[Clang] Fix lambda CheckForDefaultedFunction(...) so that it checks the CXXMethodDecl is a special member function before attempting to call DefineDefaultedFunction(...)

[shafik]

Summary

In Sema::CheckCompletedCXXClass(...) It used a lambda CheckForDefaultedFunction the CXXMethodDecl passed to CheckForDefaultedFunction may not be a special member function and so before attempting to apply functions that only apply to special member functions it needs to check. It fails to do this before calling DefineDefaultedFunction(...). This PR adds that check and test to verify we no longer crash.

This fixes https://github.com/llvm/llvm-project/issues/57431

[aaron.ballman]

LGTM aside from a minor test formatting nit. Please be sure to also write a release note for the fix.

clang/test/SemaCXX/constant-expression-cxx2a.cpp

1478–1482

[shafik]

• Fixed formatting in test
• Added release note

[shafik]

clang/lib/Sema/SemaDeclCXX.cpp

6957–6958

I realized that based on the constraints on the functions called in DefineDefaultedFunction we also should be checking isDeleted() as well but based on the way I drafted this PR I should just do a follow-up.

clang/docs/ReleaseNotes.rst

 - Fix a crash when generating code coverage information for an
   ``if consteval`` statement. This fixes
   `Issue 57377 <https://github.com/llvm/llvm-project/issues/57377>`_.
 - Fix assert that triggers a crash during template name lookup when a type was
   incomplete but was not also a TagType. This fixes
   `Issue 57387 <https://github.com/llvm/llvm-project/issues/57387>`_.
 - Fix a crash when emitting a concept-related diagnostic. This fixes
   `Issue 57415 <https://github.com/llvm/llvm-project/issues/57415>`_.
+- Fix a crash when attempting to default a virtual constexpr non-special member
+  function in a derived class. This fixes
+  `Issue 57431 <https://github.com/llvm/llvm-project/issues/57431>`_
 
 
 Improvements to Clang's diagnostics
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 - Clang will now correctly diagnose as ill-formed a constant expression where an
   enum without a fixed underlying type is set to a value outside the range of
   the enumeration's values. Due to the extended period of time this bug was
   present in major C++ implementations (including Clang), this error has the

clang/lib/Sema/SemaDeclCXX.cpp

if (!M->isInvalidDecl() && M->isExplicitlyDefaulted() &&
         // Define after any fields with in-class initializers have been parsed.
         DelayedDllExportMemberFunctions.push_back(M);
       }
     }
 
     // Define defaulted constexpr virtual functions that override a base class
     // function right away.
     // FIXME: We can defer doing this until the vtable is marked as used.
-    if (M->isDefaulted() && M->isConstexpr() && M->size_overridden_methods())
+    if (CSM != CXXInvalid && M->isDefaulted() && M->isConstexpr() &&
+        M->size_overridden_methods())
       DefineDefaultedFunction(*this, M, M->getLocation());
 
     if (!Incomplete)
       CheckCompletedMemberFunction(M);
   };
 
   // Check the destructor before any other member function. We need to
   // determine whether it's trivial in order to determine whether the claas

clang/test/SemaCXX/constant-expression-cxx2a.cpp

namespace PR45879 {
   // Only syntactic assignments change the active union member.
   constexpr bool g() { // expected-error {{never produces a constant expression}}
     C c = {.n = 1};
     c.a.operator=(B{2}.a); // expected-note 2{{member call on member 'a' of union with active member 'n' is not allowed in a constant expression}}
     return c.a.n == 2;
   }
   static_assert(g()); // expected-error {{constant expression}} expected-note {{in call}}
 }
+
+namespace GH57431 {
+class B {
+  virtual int constexpr f() = 0;
+};
+
+class D : B {
+  virtual int constexpr f() = default; // expected-error {{only special member functions and comparison operators may be defaulted}}
+};
+}