reflect: move indirect values into interface when setting interfaces

Fixes #4040

2 files changed, 29 insertions, 0 deletions

diff --git a/src/reflect/value.go b/src/reflect/value.go
index 1227ac422..16d2a17b5 100644
--- a/src/reflect/value.go
+++ b/src/reflect/value.go
@@ -1086,6 +1086,13 @@ func (v Value) Set(x Value) {
 	}
 
 	if v.typecode.Kind() == Interface && x.typecode.Kind() != Interface {
+		// move the value of x back into the interface, if possible
+		if x.isIndirect() && x.typecode.Size() <= unsafe.Sizeof(uintptr(0)) {
+			var value uintptr
+			memcpy(unsafe.Pointer(&value), x.value, x.typecode.Size())
+			x.value = unsafe.Pointer(value)
+		}
+
 		intf := composeInterface(unsafe.Pointer(x.typecode), x.value)
 		x = Value{
 			typecode: v.typecode,
diff --git a/src/reflect/value_test.go b/src/reflect/value_test.go
index 9be9789e1..6641750b9 100644
--- a/src/reflect/value_test.go
+++ b/src/reflect/value_test.go
@@ -624,6 +624,28 @@ func TestConvert(t *testing.T) {
 	}
 }
 
+func TestIssue4040(t *testing.T) {
+	var value interface{} = uint16(0)
+
+	// get the pointer to the interface value
+	inPtr := ValueOf(&value)
+
+	// dereference to get the actual value (an interface)
+	inElem := inPtr.Elem()
+
+	// create a new value of the same concrete type
+	uint16Type := TypeOf(uint16(0))
+	newUint16Value := New(uint16Type).Elem()
+	newUint16Value.Set(ValueOf(uint16(13)))
+
+	// set the new value to the interface
+	inElem.Set(newUint16Value)
+
+	if value.(uint16) != 13 {
+		t.Errorf("Failed to set interface value from uint16")
+	}
+}
+
 func equal[T comparable](a, b []T) bool {
 	if len(a) != len(b) {
 		return false