[clang-format] Fixes crash in QualifierOrder

[MyDeveloperDay]

Summary

Prevent Null Token from causing a crash

Fixes: https://github.com/llvm/llvm-project/issues/56696

[owenpan]

It seems rG492cb7bf9164 also fixed this.

[MyDeveloperDay]

Maybe I didn’t rebase correctly, let me retry and I’ll abandon if necessary, Thanks @owenpan for the pointer

clang/lib/Format/QualifierAlignmentFixer.cpp

if (Qual && Qual->is(QualifierType))
       break;
   }
 
   if (!Qual)
     return Tok;
 
   if (LastQual && Qual != LastQual && Qual->is(QualifierType)) {
     rotateTokens(SourceMgr, Fixes, Tok, Qual, /*Left=*/true);
+    if (!Qual->Next) {
+      return Qual;
+    }
     Tok = Qual->Next;
   } else if (Tok->startsSequence(tok::identifier, QualifierType)) {
     if (Tok->Next->Next && Tok->Next->Next->isOneOf(tok::identifier, tok::star,
                                                     tok::amp, tok::ampamp)) {
       // Don't swap `::iterator const` to `::const iterator`.
       if (!Tok->Previous ||
           (Tok->Previous && !Tok->Previous->is(tok::coloncolon))) {
         rotateTokens(SourceMgr, Fixes, Tok, Tok->Next, /*Left=*/true);
         Tok = Tok->Next;
       }
     } else if (Tok->startsSequence(tok::identifier, QualifierType,
                                    TT_TemplateCloser)) {
       FormatToken *Closer = Tok->Next->Next;
       rotateTokens(SourceMgr, Fixes, Tok, Tok->Next, /*Left=*/true);
       Tok = Closer;
     }
   }
-  if (Tok->is(TT_TemplateOpener) && Tok->Next &&
+  if (Tok && Tok->is(TT_TemplateOpener) && Tok->Next &&
       (Tok->Next->is(tok::identifier) || Tok->Next->isSimpleTypeSpecifier()) &&
       Tok->Next->Next && Tok->Next->Next->is(QualifierType))
     rotateTokens(SourceMgr, Fixes, Tok->Next, Tok->Next->Next, /*Left=*/true);
-  if (Tok->startsSequence(tok::identifier) && Tok->Next) {
+  if (Tok && Tok->startsSequence(tok::identifier) && Tok->Next) {
     if (Tok->Previous &&
         Tok->Previous->isOneOf(tok::star, tok::ampamp, tok::amp))
       return Tok;
     const FormatToken *Next = Tok->Next;
     // The case  `std::Foo<T> const` -> `const std::Foo<T> &&`
     while (Next && Next->isOneOf(tok::identifier, tok::coloncolon))
       Next = Next->Next;
     if (Next && Next->Previous &&

clang/unittests/Format/QualifierFixerTest.cpp

verifyFormat("template <typename T>\n"
                "  requires Concept<T const>\n"
                "void f();",
                Style);
   verifyFormat("TemplateType<const T> t;", "TemplateType<T const> t;", Style);
   verifyFormat("TemplateType<const Container> t;",
                "TemplateType<Container const> t;", Style);
 }
 
+TEST_F(QualifierFixerTest, ConstVolatile) {
+  FormatStyle Style = getLLVMStyle();
+  Style.QualifierAlignment = FormatStyle::QAS_Left;
+  Style.QualifierOrder = { "const", "volatile", "type" };
+
+  verifyFormat("bool empty() const volatile { return true; }",
+               "bool empty() const volatile { return true; }",
+               Style);
+}
+
 } // namespace format
 } // namespace clang