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When importing C declarations, one can come across a redeclaration due to two different clang modules textually including the same header. Previously, to decide visibility of a declaration from a particular module, the module in which it is redeclarated had to exactly match the one we saw before, so a struct defined in A.B.C might cause it to be hidden in A.X.Y, because we were already popping off submodule names on the "left hand side", so the module comparison was A != A.X.Y. If someone imports A.X.Y and expects that struct, they won't see it. Now, strip off submodules of the "right hand side", correctly comparing if the top-level modules match (A == A). This may have the effect of allowing more declarations to be visible than before but it prevents weird situations where struct typedefs can be hidden in overly complicated nested header includes normally found in OS system headers. rdar://problem/23588593