cgen: fix generic return assign to ComptimeSelector (fix #25125) (#25131)

vlib/v/gen/c/assign.v

@@ -442,8 +442,9 @@ fn (mut g Gen) assign_stmt(node_ ast.AssignStmt) {
 				if val.typ_key != '' {
 					val_type = g.type_resolver.get_ct_type_or_default(val.typ_key, var_type)
 				}
-			} else if val is ast.CallExpr {
+			} else if val is ast.CallExpr && val.return_type_generic.has_flag(.generic) {
 				g.assign_ct_type = g.comptime.comptime_for_field_type
+				val_type = var_type
 			}
 		} else if mut left is ast.IndexExpr && val is ast.ComptimeSelector {
 			if val.typ_key != '' {

vlib/v/tests/comptime/comptime_selector_assign_test.v

@@ -0,0 +1,25 @@
+module main
+
+struct AnyStruct[T] {
+	val T
+}
+
+fn decode_struct[T]() T {
+	mut typ := T{}
+	$for field in T.fields {
+		typ.$(field.name) = decode_field(typ.$(field.name))
+	}
+	return typ
+}
+
+fn decode_field[T](_ T) T {
+	mut field := T{}
+	return field
+}
+
+type Any = int | string | []Any
+
+fn test_main() {
+	decode_struct[AnyStruct[Any]]()
+	decode_struct[AnyStruct[[]Any]]()
+}