[clang] [C23] Implement WG14 N3037 (PR #132939)

[Erich Keane via cfe-commits]

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@@ -450,6 +453,41 @@ class StmtComparer {
 };
 } // namespace
 
+static bool
+CheckStructurallyEquivalentAttributes(StructuralEquivalenceContext &Context,
+                                      const Decl *D1, const Decl *D2,
+                                      const Decl *PrimaryDecl = nullptr) {
+  // If either declaration has an attribute on it, we treat the declarations
+  // as not being structurally equivalent.
+  // FIXME: this should be handled on a case-by-case basis via tablegen in
+  // Attr.td. There are multiple cases to consider: one declation with the
+  // attribute, another without it; different attribute syntax|spellings for
+  // the same semantic attribute, differences in attribute arguments, order
+  // in which attributes are applied, how to merge attributes if the types are
+  // structurally equivalent, etc.
+  const Attr *D1Attr = nullptr, *D2Attr = nullptr;
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erichkeane wrote:

TBH it might be nice to 'as a framework' factor this in a way to allow ONE hard-coded attribute to be equivalent.  We could pick something harmless (like `deprecated`).

So this instead of "any attribute present" becomes "any unsupported/thought of attribute" (of which we've only thought of 1 harmless one).

THOUGH in the case of `deprecated`, we probably want to warn that the 2nd one is ignored...

so maybe... eh.

https://github.com/llvm/llvm-project/pull/132939