allow SPQ cast of null to any type (#6666)

Since removal of typed nulls in #6633, a cast in SPQ of null to any type
other than null or a nullable union (i.e., a union containing null)
results in an error.  Since SQL allows a cast of null to any type, do
likewise in SPQ with the result of a cast to a non-null type being a
nullable union containing the null value.  For example, the result of
`null::int64` is `null::(int64|null)`.

Note that SUP retains the restriction that null can be cast only to null
or a nullable union.

Author: nwt

context.go

@@ -593,12 +593,12 @@ func (c *Context) WrapError(msg string, val Value) Value {
 	return NewValue(errType, b.Bytes())
 }
 
-func (c *Context) Nullable(typ Type) Type {
+func (c *Context) Nullable(typ Type) *TypeUnion {
 	var types []Type
 	if union, ok := TypeUnder(typ).(*TypeUnion); ok {
 		for _, t := range union.Types {
 			if t == TypeNull {
-				return typ
+				return union
 			}
 		}
 		types = slices.Clone(union.Types)

runtime/sam/expr/function/cast.go

@@ -50,6 +50,11 @@ func (c *cast) Cast(from super.Value, to super.Type) super.Value {
 		return from
 	case fromType.ID() == to.ID():
 		return super.NewValue(to, from.Bytes())
+	case fromType == super.TypeNull:
+		union := c.sctx.Nullable(to)
+		var b scode.Builder
+		super.BuildUnion(&b, union.TagOf(super.TypeNull), nil)
+		return super.NewValue(union, b.Bytes().Body())
 	}
 	switch to := to.(type) {
 	case *super.TypeRecord:

runtime/vam/expr/cast/cast.go

@@ -14,7 +14,15 @@ func To(sctx *super.Context, vec vector.Any, typ super.Type) vector.Any {
 	vec = vector.Under(vec)
 	switch vec.Kind() {
 	case vector.KindNull:
-		return errCastFailed(sctx, vec, typ, "")
+		union := sctx.Nullable(typ)
+		tag := union.TagOf(super.TypeNull)
+		tags := make([]uint32, vec.Len())
+		for i := range vec.Len() {
+			tags[i] = uint32(tag)
+		}
+		vecs := make([]vector.Any, len(union.Types))
+		vecs[tag] = vector.NewConst(super.Null, vec.Len())
+		return vector.NewUnion(union, tags, vecs)
 	case vector.KindError:
 		return vec
 	}

runtime/ztests/expr/cast-array.yaml

@@ -11,9 +11,9 @@ input: |
   |[1,2]|
 
 output: |
-  error({message:"cannot cast to [string]",on:null})
-  [error({message:"cannot cast to string",on:null})]
-  [error({message:"cannot cast to string",on:null})]
+  null::(null|[string])
+  [null]::[string|null]
+  [null]::[string|null]
   ["1","2"]
   ["1","2"]
   ["1","2"]

runtime/ztests/expr/cast/bool.yaml

@@ -38,7 +38,7 @@ output: |
   false
   false
   error({message:"cannot cast to bool",on:"blah"})
-  error({message:"cannot cast to bool",on:null})
+  null::(bool|null)
   true
   error("bad")
   error({message:"cannot cast to bool",on:{x:1}})

runtime/ztests/expr/cast/bytes.yaml

@@ -16,6 +16,6 @@ output: |
   0x666f6f
   0x626172
   0x
-  error({message:"cannot cast to bytes",on:null})
+  null::(bytes|null)
   error({message:"cannot cast to bytes",on:{x:"foo"}})
   error("bad")

runtime/ztests/expr/cast/duration.yaml

@@ -25,7 +25,7 @@ output: |
   -25m
   1ns
   error({message:"cannot cast to duration",on:1e+19})
-  error({message:"cannot cast to duration",on:null})
+  null::(duration|null)
   error("bad")
   error({message:"cannot cast to duration",on:{x:1}})
   error({message:"cannot cast to duration",on:[1,2]})

runtime/ztests/expr/cast/enum.yaml

@@ -13,5 +13,5 @@ output: |
   "e2"::enum(e1,e2)
   error({message:"no such symbol in enum(e1,e2)",on:"e3"})
   error({message:"cannot cast to enum",on:0})
-  error({message:"cannot cast to enum",on:null})
+  null::(null|enum(e1,e2))
   error("bad")

runtime/ztests/expr/cast/float.yaml

@@ -11,6 +11,7 @@ input: |
   2::=named
   false
   true
+  null
 
 output: |
   1.5::float16
@@ -37,6 +38,9 @@ output: |
   1.::float16
   1.::float32
   1.
+  null::(float16|null)
+  null::(float32|null)
+  null::(float64|null)
 
 ---
 
@@ -45,15 +49,13 @@ spq: cast(this, <float64>)
 vector: true
 
 input: |
-  null
   error("bad")
   {x:1}
   [1,2]
   |[1,2]|
   |{"x":1}|
 
 output: |
-  error({message:"cannot cast to float64",on:null})
   error("bad")
   error({message:"cannot cast to float64",on:{x:1}})
   error({message:"cannot cast to float64",on:[1,2]})

runtime/ztests/expr/cast/int.yaml

@@ -6,14 +6,14 @@ vector: true
 input: |
   "-1"
   "65535"
-  null
   1::uint64
   10000000000000000000::uint64
   -1.
   1.5::=named
   1e8
   false
   true
+  null
 
 output: |
   -1::int8
@@ -24,10 +24,6 @@ output: |
   error({message:"cannot cast to int16",on:"65535"})
   65535::int32
   65535
-  error({message:"cannot cast to int8",on:null})
-  error({message:"cannot cast to int16",on:null})
-  error({message:"cannot cast to int32",on:null})
-  error({message:"cannot cast to int64",on:null})
   1::int8
   1::int16
   1::int32
@@ -56,6 +52,10 @@ output: |
   1::int16
   1::int32
   1
+  null::(int8|null)
+  null::(int16|null)
+  null::(int32|null)
+  null::(int64|null)
 
 ---
 
@@ -64,15 +64,13 @@ spq: cast(this, <int64>)
 vector: true
 
 input: |
-  null
   error("bad")
   {x:1}
   [1,2]
   |[1,2]|
   |{"x":1}|
 
 output: |
-  error({message:"cannot cast to int64",on:null})
   error("bad")
   error({message:"cannot cast to int64",on:{x:1}})
   error({message:"cannot cast to int64",on:[1,2]})