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

[Marco Elver via cfe-commits]

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@@ -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
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melver wrote:

function-level comments should explain what the function does. This is explaining the implementation of the function. This is fine, but should be moved inside to document the implementation.

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