abi - Binary compatibility between VS2017 and VS2015

Question

This SO post: Is Visual-C++-2017 binary compatible with VC++-2015? clearly says that VS 2017 is binary compatible with VS 2015. It even looks like the official position.

My question is, in the past, I distinctly remember running into linker errors (I do not recall the specific set of errors) every time I try to link in a static library that was compiled with a different version of MSVC into an EXE that is being built with a newer version of MSVC.

Yet, binary (in)compatibility sounds like something that will blow up in your face at runtime, not link time.

Can someone tell me if previous versions of MSVC did indeed producer linker errors on version mismatches? How was this accomplished?

EDIT

How does this affect static libraries built with WPO/LTCG turned on? I believe these produce intermediate object files (as opposed to COFF) and Microsoft did not guarantee the format of these intermediate files to remain stagnant across different versions of the compiler.

Answer 1

As I answered on the linked question, the v140 toolset in VS 2015 and the v141 toolset in VS 2017 are binary compatible. v141 was built as a member of the same "family" as all the updates to v140 (e.g., VS 2015 Update 1, 2, 3) were all in the same family. This was an intentional design decision that helps developers to move to a new version of VS without worrying about having to make changes in their source code.

VS 2017 can support multiple toolsets. The next toolset will not be binary compatible with v140/v141. But you'll still be able to install v141 as you move your code to be compatible with the new C++ features in the next toolset.

Note that we never have supported binary compatibility across major versions. You can't link a binary built with v140 and a binary built with v130, regardless of WPO/LTCG/etc. Yes, it often works--we try to minimize breaking changes in our libraries so often it is the case that linking some code across major versions doesn't hit any errors. But eventually you'll run into something that changed and you'll see an error.

As to whether you see a link error or a runtime error, that depends on the incompatible library API that you called. If the exported shape of the API changed--the name of the function, the number of parameters--then the linker will fail to find it. If the shape is the same but the behavior has changed, you can end up with a runtime failure.

--Andrew Pardoe, MSVC tools