[4.0] SILCombine: fix propagation of existential self into witness method call #12234
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In case of a mutating method call we replaced the self argument with the source of a copy_addr. This let the call modify the wrong self value. rdar://problem/34753633
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Explanation: This fixes a miscompile when a mutating method call is called for a concrete struct-type. In some cases the method call did not modify the correct self value.
Scope of Issue: I'm wondering why this problem was not noticed earlier. It's definitely a severe bug.
Risk: Low. The fix adds a bail-out condition for exactly that case.
Reviewed By: Roman
Testing: There is a regression test for it
Radar: rdar://problem/34753633