go/src/cmd/compile/internal/types2/index.go

// If e is a valid function instantiation, indexExpr returns true.
// In that case x represents the uninstantiated function value and
// it is the caller's responsibility to instantiate the function.
func (check *Checker) indexExpr(x *operand, e *syntax.IndexExpr) (isFuncInst bool) { … }

func (check *Checker) sliceExpr(x *operand, e *syntax.SliceExpr) { … }

// singleIndex returns the (single) index from the index expression e.
// If the index is missing, or if there are multiple indices, an error
// is reported and the result is nil.
func (check *Checker) singleIndex(e *syntax.IndexExpr) syntax.Expr { … }

// index checks an index expression for validity.
// If max >= 0, it is the upper bound for index.
// If the result typ is != Typ[Invalid], index is valid and typ is its (possibly named) integer type.
// If the result val >= 0, index is valid and val is its constant int value.
func (check *Checker) index(index syntax.Expr, max int64) (typ Type, val int64) { … }

// isValidIndex checks whether operand x satisfies the criteria for integer
// index values. If allowNegative is set, a constant operand may be negative.
// If the operand is not valid, an error is reported (using what as context)
// and the result is false.
func (check *Checker) isValidIndex(x *operand, code Code, what string, allowNegative bool) bool { … }