[clang] [C++26] [Contracts] Initial parser support (PR #221139)

[Yihan Wang via cfe-commits]

================
@@ -2052,6 +2052,19 @@ class Declarator {
 
   Expr *PackIndexingExpr;
 
+public:
+  /// A function contract specifier whose predicate must be parsed after its
+  /// enclosing class is complete.
+  struct LateParsedContractSpecifier {
+    bool IsPost;
+    IdentifierInfo *ResultName = nullptr;
+    SourceLocation ResultNameLoc;
+    std::unique_ptr<CachedTokens> PredicateTokens;
+  };
+
+private:
+  SmallVector<LateParsedContractSpecifier, 2> LateParsedContractSpecifiers;
----------------
yronglin wrote:

IIUC, we should cleanup this when clearing Declarator states, the following code should be accepted, but clang rejected it with: `error: virtual specifier 'override' must appear before contract specifiers`:

```cpp
struct Base {
  virtual void g();
};

struct Derived : Base {
  void f() pre(true),
        g() __attribute__((unused)) override;
};
```

https://github.com/llvm/llvm-project/pull/221139