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See example in t/invalid-branch-playground.lisp.

Function invalid-branch &optional message

This function mark a specific part of the code to be invalid. It MUST NOT be used in a valid code.

  • On unsupported implementations, this function has no compile-time effect and just signals an error in runtime.
  • On supported implementations (currently SBCL only), the compiler signals a warning,style-warning or error (depending on the value of *INVALID-BRANCH-WARNING-LEVEL*) when the compiler fails to eliminiate it as a dead-code.

This is useful to detect a specific kind of errors, e.g. type errors, infinite loops, infinite recursion and so on. When it happens, you should fix the code, add a type restriction etc. in order to make it compile. For example, using invalid-branch in a default method of inlined-generic-function, you can perform a static type checking (only on sbcl).

Supported implementations: You can check these test results on Travis-CI.

SBCL
This feature is tested against all most recent patch versions in the past three minor versions. That is, 1.3.9, 1.2.16, 1.1.18.

Special Variable *invalid-branch-warning-level*

(declaim (type (member alexandria:simple-style-warning warn error) *invalid-branch-warning-level*))

A flag controlling the level of compile-time warning signaled when compiling a call to INVALID-BRANCH. This does not affect the runtime behavior of INVALID-BRANCH, which always signals an error.

The default value is warning.

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