Most of the cases were where a C++ file was being compiled with the C substitution.
There were a few cases of the opposite though.
LLDB seems to be the only real culprit in the LLVM codebase for these mismatches.
Rest of the LLVM presumably sticks at least language-specific options in the common substitutions
making the mistakes immediately apparent.
I found these by using Clang frontend configuration files containing language-specific options for
both C and C++ (e.g. `-std=c2y` and `-std=c++26`).
This still fails on Windows because the default language for the C++
frame is not the same as on Unix.
Also remove the LLD requirement, since we're not relying on DWARF here.
Otherwise debug-info is stripped, which influences the language of the
current frame.
Also, set explicit breakpoint because Windows seems to not obey the
debugtrap.
Log from failing test on Windows:
```
(lldb) command source -s 0 'lit-lldb-init-quiet'
Executing commands in 'D:\test\lit-lldb-init-quiet'.
(lldb) command source -C --silent-run true lit-lldb-init
(lldb) target create "main.out"
Current executable set to 'D:\test\main.out' (x86_64).
(lldb) settings set interpreter.stop-command-source-on-error false
(lldb) command source -s 0 'with-target.input'
Executing commands in 'D:\test\with-target.input'.
(lldb) expr blah
^
error: use of undeclared identifier 'blah'
note: Falling back to default language. Ran expression as 'Objective C++'.
(lldb) run
Process 29404 launched: 'D:\test\main.out' (x86_64)
Process 29404 stopped
* thread #1, stop reason = Exception 0x80000003 encountered at address 0x7ff7b3df7189
frame #0: 0x00007ff7b3df718a main.out
-> 0x7ff7b3df718a: xorl %eax, %eax
0x7ff7b3df718c: popq %rcx
0x7ff7b3df718d: retq
0x7ff7b3df718e: int3
(lldb) expr blah
^
error: use of undeclared identifier 'blah'
note: Falling back to default language. Ran expression as 'Objective C++'.
(lldb) expr -l objc -- blah
^
error: use of undeclared identifier 'blah'
note: Expression evaluation in pure Objective-C not supported. Ran expression as 'Objective C++'.
(lldb) expr -l c -- blah
^
error: use of undeclared identifier 'blah'
note: Expression evaluation in pure C not supported. Ran expression as 'ISO C++'.
```
Depends on:
* https://github.com/llvm/llvm-project/pull/162050
Since it's a 'Note' diagnostic it would only show up when expression
evaluation actually failed. This helps with expression evaluation
failure reports in mixed language environments where it's not quite
clear what language the expression ran as. It may also reduce confusion
around why the expression evaluator ran an expression in a language it
wasn't asked to run (a softer alternative to what I attempted in
https://github.com/llvm/llvm-project/pull/156648).
Here are some example outputs:
```
# Without target
(lldb) expr blah
note: Falling back to default language. Ran expression as 'Objective C++'.
# Stopped in target
(lldb) expr blah
note: Ran expression as 'C++14'.
(lldb) expr -l objc -- blah
note: Expression evaluation in pure Objective-C not supported. Ran expression as 'Objective C++'.
(lldb) expr -l c -- blah
note: Expression evaluation in pure C not supported. Ran expression as 'ISO C++'.
(lldb) expr -l c++14 -- blah
note: Ran expression as 'C++14'
(lldb) expr -l c++20 -- blah
note: Ran expression as 'C++20'
(lldb) expr -l objective-c++ -- blah
note: Ran expression as 'Objective C++'
(lldb) expr -l D -- blah
note: Expression evaluation in D not supported. Falling back to default language. Ran expression as 'Objective C++'.
```
I didn't put the diagnostic on the same line as the inline diagnostic
for now because of implementation convenience, but if reviewers deem
that a blocker I can take a stab at that again.
Also, other language plugins (namely Swift), won't immediately benefit
from this and will have to emit their own diagnistc. I played around
with having a virtual API on `UserExpression` or `ExpressionParser` that
will be called consistently, but by the time we're about to parse the
expression we are already several frames deep into the plugin. Before
(and at the beginning of) the generic `UserExpression::Parse` call we
don't have enough information to notify which language we're going to
parse in (at least for the C++ plugin).
rdar://160297649
rdar://159669244
