cgen: dereference in one place and fix

vlib/v/gen/cgen.v

@@ -796,6 +796,20 @@ fn (mut g Gen) expr_with_cast(expr ast.Expr, got_type, expected_type table.Type)
 			}
 		}
 	}
+	// Generic dereferencing logic
+	expected_sym := g.table.get_type_symbol(expected_type)
+	got_is_ptr := got_type.is_ptr()
+	expected_is_ptr := expected_type.is_ptr()
+	neither_void := table.voidptr_type !in [got_type, expected_type]
+	if got_is_ptr && !expected_is_ptr && neither_void && expected_sym.kind !in [.interface_, .placeholder] {
+		got_deref_type := got_type.deref()
+		deref_sym := g.table.get_type_symbol(got_deref_type)
+		deref_will_match := expected_type in [got_type, got_deref_type, deref_sym.parent_idx]
+		got_is_opt := got_type.flag_is(.optional)
+		if deref_will_match || got_is_opt  {
+			g.write('*')
+		}
+	}
 	// no cast
 	g.expr(expr)
 }
@@ -2070,10 +2084,6 @@ fn (mut g Gen) return_statement(node ast.Return) {
 			g.writeln(' }, sizeof($styp));')
 			return
 		}
-		if !g.fn_decl.return_type.is_ptr() && node.types[0].is_ptr() {
-			// Automatic Dereference
-			g.write('*')
-		}
 		g.expr_with_cast(node.exprs[0], node.types[0], g.fn_decl.return_type)
 	}
 	g.writeln(';')

vlib/v/gen/fn.v

@@ -618,9 +618,6 @@ fn (mut g Gen) ref_or_deref_arg(arg ast.CallArg, expected_type table.Type) {
 		if !g.is_json_fn {
 			g.write('&/*qq*/')
 		}
-	} else if !arg_is_ptr && expr_is_ptr && exp_sym.kind != .interface_ {
-		// Dereference a pointer if a value is required
-		g.write('*/*d*/')
 	}
 	g.expr_with_cast(arg.expr, arg.typ, expected_type)
 }

vlib/v/tests/sum_type_test.v

@@ -51,3 +51,37 @@ fn test_assignment_and_push() {
 		else {}
 	}
 }
+
+// Test moving structs between master/sub arrays
+
+type Master = Sub1 | Sub2
+struct Sub1 {
+mut:
+	val int
+	name string
+}
+struct Sub2 {
+	name string
+	val int
+}
+
+fn test_converting_down() {
+	mut out := []Master{}
+	out << Sub1 { val: 1, name: 'one' }
+	out << Sub2 { val: 2, name: 'two'}
+	out << Sub2 { val: 3, name: 'three'}
+
+	mut res := []Sub2{cap: out.len}
+	for d in out {
+		match d {
+			Sub2 { res << it }
+			else {}
+		}
+	}
+
+	assert res[0].val == 2
+	assert res[0].name == 'two'
+	assert res[1].val == 3
+	assert res[1].name == 'three'
+}
+