Attributes in C mode fail to consume :: when it is a result of macro expansion

[lapkow]

Issue Summary

scan_attribute() in attribute.c makes an effort to transform the individual tokens of colon colon in C mode; however, the algorithm fails when the colon colon are tokens generated by macro expansion:

There is also a failure to consume the __extension__ keyword properly.

Reproducing Source Code

#define ATTRIBUTE_IN_MACRO_WITH_COLON_COLON [[__extension__ gnu::attribute]]
void f() [[__extension__ gnu::attribute]];
void g() ATTRIBUTE_IN_MACRO_WITH_COLON_COLON;

Command-line Options

cpfe --c11 --gnu_version=160100

Type of Issue

[Front End] This code SHOULD compile with these options.

Additional Details

No response

Reproducing (Standard) Configuration(s)

No response

Reproducing (Non-standard) Configuration

No response

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