convert: Fix panic: heterogeneous tuple with null

Tuples with elements of different types can be converted to homogeneous
collections (sets or lists), so long as their elements are unifiable.
For example:

  list("a", "b")     // all elements have the same type
  list("a", 5)       // "a" and 5 can be unified to string
  list("a", 5, null) // null is a valid value for string

However, tuples with elements which are not unifiable cannot be
converted to homogeneous collections:

  list(["a"], "b")   // no common type for list(string) and string

This commit fixes a panic for this failure case, when the tuple contains
both non-unifiable types and a null value:

  list(["a"], "b", null) // should not panic

The null value was causing the unification process to result in a list
or set of dynamic type, which causes the conversion functions to pass
through the original value. This meant that in the final conversion
step, we would attempt to construct a list or set of different values,
which panics.

cty/convert/conversion_collection.go

@@ -156,34 +156,45 @@ func conversionCollectionToMap(ety cty.Type, conv conversion) conversion {
 // given tuple type and return a set of the given element type.
 //
 // Will panic if the given tupleType isn't actually a tuple type.
-func conversionTupleToSet(tupleType cty.Type, listEty cty.Type, unsafe bool) conversion {
+func conversionTupleToSet(tupleType cty.Type, setEty cty.Type, unsafe bool) conversion {
 	tupleEtys := tupleType.TupleElementTypes()
 
 	if len(tupleEtys) == 0 {
 		// Empty tuple short-circuit
 		return func(val cty.Value, path cty.Path) (cty.Value, error) {
-			return cty.SetValEmpty(listEty), nil
+			return cty.SetValEmpty(setEty), nil
 		}
 	}
 
-	if listEty == cty.DynamicPseudoType {
+	if setEty == cty.DynamicPseudoType {
 		// This is a special case where the caller wants us to find
 		// a suitable single type that all elements can convert to, if
 		// possible.
-		listEty, _ = unify(tupleEtys, unsafe)
-		if listEty == cty.NilType {
+		setEty, _ = unify(tupleEtys, unsafe)
+		if setEty == cty.NilType {
 			return nil
 		}
+
+		// If the set element type after unification is still the dynamic
+		// type, the only way this can result in a valid set is if all values
+		// are of dynamic type
+		if setEty == cty.DynamicPseudoType {
+			for _, tupleEty := range tupleEtys {
+				if !tupleEty.Equals(cty.DynamicPseudoType) {
+					return nil
+				}
+			}
+		}
 	}
 
 	elemConvs := make([]conversion, len(tupleEtys))
 	for i, tupleEty := range tupleEtys {
-		if tupleEty.Equals(listEty) {
+		if tupleEty.Equals(setEty) {
 			// no conversion required
 			continue
 		}
 
-		elemConvs[i] = getConversion(tupleEty, listEty, unsafe)
+		elemConvs[i] = getConversion(tupleEty, setEty, unsafe)
 		if elemConvs[i] == nil {
 			// If any of our element conversions are impossible, then the our
 			// whole conversion is impossible.
@@ -244,6 +255,17 @@ func conversionTupleToList(tupleType cty.Type, listEty cty.Type, unsafe bool) co
 		if listEty == cty.NilType {
 			return nil
 		}
+
+		// If the list element type after unification is still the dynamic
+		// type, the only way this can result in a valid list is if all values
+		// are of dynamic type
+		if listEty == cty.DynamicPseudoType {
+			for _, tupleEty := range tupleEtys {
+				if !tupleEty.Equals(cty.DynamicPseudoType) {
+					return nil
+				}
+			}
+		}
 	}
 
 	elemConvs := make([]conversion, len(tupleEtys))

cty/convert/public_test.go

@@ -677,6 +677,95 @@ func TestConvert(t *testing.T) {
 			}),
 			WantError: false,
 		},
+		// https://github.com/hashicorp/terraform/issues/24377:
+		{
+			Value: cty.TupleVal([]cty.Value{
+				cty.ListVal([]cty.Value{cty.StringVal("a")}),
+				cty.StringVal("b"),
+				cty.NullVal(cty.DynamicPseudoType),
+			}),
+			Type:      cty.Set(cty.DynamicPseudoType),
+			WantError: true,
+		},
+		{
+			Value: cty.TupleVal([]cty.Value{
+				cty.ListVal([]cty.Value{cty.StringVal("a")}),
+				cty.StringVal("b"),
+				cty.NullVal(cty.DynamicPseudoType),
+			}),
+			Type:      cty.List(cty.DynamicPseudoType),
+			WantError: true,
+		},
+		{
+			Value: cty.TupleVal([]cty.Value{
+				cty.ListVal([]cty.Value{cty.StringVal("a")}),
+				cty.StringVal("b"),
+			}),
+			Type:      cty.Set(cty.DynamicPseudoType),
+			WantError: true,
+		},
+		{
+			Value: cty.TupleVal([]cty.Value{
+				cty.ListVal([]cty.Value{cty.StringVal("a")}),
+				cty.StringVal("b"),
+			}),
+			Type:      cty.List(cty.DynamicPseudoType),
+			WantError: true,
+		},
+		{
+			Value: cty.TupleVal([]cty.Value{
+				cty.StringVal("a"),
+				cty.NumberIntVal(9),
+				cty.NullVal(cty.DynamicPseudoType),
+			}),
+			Type: cty.Set(cty.DynamicPseudoType),
+			Want: cty.SetVal([]cty.Value{
+				cty.StringVal("a"),
+				cty.StringVal("9"),
+				cty.NullVal(cty.DynamicPseudoType),
+			}),
+			WantError: false,
+		},
+		{
+			Value: cty.TupleVal([]cty.Value{
+				cty.StringVal("a"),
+				cty.NumberIntVal(9),
+				cty.NullVal(cty.DynamicPseudoType),
+			}),
+			Type: cty.List(cty.DynamicPseudoType),
+			Want: cty.ListVal([]cty.Value{
+				cty.StringVal("a"),
+				cty.StringVal("9"),
+				cty.NullVal(cty.DynamicPseudoType),
+			}),
+			WantError: false,
+		},
+		{
+			Value: cty.TupleVal([]cty.Value{
+				cty.NullVal(cty.DynamicPseudoType),
+				cty.NullVal(cty.DynamicPseudoType),
+				cty.NullVal(cty.DynamicPseudoType),
+			}),
+			Type: cty.Set(cty.DynamicPseudoType),
+			Want: cty.SetVal([]cty.Value{
+				cty.NullVal(cty.DynamicPseudoType),
+			}),
+			WantError: false,
+		},
+		{
+			Value: cty.TupleVal([]cty.Value{
+				cty.NullVal(cty.DynamicPseudoType),
+				cty.NullVal(cty.DynamicPseudoType),
+				cty.NullVal(cty.DynamicPseudoType),
+			}),
+			Type: cty.List(cty.DynamicPseudoType),
+			Want: cty.ListVal([]cty.Value{
+				cty.NullVal(cty.DynamicPseudoType),
+				cty.NullVal(cty.DynamicPseudoType),
+				cty.NullVal(cty.DynamicPseudoType),
+			}),
+			WantError: false,
+		},
 	}
 
 	for _, test := range tests {