convert: Strip optional attributes when creating null result values

Object types with optional attributes are only for use as a type
conversion target and are not valid to appear in the type constraint for
a null value.

cty/convert/conversion_object.go

@@ -80,13 +80,19 @@ func conversionObjectToObject(in, out cty.Type, unsafe bool) conversion {
 				}
 			}
 
+			if val.IsNull() {
+				// Strip optional attributes out of the embedded type for null
+				// values.
+				val = cty.NullVal(val.Type().WithoutOptionalAttributesDeep())
+			}
+
 			attrVals[name] = val
 		}
 
 		for name := range outOptionals {
 			if _, exists := attrVals[name]; !exists {
 				wantTy := outAtys[name]
-				attrVals[name] = cty.NullVal(wantTy)
+				attrVals[name] = cty.NullVal(wantTy.WithoutOptionalAttributesDeep())
 			}
 		}
 

cty/convert/public_test.go

@@ -1066,6 +1066,103 @@ func TestConvert(t *testing.T) {
 			}, []string{"b", "c"}),
 			WantError: `map element type is incompatible with attribute "c": object required`,
 		},
+		{
+			Value: cty.TupleVal([]cty.Value{
+				cty.ObjectVal(map[string]cty.Value{
+					"d": cty.NumberVal(big.NewFloat(10)),
+					"c": cty.ObjectVal(map[string]cty.Value{
+						"a": cty.StringVal("foo"),
+						"b": cty.BoolVal(true),
+					}),
+				}),
+				cty.ObjectVal(map[string]cty.Value{
+					"d": cty.NumberVal(big.NewFloat(5)),
+					"c": cty.NullVal(cty.ObjectWithOptionalAttrs(map[string]cty.Type{
+						"a": cty.String,
+						"b": cty.Bool,
+					}, []string{"b"})),
+				}),
+			}),
+			Type: cty.Set(cty.ObjectWithOptionalAttrs(map[string]cty.Type{
+				"c": cty.ObjectWithOptionalAttrs(map[string]cty.Type{
+					"a": cty.String,
+					"b": cty.Bool,
+				}, []string{"b"}),
+				"d": cty.Number,
+			}, []string{"c"})),
+			Want: cty.SetVal([]cty.Value{
+				cty.ObjectVal(map[string]cty.Value{
+					"d": cty.NumberVal(big.NewFloat(10)),
+					"c": cty.ObjectVal(map[string]cty.Value{
+						"a": cty.StringVal("foo"),
+						"b": cty.BoolVal(true),
+					}),
+				}),
+				cty.ObjectVal(map[string]cty.Value{
+					"d": cty.NumberVal(big.NewFloat(5)),
+					"c": cty.NullVal(cty.Object(map[string]cty.Type{
+						"a": cty.String,
+						"b": cty.Bool,
+					})),
+				}),
+			}),
+		},
+		{
+			Value: cty.TupleVal([]cty.Value{
+				cty.ObjectVal(map[string]cty.Value{
+					"d": cty.NumberVal(big.NewFloat(10)),
+					"c": cty.ObjectVal(map[string]cty.Value{
+						"a": cty.StringVal("foo"),
+						"b": cty.BoolVal(true),
+					}),
+				}),
+				cty.ObjectVal(map[string]cty.Value{
+					"d": cty.NumberVal(big.NewFloat(5)),
+				}),
+			}),
+			Type: cty.Set(cty.ObjectWithOptionalAttrs(map[string]cty.Type{
+				"c": cty.ObjectWithOptionalAttrs(map[string]cty.Type{
+					"a": cty.String,
+					"b": cty.Bool,
+				}, []string{"b"}),
+				"d": cty.Number,
+			}, []string{"c"})),
+			Want: cty.SetVal([]cty.Value{
+				cty.ObjectVal(map[string]cty.Value{
+					"d": cty.NumberVal(big.NewFloat(10)),
+					"c": cty.ObjectVal(map[string]cty.Value{
+						"a": cty.StringVal("foo"),
+						"b": cty.BoolVal(true),
+					}),
+				}),
+				cty.ObjectVal(map[string]cty.Value{
+					"d": cty.NumberVal(big.NewFloat(5)),
+					"c": cty.NullVal(cty.Object(map[string]cty.Type{
+						"a": cty.String,
+						"b": cty.Bool,
+					})),
+				}),
+			}),
+		},
+		{
+			Value: cty.MapVal(map[string]cty.Value{
+				"a": cty.StringVal("boop"),
+			}),
+			Type: cty.ObjectWithOptionalAttrs(map[string]cty.Type{
+				"a": cty.String,
+				"b": cty.String,
+				"c": cty.Object(map[string]cty.Type{
+					"d": cty.String,
+				}),
+			}, []string{"b", "c"}),
+			Want: cty.ObjectVal(map[string]cty.Value{
+				"a": cty.StringVal("boop"),
+				"b": cty.NullVal(cty.String),
+				"c": cty.NullVal(cty.Object(map[string]cty.Type{
+					"d": cty.String,
+				})),
+			}),
+		},
 		{
 			Value: cty.ListVal([]cty.Value{
 				cty.ObjectVal(map[string]cty.Value{