[PATCH] D124690: [clangd] add inlay hints for std::forward-ed parameter packs

[Sam McCall via Phabricator via cfe-commits]

sammccall added a comment.

In D124690#3492981 <https://reviews.llvm.org/D124690#3492981>, @upsj wrote:

> I will take a different approach to the matchers: For each CallExpr involving parameter packs, record the arguments (if they are a ParmVarDecl or stdForward(ParmVarDecl)) and the matching ParmVarDecl of the FunctionDecl in a map. Then I will compact that map to skip intermediate forwarding functions like emplace_back -> allocator::construct -> constructor and in a second pass resolve all previously unresolved forwarded parameters.

This makes sense to me.
This sounds more complicated, but if I'm reading correctly this multi-level forwarding isn't handled in the current version of the patch either?
At some point we'll want to extract this logic out so it can be used by hover etc, but not yet.



================
Comment at: clang-tools-extra/clangd/unittests/InlayHintTests.cpp:255
+  )cpp",
+                       ExpectedHint{"a: ", "param"});
+}
----------------
upsj wrote:
> I haven't been able to figure out why, but for some reason the `CXXConstructExpr` and `CXXTemporaryObjectExpr` are not being picked up by the `RecursiveASTVisitor`, while in a similar situation below, the `CallExpr` gets picked up by `VisitCallExpr`.
The AST here of the instantiated bar looks like
```
-FunctionDecl <line:5:1, line:7:1> line:5:4 used bar 'S *(int &&)'
  |-TemplateArgument type 'S'
  |-TemplateArgument pack
  | `-TemplateArgument type 'int'
  |-ParmVarDecl <col:8, col:18> col:18 used args 'int &&'
  `-CompoundStmt <col:24, line:7:1>
    `-ReturnStmt <line:6:3, col:23>
      `-CXXNewExpr <col:10, col:23> 'S *' Function 'operator new' 'void *(unsigned long)'
        `-CXXConstructExpr <col:14, col:23> 'S':'S' 'void (int)' list
          `-ImplicitCastExpr <col:16> 'int':'int' <LValueToRValue>
            `-DeclRefExpr <col:16> 'int':'int' lvalue ParmVar 'args' 'int &&'
```

So there's no CXXTemporaryObjectExpr (the value here is a pointer), but should be a CXXConstructExpr. 

Are you sure you're traversing bar's instantiation/specializaion, as opposed to its templated/generic FunctionDecl? The AST of the templated FunctionDecl indeed has an InitListExpr in place of the CXXConstructExpr because it's not resolved yet.


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