[llvm-bugs] [Bug 39607] New: clang-cl doesn't emit LF_NESTTYPE records for nested template instantiations

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=39607

            Bug ID: 39607
           Summary: clang-cl doesn't emit LF_NESTTYPE records for nested
                    template instantiations
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: DebugInfo
          Assignee: rnk at google.com
          Reporter: zturner at google.com
                CC: aleksandr.urakov at jetbrains.com,
                    jdevlieghere at apple.com, llvm-bugs at lists.llvm.org,
                    mosescu at google.com

struct U {
  template<typename Param>
  struct V { };
};

template<typename T>
void foo() {
  T t;
}

int main(int argc, char **argv) {
  foo<U>();
  foo<U::V<int>>();
  return 0;
}

MSVC will output this in the field list for U.


  0x1004 | LF_STRUCTURE [size = 48] `U::V<int>`
           unique name: `.?AU?$V at H@U@@`
           vtable: <no type>, base list: <no type>, field list: <no type>
           options: forward ref (-> 0x1008) | has unique name | is nested,
sizeof 0
  0x1005 | LF_FIELDLIST [size = 20]
           - LF_NESTTYPE [name = `?$V at H`, parent = 0x1004]
  0x1006 | LF_STRUCTURE [size = 32] `U`
           unique name: `.?AUU@@`
           vtable: <no type>, base list: <no type>, field list: 0x1005
           options: contains nested class | has unique name, sizeof 1

We do not output this LF_NESTTYPE at all.  This is useful when parsing debug
info in order to be able to reconstruct the DeclContext hierarchy to build an
AST from the debug info.  Without this, we won't be able to figure out what
DeclContext to put these nested template instantiations into.

With it, the algorithm is:

1) We found an LF_STRUCTURE with the contains nested class flag set.
2) We walk it's field list, find the LF_NESTTYPE.
3) It points to 0x1004, so we load the unique name for record 0x1004, call this
`actual`
4) Make a copy of the unique name from record 0x1006, insert the nested type at
offset 4, set offset 3 to actual[3], and compare the strings.
5) If they're the same, We create a definition of U::V<int> inside of U,
otherwise it's an alias (e.g. template<typename T> using V<T> = {0x1004});

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