shadow symbol-names &optional package → t

symbol-names — a designator for a list of string designators.

package — a package designator. The default is the current package.

shadow assures that symbols with names given by symbol-names are present in the package.

Specifically, package is searched for symbols with the names supplied by symbol-names. For each such name, if a corresponding symbol is not present in package (directly, not by inheritance), then a corresponding symbol is created with that name, and inserted into package as an internal symbol. The corresponding symbol, whether pre-existing or newly created, is then added, if not already present, to the shadowing symbols list of package.

(package-shadowing-symbols (make-package 'temp)) → NIL 
(find-symbol 'car 'temp) → CAR, :INHERITED 
(shadow 'car 'temp) → T 
(find-symbol 'car 'temp) → TEMP::CAR, :INTERNAL 
(package-shadowing-symbols 'temp) → (TEMP::CAR)

(make-package 'test-1) → #<PACKAGE "TEST-1"> 
(intern "TEST" (find-package 'test-1)) → TEST-1::TEST, NIL 
(shadow 'test-1::test (find-package 'test-1)) → T 
(shadow 'TEST (find-package 'test-1)) → T 
(assert (not (null (member 'test-1::test (package-shadowing-symbols 
                                           (find-package 'test-1)))))) 

(make-package 'test-2) → #<PACKAGE "TEST-2"> 
(intern "TEST" (find-package 'test-2)) → TEST-2::TEST, NIL 
(export 'test-2::test (find-package 'test-2)) → T 
(use-package 'test-2 (find-package 'test-1))    ;should not error

shadow changes the state of the package system in such a way that the package consistency rules do not hold across the change.

Current state of the package system.

package-shadowing-symbols, Section 11.1 (Package Concepts)

If a symbol with a name in symbol-names already exists in package, but by inheritance, the inherited symbol becomes shadowed₃ by a newly created internal symbol.