CLHS: Section 3.2.2.1.3

3.2.2.1.3 When Compiler Macros Are Used

The presence of a compiler macro definition for a function or macro indicates that it is desirable for the compiler to use the expansion of the compiler macro instead of the original function form or macro form. However, no language processor (compiler, evaluator, or other code walker) is ever required to actually invoke compiler macro functions, or to make use of the resulting expansion if it does invoke a compiler macro function.

When the compiler encounters a form during processing that represents a call to a compiler macro name (that is not declared notinline), the compiler might expand the compiler macro, and might use the expansion in place of the original form.

When eval encounters a form during processing that represents a call to a compiler macro name (that is not declared notinline), eval might expand the compiler macro, and might use the expansion in place of the original form.

There are two situations in which a compiler macro definition must not be applied by any language processor:

The global function name binding associated with the compiler macro is shadowed by a lexical binding of the function name.
The function name has been declared or proclaimed notinline and the call form appears within the scope of the declaration.

It is unspecified whether compiler macros are expanded or used in any other situations.