From 804c4362c1048e7ab49c26b2fbfa1a214d2e1a84 Mon Sep 17 00:00:00 2001
From: Felipe Pena <felipensp@gmail.com>
Date: Sat, 16 Aug 2025 05:40:34 -0300
Subject: [PATCH] resolver: fix generic selector field type resolver (fix
 #24570) (#25112)

---
 .../generics/generic_selector_field_test.v    | 29 +++++++++++++++++++
 vlib/v/type_resolver/generic_resolver.v       | 13 +++++++++
 2 files changed, 42 insertions(+)
 create mode 100644 vlib/v/tests/generics/generic_selector_field_test.v

diff --git a/vlib/v/tests/generics/generic_selector_field_test.v b/vlib/v/tests/generics/generic_selector_field_test.v
new file mode 100644
index 000000000..21526dc39
--- /dev/null
+++ b/vlib/v/tests/generics/generic_selector_field_test.v
@@ -0,0 +1,29 @@
+module main
+
+struct Demo[T] {
+mut:
+	val T
+}
+
+type DemoType = int | []DemoType
+
+fn test_main() {
+	assert decode[Demo[[]DemoType]]() == Demo[[]DemoType]{}
+	assert decode[Demo[DemoType]]() == Demo[DemoType]{}
+}
+
+fn decode[T]() T {
+	mut typ := T{}
+	typ.val = decode_x(typ.val)
+	return typ
+}
+
+fn decode_x[T](_ T) T {
+	mut field := T{}
+	$if T is int {
+		field = 0
+	} $else $if T is $array {
+		field = []DemoType{}
+	}
+	return field
+}
diff --git a/vlib/v/type_resolver/generic_resolver.v b/vlib/v/type_resolver/generic_resolver.v
index 212c0979c..b7b1c657d 100644
--- a/vlib/v/type_resolver/generic_resolver.v
+++ b/vlib/v/type_resolver/generic_resolver.v
@@ -342,6 +342,19 @@ pub fn (mut t TypeResolver) resolve_args(cur_fn &ast.FnDecl, func &ast.Fn, mut n
 			} else {
 				comptime_args[k] = ctyp
 			}
+		} else if mut call_arg.expr is ast.SelectorExpr
+			&& call_arg.expr.expr_type.has_flag(.generic) && call_arg.expr.expr is ast.Ident {
+			mut ctyp := t.typeof_type(call_arg.expr, call_arg.expr.typ)
+			param_typ_sym := t.table.sym(param_typ)
+			cparam_type_sym := t.table.sym(ctyp)
+			if param_typ_sym.kind == .array && cparam_type_sym.info is ast.Array {
+				comptime_args[k] = cparam_type_sym.info.elem_type
+			} else if param_typ_sym.kind == .map && cparam_type_sym.info is ast.Map {
+				comptime_args[k] = cparam_type_sym.info.key_type
+				comptime_args[k + 1] = cparam_type_sym.info.value_type
+			} else {
+				comptime_args[k] = ctyp
+			}
 		}
 	}
 	return comptime_args
-- 
2.39.5