[clang] [Clang][C++23] Implement P2448R2: Relaxing some constexpr restrictions (PR #77753)

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``````````bash
git-clang-format --diff dc974573a8a2364f24ce69c75ad80ab30753fe9a 699f47bba02012523e1862f9b15ce9de1d7511b5 -- clang/test/SemaCXX/cxx23-invalid-constexpr.cpp clang/lib/Sema/SemaDeclCXX.cpp clang/test/AST/Interp/cxx23.cpp clang/test/AST/Interp/literals.cpp clang/test/AST/Interp/shifts.cpp clang/test/CXX/basic/basic.types/p10.cpp clang/test/CXX/dcl.dcl/dcl.spec/dcl.constexpr/dtor.cpp clang/test/CXX/dcl.dcl/dcl.spec/dcl.constexpr/p3-2b.cpp clang/test/CXX/dcl.dcl/dcl.spec/dcl.constexpr/p3.cpp clang/test/CXX/dcl.dcl/dcl.spec/dcl.constexpr/p4.cpp clang/test/CXX/dcl.dcl/dcl.spec/dcl.constexpr/p5.cpp clang/test/CXX/dcl.dcl/dcl.spec/dcl.constexpr/p6.cpp clang/test/CXX/dcl.decl/dcl.fct.def/dcl.fct.def.default/p2.cpp clang/test/CXX/drs/dr13xx.cpp clang/test/CXX/drs/dr14xx.cpp clang/test/CXX/drs/dr16xx.cpp clang/test/CXX/drs/dr6xx.cpp clang/test/CXX/expr/expr.const/p2-0x.cpp clang/test/CXX/expr/expr.const/p5-26.cpp clang/test/CXX/special/class.copy/p13-0x.cpp clang/test/PCH/cxx11-constexpr.cpp clang/test/SemaCXX/builtin_vectorelements.cpp clang/test/SemaCXX/constant-expression-cxx11.cpp clang/test/SemaCXX/constant-expression-cxx14.cpp clang/test/SemaCXX/constant-expression-cxx2b.cpp clang/test/SemaCXX/constexpr-function-recovery-crash.cpp clang/test/SemaCXX/cxx1z-constexpr-lambdas.cpp clang/test/SemaCXX/cxx2a-consteval.cpp clang/test/SemaCXX/deduced-return-type-cxx14.cpp clang/test/SemaCXX/ms-constexpr-invalid.cpp clang/test/SemaCXX/ms-constexpr.cpp clang/test/SemaCXX/sizeless-1.cpp
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View the diff from clang-format here.
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``````````diff
diff --git a/clang/lib/Sema/SemaDeclCXX.cpp b/clang/lib/Sema/SemaDeclCXX.cpp
index aa49dc73e1..20c8c3ef6c 100644
--- a/clang/lib/Sema/SemaDeclCXX.cpp
+++ b/clang/lib/Sema/SemaDeclCXX.cpp
@@ -7881,8 +7881,8 @@ bool Sema::CheckExplicitlyDefaultedSpecialMember(CXXMethodDecl *MD,
               << CSM << MD->isConsteval();
         }
     // FIXME: Explain why the special member can't be constexpr.
-    if (!getLangOpts().CPlusPlus23)
-      HadError = true;
+        if (!getLangOpts().CPlusPlus23)
+          HadError = true;
   }
 
   if (First) {

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https://github.com/llvm/llvm-project/pull/77753