fix: apply declared nullability from AnyType return type to resolved concrete type

extensions/variants_test.go

@@ -25,6 +25,7 @@ func TestEvaluateTypeExpression(t *testing.T) {
 		strNull, _      = parser.ParseType("string?")
 		strNonNull, _   = parser.ParseType("string")
 		any1NonNull, _  = parser.ParseType("any1")
+		any1Nullable, _ = parser.ParseType("any1?")
 		any1listNonNull = mkFuncArgList(any1NonNull)
 
 		// Few shortcut type definitions.
@@ -89,6 +90,17 @@ func TestEvaluateTypeExpression(t *testing.T) {
 			args:     []types.Type{i64TypeReq, i64TypeReq},
 			expected: i64TypeReq,
 		},
+		{
+			// nullif with DECLARED_OUTPUT: return type is any1? (nullable).
+			// Even when both arguments are non-nullable, the result must be
+			// nullable because NULLIF returns NULL when args are equal.
+			name:     "nullif(any1, any1) -> any1? [DECLARED_OUTPUT]",
+			nulls:    extensions.DeclaredOutputNullability,
+			ret:      any1Nullable,
+			extArgs:  extensions.FuncParameterList{valArg(any1NonNull), valArg(any1NonNull)},
+			args:     []types.Type{i64TypeReq, i64TypeReq},
+			expected: &types.Int64Type{Nullability: types.NullabilityNullable},
+		},
 		{
 			name:     "element_at(list<any1>, i64) -> any1",
 			nulls:    extensions.DeclaredOutputNullability,

types/any_type.go

@@ -128,6 +128,16 @@ func (m *AnyType) ReturnType(funcParameters []FuncDefArgType, argumentTypes []Ty
 			return nil, err
 		}
 		if typ != nil {
+			// Apply the declared nullability from the return type definition.
+			// For example, nullif declares `return: any1?` — the `?` means
+			// the result is always nullable regardless of argument nullability.
+			//
+			// TODO: It is unclear if a declared non-nullable return type (e.g. `any1`)
+			// should override a nullable argument type to produce a non-nullable result.
+			// This is being resolved in https://github.com/substrait-io/substrait/issues/943
+			if m.Nullability == NullabilityNullable {
+				typ = typ.WithNullability(NullabilityNullable)
+			}
 			return typ, nil
 		}
 	}

types/any_type_test.go

@@ -25,7 +25,7 @@ func TestAnyType(t *testing.T) {
 			argName:            "any",
 			parameters:         []FuncDefArgType{&AnyType{Name: "any"}},
 			args:               []Type{decP30S9},
-			concreteReturnType: decP30S9,
+			concreteReturnType: &DecimalType{Precision: 30, Scale: 9, Nullability: NullabilityNullable},
 			nullability:        NullabilityNullable,
 			expectedString:     "any?",
 		},
@@ -325,7 +325,7 @@ func TestAnyType(t *testing.T) {
 			argName:            "any1",
 			parameters:         []FuncDefArgType{&AnyType{Name: "any1"}, &Int32Type{}},
 			args:               []Type{varchar37, &Int32Type{}},
-			concreteReturnType: varchar37,
+			concreteReturnType: &VarCharType{Length: 37, Nullability: NullabilityNullable},
 			nullability:        NullabilityNullable,
 			expectedString:     "any1?",
 		},