st.a} - ${s}'
+}
+
+fn test_generic_method_variable() {
+	mut st := Struct[int]{
+		a: 22
+	}
+	st.func = st.foo
+	ret := st.func('hello')
+	assert ret == '22 - hello'
+}