[clang] [Clang][Sema] Fix crash when diagnosing near-match for 'constexpr' redeclaration in C++11 (PR #92452)

[Krystian Stasiowski via cfe-commits]

================
@@ -9203,15 +9203,15 @@ static NamedDecl *DiagnoseInvalidRedeclaration(
         << Idx << FDParam->getType()
         << NewFD->getParamDecl(Idx - 1)->getType();
     } else if (FDisConst != NewFDisConst) {
-      SemaRef.Diag(FD->getLocation(), diag::note_member_def_close_const_match)
-          << NewFDisConst << FD->getSourceRange().getEnd()
-          << (NewFDisConst
-                  ? FixItHint::CreateRemoval(ExtraArgs.D.getFunctionTypeInfo()
-                                                 .getConstQualifierLoc())
-                  : FixItHint::CreateInsertion(ExtraArgs.D.getFunctionTypeInfo()
-                                                   .getRParenLoc()
-                                                   .getLocWithOffset(1),
-                                               " const"));
+      auto DB = SemaRef.Diag(FD->getLocation(),
+                             diag::note_member_def_close_const_match)
+                << NewFDisConst << FD->getSourceRange().getEnd();
+      if (const auto &FTI = ExtraArgs.D.getFunctionTypeInfo(); !NewFDisConst)
+        DB << FixItHint::CreateInsertion(FTI.getRParenLoc().getLocWithOffset(1),
+                                         " const");
+      else if (SourceLocation ConstLoc = FTI.getConstQualifierLoc();
----------------
sdkrystian wrote:

I opted for the "return `SourceLocation()` if no `const` qualifier is present" approach because it doesn't require the caller to check `hasMethodTypeQualifiers()` prior to calling `getConstQualifierLoc()`. Makes it less prone to being the cause of future crashes :)

https://github.com/llvm/llvm-project/pull/92452