Let's CC Jason.
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Hello,
I've been investigating a bug in gcc I came across recently and after some
difficulties, I have produced a patch that actually fixes that behavior.
However, I don't think the patch is very good and I would really
appreciate your help in making it better.
The problem is that when a const(expr) variable is used in a lambda, it
yields garbage values in gcc when the initializer is dependent. Moreover,
it should not be captured if it fulfills the requirements of a constant
expression (at least that's how I understand the definition of "odr-used"
along with the definition of default captures). finish_id_expression did
return VAR_DECL for the corresponding automatic variable in that case, but
didn't create the capture.
The idea of the fix is to postpone the decision whether or not to capture
later to the template instantiation. This is because until then, we cannot
know if the variable fulfills the conditions for a constant expression or
not. So the patch assumes that if the constant value is a VAR_DECL (which
is what I've found to happen in the debugger, although I'm not sure this
is the only case) and we are in a template declaration, it just leaves the
id expression there as is.
Then the patch fiddles with the capture list in template instantiation
code. Because new captures can be added while processing the template, we
need to concatenate them with the original list from the template, plus
there's a small hack to avoid producing the lambda-to-function-pointer
conversion operator while we still don't have the final capture list.
I have produced two tests (in the patch) and ran the testsuite in the gcc
subdirectory successfully. I've also tested the behavior on a testcase in
the attached test.cpp file (it should print ok with all possible
combinations of #defined macros).
I'd be grateful on your comments on this, especially
- whether the approach taken is a good one, and
- whether my assumptions about what's going on in the code (expressed
either in this mail and in the comments in code) are true
Regards
Jiri Palecek
gcc.diff
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test.cpp
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Dodji
