[SR-9482] Swift's filtering of "header guard" macros is too strict

[belkadan]

Previous ID	SR-9482
Radar	rdar://problem/46644027
Original Reporter	@belkadan
Type	Bug
Status	Resolved
Resolution	Done

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, ClangImporter, StarterBug
Assignee	@typesanitizer
Priority	Medium

md5: 01b81c30eea7001ee4aeaf49942f86c3

Issue Description:

The O_SYNC macro on Apple platforms is defined in its own file that looks like this:

#ifndef O_SYNC
#define O_SYNC                  0x0080          /* synch I/O file integrity */
#endif /* O_SYNC */

(with an APSL header too)

This pattern is one that Clang can use as a "header guard", to know whether it has to bother re-parsing the file when it's included again later or whether it can just skip it. Usually header guard macros are written explicitly for this purpose, but in this case O_SYNC is actually defining a useful constant. We should restrict our check for "header guard" to cases where there's exactly one token, the integer constant "1". (Macros with no expansions are already not imported into Swift.)

[belkadan]

The place where this check lives is shouldIgnoreMacro in ImportName.cpp.

[belkadan]

(This isn't perfect because sometimes the value really is 1, particularly if it's a feature flag or something, but it's a better compromise for now.)

[typesanitizer]

Fixed in GitHub PR #26150.