The previous implementation was insufficient for checking statement
attribute mutual exclusion because attributed statements do not collect
their attributes one-at-a-time in the same way that declarations do. So
the design that was attempting to check for mutual exclusion as each
attribute was processed would not ever catch a mutual exclusion in a
statement. This was missed due to insufficient test coverage, which has
now been added for the [[likely]] and [[unlikely]] attributes.
The new approach is to check all of attributes that are to be applied
to the attributed statement in a group. This required generating
another DiagnoseMutualExclusions() function into AttrParsedAttrImpl.inc.
Adds a diagnostic when the user annotates an `if constexpr` with a
likelihood attribute. The `if constexpr` statement is evaluated at compile
time so the attribute has no effect. Annotating the accompanied `else`
with a likelihood attribute has the same effect as annotating a generic
statement. Since the attribute there is most likely not intended, a
diagnostic will be issued. Since the attributes can't conflict, the
"conflict" won't be diagnosed for an `if constexpr`.
Differential Revision: https://reviews.llvm.org/D90336
This is the initial part of the implementation of the C++20 likelihood
attributes. It handles the attributes in an if statement.
Differential Revision: https://reviews.llvm.org/D85091
Aaron Ballman