panic with a dedicated error when an array access index is out-of-bounds

runtime/interpreter/errors.go

@@ -292,3 +292,19 @@ func (e *CyclicLinkError) Error() string {
 		paths,
 	)
 }
+
+// ArrayIndexOutOfBoundsError
+
+type ArrayIndexOutOfBoundsError struct {
+	Index    int
+	MaxIndex int
+	LocationRange
+}
+
+func (e ArrayIndexOutOfBoundsError) Error() string {
+	return fmt.Sprintf(
+		"array index out of bounds: got %d, expected max %d",
+		e.Index,
+		e.MaxIndex,
+	)
+}

runtime/interpreter/value.go

@@ -528,8 +528,19 @@ func (v *ArrayValue) Concat(other ConcatenatableValue) Value {
 	return NewArrayValueUnownedNonCopying(concatenated...)
 }
 
-func (v *ArrayValue) Get(_ *Interpreter, _ LocationRange, key Value) Value {
+func (v *ArrayValue) Get(_ *Interpreter, locationRange LocationRange, key Value) Value {
 	integerKey := key.(NumberValue).ToInt()
+	count := v.Count()
+
+	// Check bounds
+	if integerKey < 0 || integerKey >= count {
+		panic(ArrayIndexOutOfBoundsError{
+			Index:         integerKey,
+			MaxIndex:      count - 1,
+			LocationRange: locationRange,
+		})
+	}
+
 	return v.Values[integerKey]
 }
 

runtime/tests/interpreter/interpreter_test.go

@@ -474,6 +474,35 @@ func TestInterpretArrayIndexing(t *testing.T) {
 	)
 }
 
+func TestInterpretInvalidArrayIndexing(t *testing.T) {
+
+	t.Parallel()
+
+	inter := parseCheckAndInterpret(t, `
+       fun test(): Int {
+           let z = [0, 3]
+           return z[2]
+       }
+    `)
+
+	_, err := inter.Invoke("test")
+
+	require.Equal(t,
+		interpreter.ArrayIndexOutOfBoundsError{
+			Index:    2,
+			MaxIndex: 1,
+			LocationRange: interpreter.LocationRange{
+				Location: TestLocation,
+				Range: ast.Range{
+					StartPos: ast.Position{Offset: 71, Line: 4, Column: 19},
+					EndPos:   ast.Position{Offset: 73, Line: 4, Column: 21},
+				},
+			},
+		},
+		err,
+	)
+}
+
 func TestInterpretArrayIndexingAssignment(t *testing.T) {
 
 	t.Parallel()