cgen: fix using reference of sumtype or map as struct field (fix: #15827) (#15828)

vlib/v/gen/c/auto_str_methods.v

@@ -921,8 +921,8 @@ fn (mut g Gen) gen_str_for_struct(info ast.Struct, styp string, str_fn_name stri
 			funcprefix += 'isnil(it.${c_name(field.name)})'
 			funcprefix += ' ? _SLIT("nil") : '
 			// struct, floats and ints have a special case through the _str function
-			if sym.kind !in [.struct_, .alias, .enum_, .interface_] && !field.typ.is_int_valptr()
+			if sym.kind !in [.struct_, .alias, .enum_, .sum_type, .map, .interface_]
-				&& !field.typ.is_float_valptr() {
+				&& !field.typ.is_int_valptr() && !field.typ.is_float_valptr() {
 				funcprefix += '*'
 			}
 		}

vlib/v/gen/c/testdata/ref_sumtype_as_struct_field.out

@@ -0,0 +1,3 @@
+SumTypePtr{
+    ptr: &nil
+}

vlib/v/gen/c/testdata/ref_sumtype_as_struct_field.vv

@@ -0,0 +1,14 @@
+struct S1 {}
+
+struct S2 {}
+
+type T1 = S1 | S2
+
+struct SumTypePtr {
+	ptr &T1 = unsafe { nil }
+}
+
+fn main() {
+	ptr := SumTypePtr{}
+	println(ptr)
+}