Here is a g++ error message that took me an embarrassingly long time to figure out:

In file included from /home/llvm-project/llvm/lib/IR/Constants.cpp:15:
/home/llvm-project/llvm/lib/IR/LLVMContextImpl.h:447:11: error: explicit specialization in non-namespace scope ‘struct llvm::MDNodeKeyImpl<llvm::DIBasicType>’
 template <> struct MDNodeKeyImpl<DIStringType> {
           ^

This is the code:

template <> struct MDNodeKeyImpl<DIStringType> {
  unsigned Tag;
  MDString *Name;
  Metadata *StringLength;
  Metadata *StringLengthExp;
  Metadata *StringLocationExp;
  uint64_t SizeInBits;
  uint32_t AlignInBits;
  unsigned Encoding;

This specialization isn’t materially different than the one that preceded it:

template <> struct MDNodeKeyImpl<DIBasicType> {
  unsigned Tag;
  MDString *Name;
  MDString *PictureString;
  uint64_t SizeInBits;
  uint32_t AlignInBits;
  unsigned Encoding;
  unsigned Flags;
  Optional<DIBasicType::DecimalInfo> DecimalAttrInfo;

  MDNodeKeyImpl(unsigned Tag, MDString *Name, MDString *PictureString,
               uint64_t SizeInBits, uint32_t AlignInBits, unsigned Encoding,
                unsigned Flags,
                Optional<DIBasicType::DecimalInfo> DecimalAttrInfo)
      : Tag(Tag), Name(Name), PictureString(PictureString),
        SizeInBits(SizeInBits), AlignInBits(AlignInBits), Encoding(Encoding),
        Flags(Flags), DecimalAttrInfo(DecimalAttrInfo) {}
  MDNodeKeyImpl(const DIBasicType *N)
      : Tag(N->getTag()), Name(N->getRawName()), PictureString(N->getRawPictureString()), SizeInBits(N->getSizeInBits()),
        AlignInBits(N->getAlignInBits()), Encoding(N->getEncoding()),
        Flags(N->getFlags(), DecimalAttrInfo(N->getDecimalInfo()) {}

  bool isKeyOf(const DIBasicType *RHS) const {
    return Tag == RHS->getTag() && Name == RHS->getRawName() &&
           PictureString == RHS->getRawPictureString() &&
           SizeInBits == RHS->getSizeInBits() &&
           AlignInBits == RHS->getAlignInBits() &&
           Encoding == RHS->getEncoding() && Flags == RHS->getFlags() &&
           DecimalAttrInfo == RHS->getDecimalInfo();
  }

  unsigned getHashValue() const {
    return hash_combine(Tag, Name, SizeInBits, AlignInBits, Encoding);
  }
};

However, there is an error hiding above it on this line:

        Flags(N->getFlags(), DecimalAttrInfo(N->getDecimalInfo()) {}

i.e. a single missing brace in the initializer for the Flags member, a consequence of a cut and paste during rebase that clobbered that one character, when adding a comma after it.

It turns out that the compiler was giving me a hint that something was wrong before this in the message:

error: explicit specialization in non-namespace scope

as it states that the scope is:

‘struct llvm::MDNodeKeyImpl’

which is the previous class definition. Inspection of the code made me think that the scope was ‘namespace llvm {…}’, and I’d gone looking for a rebase error that would have incorrectly terminated that llvm namespace scope. This is a classic example of not paying enough attention to what is in front of you, and going off looking based on hunches instead. I didn’t understand the compiler message, but in retrospect, non-namespace scope meant that something in that scope was incomplete. The compiler wasn’t smart enough to tell me that the previous specialization was completed due to the missing brace, but it did tell me that something was wrong in that previous specialization (which was explicitly named), and I didn’t look at that because of my “what the hell does that mean” reaction to the compilation error message.

In this case, I was building on RHEL8.3, which uses an ancient GCC toolchain. I wonder if newer versions of g++ fare better (i.e.: a message like “possibly unterminated brace on line …” would have been much nicer)? I wasn’t able to try with clang++ as I was building llvm+clang+lldb (V14), and had uninstalled all of the llvm related toolchain to avoid interference.