<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/63138>63138</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            LibTooling: TypeName::getFullyQualifiedTypeName no longer respecting 'SuppressDefaultTemplateArgs' in PrintingPolicy
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          ansonmiu0214
      </td>
    </tr>
</table>

<pre>
    ### Context

The [`clang::TypeName::getFullyQualifiedName` class method](https://clang.llvm.org/doxygen/namespaceclang_1_1TypeName.html#aa956321ff24d3b3d1265b8ee4dc031c0) generates and returns the fully qualified name for the parametrized `type`, based on the parametrized AST context and [`PrintingPolicy`](https://clang.llvm.org/doxygen/structclang_1_1PrintingPolicy.html).

The `PrintingPolicy` is a record that "describes how types, statements, expressions, and declarations should be printed". One of the record fields is [`SuppressDefaultTemplateArgs`](https://clang.llvm.org/doxygen/structclang_1_1PrintingPolicy.html#a198855bf8d8d508ebb5f729616a1380a), which "[attempts] to suppress template arguments that match the default argument for the parameter".

### Problem

When using the Clang-**12** libTooling API, the `getFullyQualifiedTypeName` class method correctly respects the `SuppressDefaultTemplateArgs` flag in the `PrintingPolicy`:

- if we have some type `std::string`,
- the canonical type is resolved to be `std::basic_string<char, std::char_traits<char>, std::allocator<char>>`
- but calling `getFullyQualifiedTypeName` with `SuppressDefaultTemplateArgs` _enabled_ correctly prints `std::basic_string<char>`, because the trailing template arguments match the [defaults](https://en.cppreference.com/w/cpp/string/basic_string)

When using the latest Clang libTooling API, the behavior above is no longer the case - even with `SuppressDefaultTemplateArgs` enabled in the `PrintingPolicy` passed to the `getFullyQualifiedTypeName` class method, invoking this class method on `std::basic_string<char, std::char_traits<char>, std::allocator<char>>` incorrectly returns `std::basic_string<char, std::char_traits<char>, std::allocator<char>>`.

I've traced the regression back to first occurring in the Clang-13 release:

Clang version | `type` argument to `getFullyQualifiedTypeName` | `policy` argument to `getFullyQualifiedTypeName` | output
--- | --- | --- | ---
< 13 | `std::basic_string<char, std::char_traits<char>, std::allocator<char>>` | `SuppressDefaultTemplateArgs: 1` | `std::basic_string<char>`
\>= 13 | `std::basic_string<char, std::char_traits<char>, std::allocator<char>>`  | `SuppressDefaultTemplateArgs: 1` | `std::basic_string<char, std::char_traits<char>, std::allocator<char>>`
</pre>
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