Fixes#163498
---
This PR addresses the issue of confusing diagnostics for lambdas with
init-captures appearing inside braced initializers.
Cases such as:
```cpp
S s{[a(42), &] {}};
```
were misparsed as C99 array designators, producing unrelated
diagnostics, such as `use of undeclared identifier 'a'`, and `expected
']'`
---
bb9bd5f263/clang/lib/Parse/ParseInit.cpp (L470)bb9bd5f263/clang/lib/Parse/ParseInit.cpp (L74)bb9bd5f263/clang/include/clang/Parse/Parser.h (L4652-L4655)24c22b7de6/clang/lib/Parse/ParseExprCXX.cpp (L871-L879)
The tentative parser now returns `Incomplete` for partially valid lambda
introducers, preserving the `lambda` interpretation and allowing the
proper diagnostic to be issued later.
---
Clang now correctly recognizes such constructs as malformed lambda
introducers and emits the expected diagnostic — for example,
“capture-default must be first” — consistent with direct initialization
cases such as:
```cpp
S s([a(42), &] {});
```
Currently a capture-default which is not the first element in the lambda-capture
is diagnosed with a generic expected variable name or 'this' in lambda capture
list, which is true but not very helpful.
If we don't have already parsed a capture-default then a lone "&" or "=" is
likely to be a misplaced capture-default, so diagnose it as such.
Differential Revision: https://reviews.llvm.org/D83681
Reviewed By: aaron.ballman
