From Jason Turner

[cpp.import]

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+ ## Header unit importation <a id="cpp.import">[[cpp.import]]</a>
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+
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+ ``` bnf
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+ pp-import:
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+ exportₒₚₜ import header-name pp-tokensₒₚₜ ';' new-line
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+ exportₒₚₜ import header-name-tokens pp-tokensₒₚₜ ';' new-line
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+ exportₒₚₜ import pp-tokens ';' new-line
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+ ```
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+
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+ A *pp-import* shall not appear in a context where `import` or (if it is
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+ the first token of the *pp-import*) `export` is an identifier defined as
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+ an object-like macro.
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+
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+ The preprocessing tokens after the `import` preprocessing token in the
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+ `import` *control-line* are processed just as in normal text (i.e., each
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+ identifier currently defined as a macro name is replaced by its
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+ replacement list of preprocessing tokens). An `import` directive
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+ matching the first two forms of a *pp-import* instructs the preprocessor
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+ to import macros from the header unit [[module.import]] denoted by the
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+ *header-name*. The *point of macro import* for the first two forms of
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+ *pp-import* is immediately after the *new-line* terminating the
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+ *pp-import*. The last form of *pp-import* is only considered if the
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+ first two forms did not match.
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+
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+ If a *pp-import* is produced by source file inclusion (including by the
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+ rewrite produced when a `#include` directive names an importable header)
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+ while processing the *group* of a *module-file*, the program is
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+ ill-formed.
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+
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+ In all three forms of *pp-import*, the `import` and `export` (if it
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+ exists) preprocessing tokens are replaced by the *import-keyword* and
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+ *export-keyword* preprocessing tokens respectively.
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+
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+ [*Note 1*: This makes the line no longer a directive so it is not
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+ removed at the end of phase 4. — *end note*]
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+
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+ Additionally, in the second form of *pp-import*, a *header-name* token
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+ is formed as if the *header-name-tokens* were the *pp-tokens* of a
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+ `#include` directive. The *header-name-tokens* are replaced by the
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+ *header-name* token.
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+
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+ [*Note 2*: This ensures that imports are treated consistently by the
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+ preprocessor and later phases of translation. — *end note*]
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+
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+ Each `#define` directive encountered when preprocessing each translation
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+ unit in a program results in a distinct *macro definition*.
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+
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+ [*Note 3*: A predefined macro name [[cpp.predefined]] is not introduced
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+ by a `#define` directive. Implementations providing mechanisms to
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+ predefine additional macros are encouraged to not treat them as being
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+ introduced by a `#define` directive. — *end note*]
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+
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+ Importing macros from a header unit makes macro definitions from a
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+ translation unit visible in other translation units. Each macro
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+ definition has at most one point of definition in each translation unit
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+ and at most one point of undefinition, as follows:
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+
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+ - The *point of definition* of a macro definition within a translation
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+ unit is the point at which its `#define` directive occurs (in the
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+ translation unit containing the `#define` directive), or, if the macro
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+ name is not lexically identical to a keyword [[lex.key]] or to the
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+ *identifier*s `module` or `import`, the first point of macro import of
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+ a translation unit containing a point of definition for the macro
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+ definition, if any (in any other translation unit).
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+ - The *point of undefinition* of a macro definition within a translation
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+ unit is the first point at which a `#undef` directive naming the macro
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+ occurs after its point of definition, or the first point of macro
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+ import of a translation unit containing a point of undefinition for
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+ the macro definition, whichever (if any) occurs first.
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+
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+ A macro directive is *active* at a source location if it has a point of
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+ definition in that translation unit preceding the location, and does not
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+ have a point of undefinition in that translation unit preceding the
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+ location.
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+
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+ If a macro would be replaced or redefined, and multiple macro
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+ definitions are active for that macro name, the active macro definitions
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+ shall all be valid redefinitions of the same macro [[cpp.replace]].
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+
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+ [*Note 4*: The relative order of *pp-import*s has no bearing on whether
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+ a particular macro definition is active. — *end note*]
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+
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+ [*Example 1*:
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+
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+ Importable header \`"a.h"\`
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+
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+ ``` cpp
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+ #define X 123 // #1
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+ #define Y 45 // #2
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+ #define Z a // #3
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+ #undef X // point of undefinition of #1 in "a.h"
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+ ```
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+
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+ Importable header \`"b.h"\`
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+
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+ ``` cpp
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+ import "a.h"; // point of definition of #1, #2, and #3, point of undefinition of #1 in "b.h"
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+ #define X 456 // OK, #1 is not active
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+ #define Y 6 // error: #2 is active
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+ ```
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+
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+ Importable header \`"c.h"\`
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+
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+ ``` cpp
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+ #define Y 45 // #4
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+ #define Z c // #5
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+ ```
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+
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+ Importable header \`"d.h"\`
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+
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+ ``` cpp
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+ import "a.h"; // point of definition of #1, #2, and #3, point of undefinition of #1 in "d.h"
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+ import "c.h"; // point of definition of #4 and #5 in "d.h"
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+ int a = Y; // OK, active macro definitions #2 and #4 are valid redefinitions
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+ int c = Z; // error: active macro definitions #3 and #5 are not valid redefinitions of Z
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+ ```
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+
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+ — *end example*]
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+