function/stdlib: ElementFunc handles negative index into tuple

An earlier commit added support for negative indices but only dealt with
the Impl function. Since the length of a tuple is part of its type, the
Type callback also needs to deal with the possibility of negative indices
when deciding what type to return.

CHANGELOG.md

@@ -1,5 +1,6 @@
 # 1.16.2 (Unreleased)
 
+* `function/stdlib`: `ElementFunc` no longer crashes when asked for a negative index into a tuple. This fixes a miss in the negative index support added back in v1.15.0. ([#200](https://github.com/zclconf/go-cty/pull/200))
 
 # 1.16.1 (January 13, 2025)
 

cty/function/stdlib/collection.go

@@ -170,6 +170,9 @@ var ElementFunc = function.New(&function.Spec{
 				return cty.DynamicPseudoType, errors.New("cannot use element function with an empty list")
 			}
 			index = index % len(etys)
+			if index < 0 {
+				index += len(etys)
+			}
 			return etys[index], nil
 		default:
 			return cty.DynamicPseudoType, fmt.Errorf("cannot read elements from %s", listTy.FriendlyName())

cty/function/stdlib/collection_test.go

@@ -1138,6 +1138,18 @@ func TestElement(t *testing.T) {
 		cty.StringVal("brown").Mark("fox"),
 		cty.UnknownVal(cty.String),
 	})
+	tuple := cty.TupleVal([]cty.Value{
+		cty.StringVal("the"),
+		cty.UnknownVal(cty.String),
+		cty.StringVal("brown"),
+		cty.False,
+	})
+	unknownTuple := cty.UnknownVal(cty.Tuple([]cty.Type{
+		cty.String,
+		cty.String,
+		cty.String,
+		cty.Bool,
+	}))
 
 	tests := []struct {
 		List  cty.Value
@@ -1253,6 +1265,92 @@ func TestElement(t *testing.T) {
 			cty.StringVal("fox"),
 			true,
 		},
+
+		{
+			tuple,
+			cty.NumberIntVal(0),
+			cty.StringVal("the"),
+			false,
+		},
+		{
+			tuple,
+			cty.NumberIntVal(1),
+			cty.UnknownVal(cty.String),
+			false,
+		},
+		{
+			tuple,
+			cty.NumberIntVal(3),
+			cty.False,
+			false,
+		},
+		{
+			tuple,
+			cty.NumberIntVal(4),
+			cty.StringVal("the"),
+			false,
+		},
+		{
+			tuple,
+			cty.NumberIntVal(10),
+			cty.StringVal("brown"),
+			false,
+		},
+		{
+			tuple,
+			cty.NumberIntVal(-1),
+			cty.False,
+			false,
+		},
+		{
+			tuple,
+			cty.NumberIntVal(-6),
+			cty.StringVal("brown"),
+			false,
+		},
+
+		{
+			unknownTuple,
+			cty.NumberIntVal(0),
+			cty.UnknownVal(cty.String),
+			false,
+		},
+		{
+			unknownTuple,
+			cty.NumberIntVal(1),
+			cty.UnknownVal(cty.String),
+			false,
+		},
+		{
+			unknownTuple,
+			cty.NumberIntVal(3),
+			cty.UnknownVal(cty.Bool),
+			false,
+		},
+		{
+			unknownTuple,
+			cty.NumberIntVal(4),
+			cty.UnknownVal(cty.String),
+			false,
+		},
+		{
+			unknownTuple,
+			cty.NumberIntVal(10),
+			cty.UnknownVal(cty.String),
+			false,
+		},
+		{
+			unknownTuple,
+			cty.NumberIntVal(-1),
+			cty.UnknownVal(cty.Bool),
+			false,
+		},
+		{
+			unknownTuple,
+			cty.NumberIntVal(-6),
+			cty.UnknownVal(cty.String),
+			false,
+		},
 	}
 
 	for _, test := range tests {