checker: fix generics fn return generics fn type (fix #10085) (#10088)

vlib/v/ast/types.v

@@ -426,7 +426,7 @@ pub mut:
 }
 
 pub struct FnType {
-pub:
+pub mut:
 	is_anon  bool
 	has_decl bool
 	func     Fn

vlib/v/checker/check_types.v

@@ -508,6 +508,15 @@ pub fn (mut c Checker) infer_fn_generic_types(f ast.Fn, mut call_expr ast.CallEx
 
 			if param.typ.has_flag(.generic) && param_type_sym.name == gt_name {
 				to_set = c.table.mktyp(arg.typ)
+				mut sym := c.table.get_type_symbol(arg.typ)
+				if mut sym.info is ast.FnType {
+					if !sym.info.is_anon {
+						sym.info.func.name = ''
+						idx := c.table.find_or_register_fn_type(c.mod, sym.info.func,
+							true, false)
+						to_set = ast.new_type(idx).derive(arg.typ)
+					}
+				}
 				if arg.expr.is_auto_deref_var() {
 					to_set = to_set.deref()
 				}

vlib/v/tests/generics_with_generics_fn_return_generics_fn_type_test.v

@@ -0,0 +1,60 @@
+fn neg(a int) int {
+	return -a
+}
+
+fn normal_v1(func fn (int) int) fn (int) int {
+	assert typeof(func).name == typeof(neg).name
+	return func
+}
+
+fn normal_v2(func fn (int) int) fn (int) int {
+	f := func
+	assert typeof(f).name == typeof(neg).name
+	return f
+}
+
+fn generic_v1<T>(func T) T {
+	assert T.name == typeof(neg).name
+	assert typeof(func).name == typeof(neg).name
+	return func
+}
+
+fn generic_v2<T>(func T) T {
+	assert T.name == typeof(neg).name
+	f := func
+	assert typeof(f).name == typeof(neg).name
+	return f
+}
+
+fn mixed_v1<T>(func T) fn (int) int {
+	assert T.name == typeof(neg).name
+	assert typeof(func).name == typeof(neg).name
+	return func
+}
+
+fn mixed_v2<T>(func T) fn (int) int {
+	assert T.name == typeof(neg).name
+	f := func
+	assert typeof(f).name == typeof(neg).name
+	return f
+}
+
+fn test_generics_with_generics_fn_return_type() {
+	mut f := neg
+	assert f(1) == -1
+
+	f = normal_v1(neg)
+	assert f(2) == -2
+	f = normal_v2(neg)
+	assert f(3) == -3
+
+	f = generic_v1(neg)
+	assert f(4) == -4
+	f = generic_v2(neg)
+	assert f(5) == -5
+
+	f = mixed_v1(neg)
+	assert f(6) == -6
+	f = mixed_v2(neg)
+	assert f(7) == -7
+}