HighCommander4 wrote:
Testcase for reference:
```c++
constexpr auto v = 0;
template <decltype(v) t> class c {};
```
This is `TSI->getType()`, i.e. the type of the NTTP as written in the source:
```
DecltypeType 0x55bf2ddb8fa0 'decltype(v)' sugar
|-DeclRefExpr 0x55bf2ddb8f80 'const int' lvalue Var 0x55bf2ddb8d30 'v' 'const
int' non_odr_use_unevaluated
`-QualType 0x55bf2ddb8ef1 'const int' const
`-AutoType 0x55bf2ddb8ef0 'int' sugar
`-BuiltinType 0x55bf2dd88930 'int'
```
It's a `DecltypeType` because that's how the type is written. The value of the
decltype is "the declared type of `v`", which is a `QualType` because `v` is
`const`, wrapping an `AutoType` because `v` is declared with `auto`, wrapping a
`BuiltinType` because `int` is what the `auto` is deduced to.
When computing the type that the NTTP's `getType()` will return, however,
`[temp.param]p5` needs to be obeyed: "The top-level cv-qualifiers on the
template-parameter are ignored when determining its type.", which happens
[here](https://searchfox.org/llvm/rev/4fb7bf3323ea54d929f3846e6cd28d9dda1b891c/clang/lib/Sema/SemaTemplate.cpp#1477-1479).
This requires removing the `DecltypeType` sugar, so that `getType()` is just:
```
AutoType 0x55bf2ddb8ef0 'int' sugar
`-BuiltinType 0x55bf2dd88930 'int'
```
This type will naturally return true for `getContainedAutoType()`, but the
original `DecltypeType` will not. This is
[deliberate](https://searchfox.org/llvm/rev/4fb7bf3323ea54d929f3846e6cd28d9dda1b891c/clang/include/clang/AST/TypeBase.h#2960-2965):
"This looks through declarators like pointer types, but not through decltype
or typedefs." (The intention is to find an `auto` type written at the
variable's declaration, not a referenced `auto` type that's written somewhere
else, as is the case here.)
Conclusions:
* The AST modeling here seems reasonable
* The patch produces the desired outcome (we do _not_ want this call to
produce a highlighting token for the `AutoType` returned by `D->getType()`,
because that's actually an `auto` type written in a different declaration)
https://github.com/llvm/llvm-project/pull/207323
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