[clang] [clang] Fix assertion crash in alloc_size structural equivalence check (#199407) (PR #199980)

[Marco Elver via cfe-commits]

================
@@ -306,6 +306,19 @@ equalAttrArgs(T A1, T A2, StructuralEquivalenceContext &Context) {
   return A1 == A2;
 }
 
+// ParamIdx has an explicit specialization because it can be invalid (when
+// representing an optional parameter that was not specified).  Two invalid
+// ParamIdx values compare equal; an invalid value is not equal to any valid
+// one.  ParamIdx::operator== asserts both sides are valid, so we must guard
+// against the invalid case here before delegating to operator==.
+template <>
+bool equalAttrArgs<ParamIdx>(ParamIdx P1, ParamIdx P2,
+                             StructuralEquivalenceContext &) {
+  if (!P1.isValid() || !P2.isValid())
----------------
melver wrote:

`P1.isValid() == P2.isValid()` is clever, but prefer explicit logical flow for clarity.

```
 if (P1.isValid() != P2.isValid())
   return false;
 if (!P1.isValid())
   return true;
...
```

https://github.com/llvm/llvm-project/pull/199980