parser: fixed struct methods and receiver use different generic names…

… than the generic names defined (fix #20365 #20363) (#20397)

vlib/v/parser/fn.v

@@ -375,11 +375,16 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 	if is_method && rec.typ.has_flag(.generic) {
 		sym := p.table.sym(rec.typ)
 		if sym.info is ast.Struct {
+			decl_generic_names := p.types_to_names(sym.info.generic_types, p.tok.pos(),
+				'sym.info.generic_types') or { return ast.FnDecl{
+				scope: unsafe { nil }
+			} }
 			fn_generic_names := generic_names.clone()
-			generic_names = p.types_to_names(sym.info.generic_types, p.tok.pos(), 'sym.info.generic_types') or {
-				return ast.FnDecl{
-					scope: unsafe { nil }
-				}
+			generic_names = p.table.generic_type_names(rec.typ)
+			if decl_generic_names.len != generic_names.len {
+				plural := if decl_generic_names.len == 1 { '' } else { 's' }
+				p.error_with_pos('expected ${decl_generic_names.len} generic parameter${plural}, got ${generic_names.len}',
+					rec.type_pos)
 			}
 			for gname in fn_generic_names {
 				if gname !in generic_names {

vlib/v/tests/generics_method_with_diff_generic_names_test.v

@@ -0,0 +1,34 @@
+// for issue: 20363
+// The generic name used by the methods and receiver is different from the generic name defined by the struct.
+struct Foo[T] {
+	field T
+}
+
+fn new_foo[U](arg U) Foo[U] {
+	return Foo[U]{arg}
+}
+
+fn test_using_diff_generic_names() {
+	foo := new_foo(1)
+	foo.method_1('hello')
+	foo.method_2('hello')
+}
+
+fn (f Foo[U]) method_1(arg string) {
+	assert arg == 'hello'
+}
+
+fn (f Foo[U]) method_2[U](arg string) {
+	assert arg == 'hello'
+}
+
+// for issue: 20365
+// The generic name used by the `str()` method and receiver is different from the generic name defined by the struct.
+fn (f Foo[U]) str[U]() string {
+	return 'hello'
+}
+
+fn test_str_override() {
+	foo := new_foo(1)
+	assert '${foo}' == 'hello'
+}