_Generic accepts an expression operand whose type is matched against a
list of associations. The expression operand is unevaluated, but the
type matched is the type after lvalue conversion. This conversion loses
type information, which makes it more difficult to match against
qualified or incomplete types.
This extension allows _Generic to accept a type operand instead of an
expression operand. The type operand form does not undergo any
conversions and is matched directly against the association list.
This extension is also supported in C++ as we already supported
_Generic selection expressions there.
The RFC for this extension can be found at:
https://discourse.llvm.org/t/rfc-generic-selection-expression-with-a-type-operand/70388
Differential Revision: https://reviews.llvm.org/D149904
Currently, Clang accepts this code in C mode (where the tag is required
to be used) but rejects it in C++ mode thinking that the association is
defining a new type.
void foo(void) {
struct S { int a; };
_Generic(something, struct S : 1);
}
Clang thinks this in C++ because it sees struct S : when parsing the
class specifier and decides that must be a type definition (because the
colon signifies the presence of a base class type). This patch adds a
new declarator context to represent a _Generic association so that we
can distinguish these situations properly.
Fixes#55562
Differential Revision: https://reviews.llvm.org/D126969
New diagnostics were added for unreachable generic selection expression
associations in ca75ac5f04f269def97e6844c2f5c9596b29c84c, but it did
not account for a difference in behavior between C and C++ regarding
lvalue to rvalue conversions. So we would issue diagnostics about a
selection being unreachable and then reach it. This corrects the
diagnostic behavior in that case.
Differential Revision: https://reviews.llvm.org/D125882
arguments of types by default.
This somewhat improves the worst-case printing of types like
std::string, std::vector, etc., where many irrelevant default arguments
can be included in the type as printed if we've lost the type sugar.
