Created attachment 1349475 [details]
Test case

Description of problem:
When optimisations are enabled, g++ fails to compile an overridden virtual method which takes a variable number of arguments, if there is another base class with virtual functions before the one with the overridden function in the base clause.

Version-Release number of selected component (if applicable):
gcc-7.2.1-2.fc26

How reproducible:
Occurs every time.

Steps to Reproduce:
1. Download test case attachment to a file called test.cpp
2. run g++ -O1 -c test.cpp

Actual results:
Compilation fails with the output:
test.cpp:3:34: error: generic thunk code fails for method ‘virtual void<unnamed class>::_ZThn8_N3._01cEz(...)’ which uses ‘...’
 class : a, b { void c(...) {} } d;
                                  ^
Expected results:
An object file is produced with no error.

Additional info:
I think this is a regression and it previously worked on GCC 6.
It can be worked around by adding the attribute [[gnu::optimize("O0")]] to any offending function definitions.