<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/85767>85767</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            CTAD: missing aggregate deduction guides for alias templates
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            c++20,
            clang:frontend
      </td>
    </tr>

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

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

<pre>
    The following code is an example for aggregate deduction guides.

```
template <typename T1, typename T2>
struct Foo { 
    T1 t1;
    T2 t2;
};

template<typename X, typename Y>
using A = Foo<X, Y>;

A a = {1, 2};
```

The related AST `DeductionGuideDecl` nodes look like:

```
|-FunctionTemplateDecl 0x5633ce70b420 <line:7:1, line:8:19> col:1 implicit <deduction guide for A>
| |-TemplateTypeParmDecl 0x5633ce70ad90 <line:7:10, col:19> col:19 typename depth 0 index 0 X
| |-TemplateTypeParmDecl 0x5633ce70ae18 <col:22, col:31> col:31 typename depth 0 index 1 Y
| `-CXXDeductionGuideDecl 0x5633ce70b360 <col:1, line:8:19> col:1 implicit <deduction guide for A> 'auto () -> Foo<type-parameter-0-0, type-parameter-0-1>'
|-FunctionTemplateDecl 0x5633ce70c458 <line:7:1, line:8:19> col:1 implicit <deduction guide for A>
| |-TemplateTypeParmDecl 0x5633ce70b480 <line:7:10, col:19> col:19 typename depth 0 index 0 X
| |-TemplateTypeParmDecl 0x5633ce70b508 <col:22, col:31> col:31 typename depth 0 index 1 Y
| `-CXXDeductionGuideDecl 0x5633ce70c390 <col:1, line:8:19> col:1 implicit <deduction guide for A> 'auto (Foo<type-parameter-0-0, type-parameter-0-1>) -> Foo<type-parameter-0-0, type-parameter-0-1>'
| `-ParmVarDecl 0x5633ce70b8e8 <line:2:8> col:8 'Foo<type-parameter-0-0, type-parameter-0-1>'
`-FunctionTemplateDecl 0x5633ce70c748 <line:1:1, line:2:8> col:8 implicit <deduction guide for Foo>
 |-TemplateTypeParmDecl 0x5633ce6eac18 <line:1:11, col:20> col:20 referenced typename depth 0 index 0 T1
  |-TemplateTypeParmDecl 0x5633ce6eacd8 <col:24, col:33> col:33 referenced typename depth 0 index 1 T2
 |-CXXDeductionGuideDecl 0x5633ce70c678 <line:2:8> col:8 implicit <deduction guide for Foo> 'auto (T1, T2) -> Foo<T1, T2>'
  | |-ParmVarDecl 0x5633ce70c588 <col:8> col:8 'T1'
  | `-ParmVarDecl 0x5633ce70c608 <col:8> col:8 'T2'
  `-CXXDeductionGuideDecl 0x5633ce70ce68 <col:8> col:8 implicit used <deduction guide for Foo> 'auto (int, int) -> Foo<int, int>' implicit_instantiation
    |-TemplateArgument type 'int'
    | `-BuiltinType 0x5633ce699230 'int'
    |-TemplateArgument type 'int'
 | `-BuiltinType 0x5633ce699230 'int'
    |-ParmVarDecl 0x5633ce70cad8 <col:8> col:8 'int'
    `-ParmVarDecl 0x5633ce70cb88 <col:8> col:8 'int'
```

We have issues:
- we don't have the aggregate deduction guide decl for the alias `A`, we should synthesize one from the Foo's `auto (T1, T2) -> Foo<T1, T2>`;
- the template argument deduction is performed on deduction guide for `Foo`, which should be performed on use this synthesized guide decl (the instantiation should not be performed on deduction guide for `A`);

</pre>
<img width="1px" height="1px" alt="" src="http://email.email.llvm.org/o/eJzEl82O2zYUhZ-G3lx4QJGyKC288E_cbYEa7WRVUNKVxUYiDZFKZvr0BekfyXY84yRtCgS2RZPn3Hv4ZUxKa9VOI87JbEkYq80nVJowRmbriexdbbr5YWySm_J1vq0RKtM05ovSOyhMiaAsSA34Itt947_sQO52He6kQyix7AunjIZdr0q0T4SuCV0cXxN6_BceHbb7xi8ifOVe96hli7CNCFvB8MgI_3CYbl3XFw42xgARSzgMAgBsI3AR4cvRCAPHziNErIfPF85j4-cL349n2976xhdA-Np7E74KM8OES9UFyDCLiGVogl0YX_Z-ePXhdugrKWHx2xZIQtenAH_x-a2xaEhCQZsSLTTGfIJGfULCF2_kSsRquul1UNkeO_VCQF9mCecFCprHjPrYG6W9mCB8EUo-Pqf-OSP8AxSm8Z9BtftGFcr5RVd7HAhYnPMiYgW-gpPz9nWPv8quvapAltlNBdSXcHQcu2fDvpS4dzVQULrEF6Dw_G2mGKXe9KDL2ODHo8GPR_f8Ivg48kvodPX8fLth45x5QgfDH48YCBOydwYISwnLYOrHDlT6kqd72ckWHXZTOqUnoC9GfZ-EiYdJKeJZ-v-Sksfpzycln9GfS0rBs_-MlO8A5N9AK_Tts_1ddtfxpjimih3aPHWY-uJ_wDqh71It4rF_dBX5TT3vJR6KPWL9Pl4JyiK68Y8GxhgdzBmFDivsUBdY3sd7G51-_h6yL8d0xyO6-Yhu_oBz5H-ih77fxTwRb278Y0GP4T4cGLbsitnz8AiMkE0o8-tUFrN0FMs1kF7xUugu3kVC3xJiY6FH_jZgck_unFdvsXw8NKWdjye8XcQ2-iIEdzb4U2nrpHZKevHhqDWmbdHt-ha1C6x4u6AmLiaHhpe9apzSns6ByixjnN5Z9aDF9-vf2UdZvrGPNzr3ecjfAmus89Vj4h8ItfzsD962R3s-_E3hC0JpNGHCHSa4Gu-fxaH0RXkewrRGSetLDpZs5bVsbfqmBPuqXY1W_Y1gNELVmTYs8YQwERZ9w3--hJ7PwNMgcz72y9NeDoUqC3vsKtO1WILRNy346klCvcux6loV9anwHC9X99ZHouyoo3KcBWGpL-iC7JOWNu5G7041hwSzc5uTcs7LjGdygvNIRDSN4zQRk3qeJ5Vk2SwXVSWrNJcxFVLiTCQVz0VOcaLmjLKY8iiLaBxR8SQFreJIyCiu0ryiMYkptlI1T03zuX0y3W4SmJinM5GISSNzbOzxYlcQtiRsySgJpxc_0ki9I3xRdUY71OXx0tfNvdg073eWxLRR1tlB3inX4Hy1XawJX0CrbLgN3b_vHa6Dga3TPttJ3zXz2rl9QJdtCNvslKv7_KkwLWEbb3Z8m-478xcWjrDNEXa2Cb39EwAA__8PA10Q">