cgen: fix error for generics struct that parent has str to string (#14037)

vlib/v/gen/c/auto_str_methods.v

@@ -896,7 +896,12 @@ fn (mut g Gen) gen_str_for_struct(info ast.Struct, styp string, str_fn_name stri
 		sftyp := g.typ(field.typ)
 		mut field_styp := sftyp.replace('*', '')
 		field_styp_fn_name := if sym_has_str_method {
-			'${field_styp}_str'
+			mut field_fn_name := '${field_styp}_str'
+			if sym.info is ast.Struct {
+				field_fn_name = g.generic_fn_name(sym.info.concrete_types, field_fn_name,
+					false)
+			}
+			field_fn_name
 		} else {
 			g.get_str_fn(field.typ)
 		}

vlib/v/tests/generics_struct_parent_has_str_to_string_test.v

@@ -0,0 +1,36 @@
+import datatypes
+
+const (
+	w = 64
+	h = 32
+)
+
+interface Display {
+mut:
+	pixel(x int, y int, val bool)
+	clear()
+	refresh()
+}
+
+struct Vm {
+mut:
+	display Display
+	stack   datatypes.Stack<u16>
+}
+
+struct Pattern {
+	pattern []byte
+	handler fn (mut m Vm)
+}
+
+fn new_pattern(pattern string, handler fn (mut m Vm)) Pattern {
+	return Pattern{pattern.runes().map(byte('0x$it'.int())), handler}
+}
+
+fn test_generics_struct_parent_has_str_to_string() {
+	p := new_pattern('00E0', fn (mut m Vm) {
+		println(m)
+	})
+	println(p)
+	assert true
+}