Bug 19028 – Emit warning for identifiers starting with `__`

Identifiers starting with `__` are reserved for compiler and their behaviour is implementation-defined. But compilers do not show warning or error message when you try to use such names. Furthermore, some functions ignore these identifiers and may produce hidden bugs in a code. For example: void main() { writeln([__traits(allMembers, S)]); // will print only ["a"] } struct S { int a; int __refCount; } Documentation of `allMembers` says nothing about this behaviour. Links: https://dlang.org/spec/lex.html#identifiers https://dlang.org/spec/traits.html#allMembers https://github.com/dlang/dmd/blob/2ce74fc1180a3b0ed5e5c29aa79bf3773af5d5b7/src/dmd/traits.d#L1385 https://forum.dlang.org/thread/[email protected]

My suggestion is to only emit warnings for packages that are not owned by the language. In other words, the language should be free to add these types of symbols in std.*, rt.*, core.*, and object without complaint.

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