I think you're saying that a and b are actually typedefs to the same type. Yes, in that case I believe C++ compilers are smart enough to share code.
I think GP is talking about something different. Let's say you have two completely unrelated types `a` and `b`, but, for whatever reason, `foo<a>` and `foo<b>` happen to compile to exactly the same machine code. (For a concrete example, say `a` and `b` have the same size and alignment requirements, and say `foo` is `std::vector::reserve`.) Then either the compiler or the linker can “merge” the duplicated code for `foo<a>` and `foo<b>`.
Example: You can have 2 struct types that each only contain an int with the same member name. They would [likely] have the same binary representation. You could then create 2 vector types specialized for each struct type. The code for both vectors would handle the data in the same way - but the compiler would view the vector types as _STRICTLY_ separate types.
I do not think this code would be uniquely shared - the compiler would generate code for both vector types.
It's just a theory I have that this is low-hanging fruit to reduce C++ binary size.
You're less likely to define the same structure and methods of a type in exactly the same way as someone else in C because we don't have the STL (ofc). So it's probably a worse issue in C having a hundred different implementations of a linked list or some array type across your system in each program.
I just think there's an opportunity to aggressively dedupe object code for "representation-identical" types in C++.
Someone further up the chain mentioned this sort of deduplication might be done by the linker, but the code for the duplicated type would still exist in the binary before linking.. An old but probably negligible complaint about C++ is that it creates larger binaries than if one were writing C. I think this is something overlooked because C++ has a focus on type strictness - perhaps the linker has a better way of seeing through abstractions..
I've identified where I think a problem lies.. now I just have to find proof it :3
Unrelated: I wonder if identical libraries that exist in separate filesystem trees (like in Docker) are deduped in memory, or if it's done by inode.