https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88086
Bug ID: 88086
Summary: gcc only allows valid expressions as unknown C++
attribute argument clause
Product: gcc
Version: 8.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: bruno-gcc at defraine dot net
Target Milestone: ---
GCC raises an error when the argument of an (unknown) C++ attribute contains an
undeclared identifier. Tested with -std=c++17:
int i [[vendor::attr(somename)]];
// error: 'somename' was not declared in this scope
More in general, GCC seem to expect a valid expression as the argument:
int i [[vendor::attr(123?456)]];
// error: expected ':' before ')' token
// error: expected primary-expression before ')' token
Similarly, a specific example listed as "well-formed implementation-defined
attribute" in the standard also raises errors in GCC:
int i [[vendor::attr([[]])]]; // well-formed implementation-defined attribute
// error: expected identifier before '[' token
// In lambda function:
// error: expected '{' before ')' token
The treatment by GCC of the attribute argument as an expression does not seem
in line with the C++ standard (section 10.6.1 Attribute syntax and semantics),
which defines the attribute argument as any balanced token sequence, and which
also specifies that "any attribute-token that is not recognized by the
implementation is ignored". From this, you would expect a conforming compiler
to skip the balanced tokens in the argument of an unknown attribute.
This also seems to be what other compilers (clang, MSVC,...) are doing.
