Incorrect paste of placeholder with '##'.

[mrolle45]

If a macro definition contains x ## ## and x is a parameter and the macro is called with an empty argument, then clang ignores the x ## when expanding the macro, leaving just ##. It then tries to paste ## after the preceding token!
To avoid confusion, let paste be a ## token in an odd numbered position in a sequence of ## tokens, and let hashhash be a ## token in an even numbered position in a sequence of ## tokens.
In most cases, clang treats hashhash as an ordinary token, not as a paste operator.
But if x is a parameter for an empty argument, foo x ## bar foo x ## ## bar foo x ## ## ## bar is expanded to foo bar foo ## bar foo ## ## bar. clang interprets this as foo bar foo pastebar foo paste hashhash bar. The result output is foo bar foobar foo ## bar.
Since the first ## got removed, the positions of paste and hashhash are swapped, and clang should produce foo bar foo ## bar foo ## bar.
I verified this result by running clang -E on the file

#define F(x) foo x ## bar foo x ## ## bar foo x ## ## ## bar
F()

Suggested fix

Rather than delete the x ## from the expansion, replace it with a new "placeholder" token type. Then follow the Standard rules for pasting with a placeholder, and remove any remaining placeholders after doing the pasting. There will be no cases of consecutive ## tokens
The result output might have an extra '##' in some cases, but it would be consistent with the result for a non-empty macro argument.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (mrolle45)

If a macro definition contains `x ## ##` and `x` is a parameter and the macro is called with an empty argument, then clang ignores the `x ##` when expanding the macro, leaving just `##`. It then tries to paste `##` after the preceding token! To avoid confusion, let **paste** be a ## token in an odd numbered position in a sequence of ## tokens, and let **hashhash** be a ## token in an even numbered position in a sequence of ## tokens. In most cases, clang treats **hashhash** as an ordinary token, not as a paste operator. But if `x` is a parameter for an empty argument, `foo x ## bar foo x ## ## bar foo x ## ## ## bar ` is expanded to `foo bar foo ## bar foo ## ## bar`. clang interprets this as `foo bar foo `**paste**` bar foo `**hashhash**` bar`. The result output is `foo bar foobar foo ## bar`.

[chrchr-github]

#112122 seems related.

[zygoloid]

Closing as duplicate of #1112122.

@mrolle45 Please can you keep any further discussion on issue #112122? Splitting the discussion across multiple issues makes it harder to follow.

[mrolle45]

OK. I'll add comment to #112122. Note that the example I have here is a different problem from the one in #112122, as it involves placeholders.