[clang] Fix crash on defaulted comparison recovery in Sema and AST (PR #210668)

[Oliver Hunt via cfe-commits]

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@@ -0,0 +1,16 @@
+// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify %s
+
+namespace std {
+  struct strong_ordering {
+    int n;
+    static const strong_ordering less, equal, greater;
+  };
+  constexpr strong_ordering strong_ordering::less{-1}, strong_ordering::equal{0}, strong_ordering::greater{1};
+  struct reverse_compare {
+    constexpr explicit reverse_compare(std::strong_ordering o) : n(-o.n) {} // expected-error {{member initializer 'n' does not name a non-static data member or base class}}
+  } // expected-error {{expected ';' after struct}}
+  struct B { // expected-note 2{{definition of 'std::B' is not complete until the closing '}'}}
+    friend reverse_compare operator<=>(const B&, const B&) = default; // expected-note {{while rewriting comparison as call to 'operator<=>' declared here}}
+    static_assert(B{1, 2, 3, 4, 5} >= B{1, 2, 0, 40, 5}); // expected-error 2{{invalid use of incomplete type 'B'}} expected-error {{invalid operands to binary expression ('reverse_compare' and 'int')}}
+  } // expected-error {{expected ';' after struct}}
+}
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ojhunt wrote:

I'm unsure that this test is minimal. Could you reduce it and extend it to cover related code?

https://github.com/llvm/llvm-project/pull/210668