diff --git a/rust/candid/src/lib.rs b/rust/candid/src/lib.rs
index d9df4e0d0..c22779d0c 100644
--- a/rust/candid/src/lib.rs
+++ b/rust/candid/src/lib.rs
@@ -288,7 +288,7 @@ macro_rules! Encode {
         Encode!(@PutValue builder $($x,)*)
     }};
     ( @PutValue $builder:ident $x:expr, $($tail:expr,)* ) => {{
-        $builder.arg($x).and_then(|mut builder| Encode!(@PutValue builder $($tail,)*))
+        $builder.arg($x).and_then(|builder| Encode!(@PutValue builder $($tail,)*))
     }};
     ( @PutValue $builder:ident ) => {{
         $builder.serialize_to_vec()
diff --git a/rust/candid/src/parser/typing.rs b/rust/candid/src/parser/typing.rs
index 1beb0c9d5..82165ba67 100644
--- a/rust/candid/src/parser/typing.rs
+++ b/rust/candid/src/parser/typing.rs
@@ -44,6 +44,13 @@ impl TypeEnv {
             t => Ok(t),
         }
     }
+    pub fn trace_type<'a>(&'a self, t: &'a Type) -> Result<Type> {
+        match t {
+            Type::Var(id) => self.trace_type(self.find_type(id)?),
+            Type::Knot(ref id) => self.trace_type(&crate::types::internal::find_type(id).unwrap()),
+            t => Ok(t.clone()),
+        }
+    }
     pub fn as_func<'a>(&'a self, t: &'a Type) -> Result<&'a Function> {
         match t {
             Type::Func(f) => Ok(f),
diff --git a/rust/candid/src/parser/value.rs b/rust/candid/src/parser/value.rs
index 6c6cc5e9f..605a41d9d 100644
--- a/rust/candid/src/parser/value.rs
+++ b/rust/candid/src/parser/value.rs
@@ -154,7 +154,6 @@ impl IDLValue {
                 let ty = crate::types::internal::find_type(id).unwrap();
                 self.annotate_type(from_parser, env, &ty)?
             }
-            (IDLValue::Null, Type::Opt(_)) if from_parser => IDLValue::None,
             (IDLValue::Float64(n), Type::Float32) if from_parser => IDLValue::Float32(*n as f32),
             (IDLValue::Number(str), t) if from_parser => match t {
                 Type::Int => IDLValue::Int(str.parse::<Int>()?),
@@ -177,8 +176,9 @@ impl IDLValue {
             (_, Type::Reserved) => IDLValue::Reserved,
             (IDLValue::Null, Type::Null) => IDLValue::Null,
             (IDLValue::Bool(b), Type::Bool) => IDLValue::Bool(*b),
-            (IDLValue::Int(i), Type::Int) => IDLValue::Int(i.clone()),
             (IDLValue::Nat(n), Type::Nat) => IDLValue::Nat(n.clone()),
+            (IDLValue::Int(i), Type::Int) => IDLValue::Int(i.clone()),
+            (IDLValue::Nat(n), Type::Int) => IDLValue::Int(n.clone().into()),
             (IDLValue::Nat8(n), Type::Nat8) => IDLValue::Nat8(*n),
             (IDLValue::Nat16(n), Type::Nat16) => IDLValue::Nat16(*n),
             (IDLValue::Nat32(n), Type::Nat32) => IDLValue::Nat32(*n),
@@ -190,11 +190,23 @@ impl IDLValue {
             (IDLValue::Float64(n), Type::Float64) => IDLValue::Float64(*n),
             (IDLValue::Float32(n), Type::Float32) => IDLValue::Float32(*n),
             (IDLValue::Text(s), Type::Text) => IDLValue::Text(s.to_owned()),
+            // opt parsing. NB: Always succeeds!
+            (IDLValue::Null, Type::Opt(_)) => IDLValue::None,
+            (IDLValue::Reserved, Type::Opt(_)) => IDLValue::None,
             (IDLValue::None, Type::Opt(_)) => IDLValue::None,
-            (IDLValue::Opt(v), Type::Opt(ty)) => {
-                let v = v.annotate_type(from_parser, env, ty)?;
-                IDLValue::Opt(Box::new(v))
+            // liberal decoding of optionals
+            (IDLValue::Opt(v), Type::Opt(ty)) => v
+                .annotate_type(from_parser, env, ty)
+                .map(|v| IDLValue::Opt(Box::new(v)))
+                .unwrap_or(IDLValue::None),
+            // try consituent type
+            (v, Type::Opt(ty)) if !matches!(env.trace_type(ty)?, Type::Null|Type::Reserved|Type::Opt(_)) => {
+                v.annotate_type(from_parser, env, ty)
+                    .map(|v| IDLValue::Opt(Box::new(v)))
+                    .unwrap_or(IDLValue::None)
             }
+            // fallback
+            (_, Type::Opt(_)) => IDLValue::None,
             (IDLValue::Vec(vec), Type::Vec(ty)) => {
                 let mut res = Vec::new();
                 for e in vec.iter() {
@@ -210,7 +222,13 @@ impl IDLValue {
                 for Field { id, ty } in fs.iter() {
                     let val = fields
                         .get(&id)
-                        .ok_or_else(|| Error::msg(format!("field {} not found", id)))?;
+                        .cloned()
+                        .or_else(|| match env.trace_type(ty).unwrap() {
+                            Type::Opt(_) => Some(&IDLValue::None),
+                            Type::Reserved => Some(&IDLValue::Reserved),
+                            _ => None,
+                        })
+                        .ok_or_else(|| Error::msg(format!("required field {} not found", id)))?;
                     let val = val.annotate_type(from_parser, env, ty)?;
                     res.push(IDLField {
                         id: id.clone(),
diff --git a/rust/candid/src/types/internal.rs b/rust/candid/src/types/internal.rs
index 00531bab9..ae4445ece 100644
--- a/rust/candid/src/types/internal.rs
+++ b/rust/candid/src/types/internal.rs
@@ -351,7 +351,7 @@ thread_local! {
 pub(crate) fn find_type(id: &TypeId) -> Option<Type> {
     ENV.with(|e| match e.borrow().get(id) {
         None => None,
-        Some(t) => Some((*t).clone()),
+        Some(t) => Some(t.clone()),
     })
 }
 
diff --git a/rust/candid/src/types/number.rs b/rust/candid/src/types/number.rs
index 605a494c7..7546345a1 100644
--- a/rust/candid/src/types/number.rs
+++ b/rust/candid/src/types/number.rs
@@ -26,6 +26,14 @@ impl From<BigUint> for Nat {
     }
 }
 
+impl From<Nat> for Int {
+    #[inline(always)]
+    fn from(n: Nat) -> Self {
+        let i: BigInt = n.0.into();
+        i.into()
+    }
+}
+
 impl From<Int> for BigInt {
     #[inline(always)]
     fn from(i: Int) -> Self {
diff --git a/test/construct.test.did b/test/construct.test.did
index 59f50597f..a517a7ea2 100644
--- a/test/construct.test.did
+++ b/test/construct.test.did
@@ -6,6 +6,8 @@ Encoding tests for construct types
 type Opt = opt Opt;
 type Vec = vec Vec;
 type EmptyRecord = record { EmptyRecord };
+type OptRec = opt record { OptRec };
+
 type List = opt record { head : int; tail : List };
 type List1 = opt List2;
 type List2 = record { head : int; tail : List1 };
@@ -47,7 +49,7 @@ assert blob "DIDL\02\6e\01\6e\00\01\00\01\01\00" == "(opt opt null)" : (Opt) "op
 // vector
 assert blob "DIDL\01\6d\7c\01\00\00" == "(vec {})"                          : (vec int) "vec: empty";
 assert blob "DIDL\01\6d\7c\01\00\02\01\02" == "(vec { 1; 2 })"              : (vec int) "vec";
-assert blob "DIDL\01\6d\7b\01\00\02\01\02" == "(blob \"\01\02\")"           : (vec nat8) "vec: blob";
+assert blob "DIDL\01\6d\7b\01\00\02\01\02" == "(blob \"\\01\\02\")"         : (vec nat8) "vec: blob";
 assert blob "DIDL\01\6d\00\01\00\00" == "(vec {})"                          : (Vec) "vec: recursive vector";
 assert blob "DIDL\01\6d\00\01\00\02\00\00" == "(vec { vec {}; vec {} })"    : (Vec) "vec: tree";
 assert blob "DIDL\01\6d\00\01\00\02\00\00" == "(vec { vec {}; vec {} })"    : (vec vec empty) "vec: non-recursive tree";
@@ -85,14 +87,47 @@ assert blob "DIDL\01\6c\02\00\7c\01\7e\02\7e\01\00\2a\01\00"                   !
 assert blob "DIDL\01\6c\02\00\01\01\7e\01\00\00\00"                            !: (record {}) "record: type out of range";
 assert blob "DIDL\01\6c\02\00\01\01\7e\01\7c\2a"                               !: (reserved) "record: type out of range";
 
+// opt
+assert blob "DIDL\00\01\7f" == "(null)"                                         : (opt empty) "opt: parsing null : null";
+assert blob "DIDL\01\6e\6f\01\00\00" == "(null)"                                : (opt empty) "opt: parsing null : opt empty";
+assert blob "DIDL\01\6e\7e\01\00\00" == "(null)"                                : (opt bool) "opt: parsing null : opt bool ";
+assert blob "DIDL\01\6e\7e\01\00\01\00" == "(opt false)"                        : (opt bool) "opt: parsing opt false : opt bool";
+assert blob "DIDL\01\6e\7e\01\00\01\01" == "(opt true)"                         : (opt bool) "opt: parsing opt true : opt bool";
+assert blob "DIDL\01\6e\7e\01\00\01\02"                                        !: (opt bool) "opt: parsing invalid bool at opt bool";
+
+// nested opt
+assert blob "DIDL\02\6e\01\6e\7e\01\00\00" == "(null)"                          : (opt opt bool) "opt: parsing null : opt opt bool";
+assert blob "DIDL\02\6e\01\6e\7e\01\00\01\00" == "(opt null)"                   : (opt opt bool) "opt: parsing opt null : opt opt bool";
+assert blob "DIDL\02\6e\01\6e\7e\01\00\01\01\00" == "(opt opt false)"           : (opt opt bool) "opt: parsing opt opt false : opt opt bool";
+
+// opt subtype to constituent
+assert blob "DIDL\00\01\7e\00" == "(opt false)"   : (opt bool) "opt: parsing false : opt bool";
+assert blob "DIDL\00\01\7e\01" == "(opt true)"    : (opt bool) "opt: parsing true : opt bool";
+assert blob "DIDL\00\01\7e\02"                   !: (opt bool) "opt: parsing invalid bool at opt bool";
+assert blob "DIDL\01\6e\7f\01\00\00" == "(null)"  : (opt opt null) "opt: parsing (null : opt null) at opt opt null gives null, not opt null";
+assert blob "DIDL\00\01\7e\01" == "(null)"        : (opt opt bool) "opt: parsing true : opt opt bool gives null";
+assert blob "DIDL\00\01\7e\01" == "(null)"        : (Opt) "opt: parsing (true : bool) at \"fix opt\" gives null";
+assert blob "DIDL\01\6c\01\00\00\01\00"          !: (OptRec) "opt: parsing \"fix record\" at \"fix record opt\" fails";
+
+// special opt subtyping
+assert blob "DIDL\00\01\70" == "(null)"  : (opt nat) "reserved <: opt nat";
+assert blob "DIDL\01\6e\7e\01\00\00"    == "(null)"  : (opt nat) "null : opt bool <: opt nat";
+assert blob "DIDL\01\6e\7e\01\00\01\01" == "(null)"  : (opt nat) "opt true : opt bool <: opt nat";
+assert blob "DIDL\01\6e\7e\01\00\01\02"             !: (opt nat) "opt bool <: opt nat with invalid boolean value";
+assert blob "DIDL\02\6e\01\6e\7e\01\00\01\01\01" == "(null)"  : (opt nat) "opt opt true : opt opt bool <: opt nat";
+assert blob "DIDL\02\6e\01\6e\7e\01\00\01\01\01" == "(opt null)"  : (opt opt nat) "opt opt true : opt opt bool <: opt opt nat";
+assert blob "DIDL\02\6e\01\6b\01\00\7e\01\00\01\00\01" == "(null)"  : (opt variant { 0 : int }) "opt: recovered coercion error under variant";
+
+// special record field rules
+assert blob "DIDL\01\6c\00\01\00" == "(record { foo = null })" : (record { foo : opt bool }) "missing optional field";
+assert blob "DIDL\01\6c\00\01\00" == "(record { foo = \"☃\" })" : (record { foo : reserved }) "missing reserved field";
+
 // variant
 assert "(variant {})"                                                          !: (variant {}) "variant: no empty value";
 assert blob "DIDL\01\6b\00\01\00"                                              !: (variant {}) "variant: no empty value";
 assert blob "DIDL\01\6b\01\00\7f\01\00\00" == "(variant {0})"                   : (variant {0}) "variant: numbered field";
 assert blob "DIDL\01\6b\01\00\7f\01\00\00\2a"                                  !: (variant {0:int}) "variant: type mismatch";
-
-// TODO Should this pass?
-assert blob "DIDL\01\6b\02\00\7f\01\7c\01\00\01\2a"                             : (variant {0:int; 1:int}) "??? variant: type mismatch";
+assert blob "DIDL\01\6b\02\00\7f\01\7c\01\00\01\2a"                             : (variant {0:int; 1:int}) "variant: type mismatch in unused tag";
 assert blob "DIDL\01\6b\01\00\7f\01\00\00" == "(variant {0})"                   : (variant {0;1}) "variant: ignore field";
 assert blob "DIDL\01\6b\02\00\7f\01\7f\01\00\00" == "(variant {0})"             : (variant {0}) "variant: ignore field";
 assert blob "DIDL\01\6b\02\00\7f\01\7f\01\00\01" == "(variant {1})"             : (variant {1;2}) "variant: change index";
@@ -101,6 +136,8 @@ assert blob "DIDL\01\6b\01\00\7f\01\00\01"                                     !
 assert blob "DIDL\01\6b\02\00\7f\00\7f\01\00\00"                               !: (variant {0}) "variant: duplicate fields";
 assert blob "DIDL\01\6b\03\00\7f\01\7f\01\7f\01\00\00"                         !: (variant {0}) "variant: duplicate fields";
 assert blob "DIDL\01\6b\02\01\7f\00\7f\01\00\00"                               !: (variant {0;1}) "variant: unsorted";
+assert blob "DIDL\01\6b\03\00\7f\01\7f\ff\ff\ff\ff\0f\7f\01\00\00"              : (variant {0;1;4294967295}) "variant: ok with maxInt";
+assert blob "DIDL\01\6b\03\00\7f\ff\ff\ff\ff\0f\7f\01\7f\01\00\00"             !: (variant {0;1;4294967295}) "variant: unsorted with maxInt";
 assert blob "DIDL\01\6b\02\b3\d3\c9\01\7f\e6\fd\d5\01\7f\01\00\00" == "(variant { Bar })" : (variant { Foo; Bar; }) "variant: enum";
 assert blob "DIDL\01\6b\01\cd\84\b0\05\7f\01\00\00" == "(variant { \"☃\" })"    : (variant { "☃" }) "variant: unicode field";
 assert blob "DIDL\01\6b\02\bc\8a\01\71\c5\fe\d2\01\71\01\00\00\04\67\6f\6f\64"
diff --git a/test/prim.test.did b/test/prim.test.did
index 90d055d53..2035f9ec9 100644
--- a/test/prim.test.did
+++ b/test/prim.test.did
@@ -52,6 +52,12 @@ assert blob "DIDL\00\01\7c\80\7f" == "(-128)"   : (int) "int: leb not overlong w
 assert blob "DIDL\00\01\7c\80\80\98\f4\e9\b5\ca\ea\00" == "(60000000000000000)"  : (int) "int: big number";
 assert blob "DIDL\00\01\7c\80\80\e8\8b\96\ca\b5\95\7f" == "(-60000000000000000)" : (int) "int: negative big number";
 
+assert blob "DIDL\00\01\7d\00" == "(0)"         : (int) "nat <: int: 0";
+assert blob "DIDL\00\01\7d\01" == "(1)"         : (int) "nat <: int: 1";
+assert blob "DIDL\00\01\7d\7f" == "(127)"       : (int) "nat <: int: leb (two bytes)";
+assert blob "DIDL\00\01\7d\80\01"  == "(128)"   : (int) "nat <: int: leb (two bytes)";
+assert blob "DIDL\00\01\7d\ff\7f"  == "(16383)" : (int) "nat <: int: leb (two bytes, all bits)";
+
 assert blob "DIDL\00\01\7b\00" == "(0)"         : (nat8) "nat8: 0";
 assert blob "DIDL\00\01\7b\01" == "(1)"         : (nat8) "nat8: 1";
 assert blob "DIDL\00\01\7b\ff" == "(255)"       : (nat8) "nat8: 255";