There are situations where DiscardResult is set, but we still wish to
emit a float value, e.g. when we will discard the result of an operation
involving such a float value.
Fixes llvm#176902: [clang][bytecode] crashes on ill-formed
_Static_assert comparing complex value
This patch resolves a crash in Clang's constant evaluation when handling
complex number comparisons in discarded expressions, such as those
involving short-circuiting logical operators. The crash occurred due to
unnecessary evaluation of the comparison in the experimental constant
interpreter.
The issue was originally observed and minimized in the following
example:
```cpp
#define EVAL(a, b) _Static_assert(a == b, "")
void foo() {
EVAL(; + 0, 1i);
}
```
This patch ensures that such comparisons are handled correctly without
triggering assertions when the result is discarded.
Tests
A regression test has been added to verify that complex comparisons in
discarded expressions no longer cause crashes during constant
evaluation.
Local verification:
llvm-project-main/clang/lib/AST/ByteCode/constant-eval-complex-discard.c
passes.
Full ninja check-clang may fail locally due to a known GCC ICE when
building Clang unittests; this is unrelated to the change itself.
Per [decl.ref],
> Because a null pointer value or a pointer past the end of an object
does not point to an object, a reference in a well-defined program
cannot refer to such things.
Note this does not fixes the new bytecode interpreter.
Fixes#48665
