stdlib: Fix set function crashes with empty sets

If one or more arguments to the stdlib set functions was an empty set of
dynamic pseudo type, the functions would panic due to incompatible set
rules.

We can special case empty dynamic pseudo type sets to be ignored through
type unification, because they are always capable of being converted to
any other type.

Author: alisdair

cty/function/stdlib/set.go

@@ -163,8 +163,23 @@ func SetSymmetricDifference(sets ...cty.Value) (cty.Value, error) {
 func setOperationReturnType(args []cty.Value) (ret cty.Type, err error) {
 	var etys []cty.Type
 	for _, arg := range args {
-		etys = append(etys, arg.Type().ElementType())
+		ty := arg.Type().ElementType()
+
+		// Do not unify types for empty dynamic pseudo typed collections. These
+		// will always convert to any other concrete type.
+		if arg.LengthInt() == 0 && ty.Equals(cty.DynamicPseudoType) {
+			continue
+		}
+
+		etys = append(etys, ty)
 	}
+
+	// If all element types were skipped (due to being empty dynamic collections),
+	// the return type should also be a set of dynamic pseudo type.
+	if len(etys) == 0 {
+		return cty.Set(cty.DynamicPseudoType), nil
+	}
+
 	newEty, _ := convert.UnifyUnsafe(etys)
 	if newEty == cty.NilType {
 		return cty.NilType, fmt.Errorf("given sets must all have compatible element types")

cty/function/stdlib/set_test.go

@@ -55,6 +55,27 @@ func TestSetUnion(t *testing.T) {
 				cty.StringVal("c"),
 			}),
 		},
+		{
+			[]cty.Value{
+				cty.SetVal([]cty.Value{cty.True}),
+				cty.SetValEmpty(cty.DynamicPseudoType),
+			},
+			cty.SetVal([]cty.Value{cty.True}),
+		},
+		{
+			[]cty.Value{
+				cty.SetVal([]cty.Value{cty.EmptyObjectVal}),
+				cty.SetValEmpty(cty.DynamicPseudoType),
+			},
+			cty.SetVal([]cty.Value{cty.EmptyObjectVal}),
+		},
+		{
+			[]cty.Value{
+				cty.SetValEmpty(cty.DynamicPseudoType),
+				cty.SetValEmpty(cty.DynamicPseudoType),
+			},
+			cty.SetValEmpty(cty.DynamicPseudoType),
+		},
 	}
 
 	for _, test := range tests {
@@ -117,6 +138,27 @@ func TestSetIntersection(t *testing.T) {
 				cty.StringVal("b"),
 			}),
 		},
+		{
+			[]cty.Value{
+				cty.SetVal([]cty.Value{cty.True}),
+				cty.SetValEmpty(cty.DynamicPseudoType),
+			},
+			cty.SetValEmpty(cty.Bool),
+		},
+		{
+			[]cty.Value{
+				cty.SetVal([]cty.Value{cty.EmptyObjectVal}),
+				cty.SetValEmpty(cty.DynamicPseudoType),
+			},
+			cty.SetValEmpty(cty.EmptyObject),
+		},
+		{
+			[]cty.Value{
+				cty.SetValEmpty(cty.DynamicPseudoType),
+				cty.SetValEmpty(cty.DynamicPseudoType),
+			},
+			cty.SetValEmpty(cty.DynamicPseudoType),
+		},
 	}
 
 	for _, test := range tests {
@@ -169,6 +211,21 @@ func TestSetSubtract(t *testing.T) {
 				cty.StringVal("b"),
 			}),
 		},
+		{
+			cty.SetVal([]cty.Value{cty.StringVal("a")}),
+			cty.SetValEmpty(cty.DynamicPseudoType),
+			cty.SetVal([]cty.Value{cty.StringVal("a")}),
+		},
+		{
+			cty.SetVal([]cty.Value{cty.EmptyObjectVal}),
+			cty.SetValEmpty(cty.DynamicPseudoType),
+			cty.SetVal([]cty.Value{cty.EmptyObjectVal}),
+		},
+		{
+			cty.SetValEmpty(cty.DynamicPseudoType),
+			cty.SetValEmpty(cty.DynamicPseudoType),
+			cty.SetValEmpty(cty.DynamicPseudoType),
+		},
 	}
 
 	for _, test := range tests {