ayzhao added inline comments.
================
Comment at: clang/lib/Sema/SemaExprCXX.cpp:1582-1596
+ MultiExprArg ExprsToPass;
+ if (Exprs.size() == 1 && isa<CXXParenListInitExpr>(Exprs[0])) {
+ // C++20 [expr.static.cast]p4:
+ // An expression E can be explicitly converted to a type T...if T is an
+ // aggregate type ([dcl.init.aggr]) having a first element x and there is
+ // an implicit conversion sequence from E to the type of x
+ //
----------------
rsmith wrote:
> The input to this function should be a syntactic initializer such as a
> `ParenListExpr`, not an already-type-checked semantic initializer such as a
> `CXXParenListInitExpr`. The right place to do this unwrapping is in
> `TreeTransform::TransformInitializer`, where we unwrap various other kinds of
> semantic initializer.
> The right place to do this unwrapping is in
> `TreeTransform::TransformInitializer`, where we unwrap various other kinds of
> semantic initializer.
So the `CXXParenListInitExpr` for `S` never gets passed to
`TreeTransform::TransformInitializer` - rather,
`TransformCXXParenListInitExpr(...)` calls `TransformExprs(...)` on its
subexpressions. If the subexpression happens to be a `CXXConstructExpr`, then
`TransformCXXConstructExpr` will call `TransformInitializer(...)`, which is
[why we saw the CXXConstructExpr getting
deleted](https://github.com/llvm/llvm-project/blob/4c483a046d2ff29ec2fd5bad6305f97424a2b880/clang/lib/Sema/TreeTransform.h#L13030-L13036)
Interestingly enough, if we initialize `S` with a braced init-list (i.e.
`S{(V&&) Vec}`), we never end up calling `TransformCXXConstructExpr(...)` since
[we revert to the syntatic
form.](https://github.com/llvm/llvm-project/blob/4c483a046d2ff29ec2fd5bad6305f97424a2b880/clang/lib/Sema/TreeTransform.h#L11584-L11585).
> The input to this function should be a syntactic initializer such as a
> `ParenListExpr`, not an already-type-checked semantic initializer such as a
> `CXXParenListInitExpr`.
We can probably do this by returning a `ParenListExpr` in
`TransformCXXParenListInitExpr(...)`. This does cause some problems though. If
we use a `ParenListExpr`, how would we know when to bail out in line 1551
(`!isa<InitListExpr>(Exprs[0]) && !isa<CXXParenListInitExpr>(Exprs[0])`)?
Surely, not every instance of a `ParenListExpr` would be caused by a
`CXXParenListInitExpr`, and if this were the case, that would be a pretty
brittle assumption. Furthermore, we never create a `ParenListExpr` for a
corresponding `CXXParenListInitExpr`.
As an alternative, we could probably do something similar to `InitListExpr`,
which has separate semantic and syntactic forms. Would that work?
Also, in general, are the `TransformFooExpr(...)` functions supposed to return
semantic or syntactic results? I seem to see a mix of both...
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D146465/new/
https://reviews.llvm.org/D146465
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