The following code is miscompiled: ``` // File a.d module a; private struct A { int a; } extern(C++) int foo(T)(T t) { return T.sizeof; } int return4() { A a; return foo(a); } ``` ``` // File b.d module b; import a; private struct A { int[100] a; } void main() { import std.stdio; A a; writeln(foo(a), " =? ", A.sizeof); writeln(return4()); } ``` Compile and run: > dmd a.d b.d > ./a 4 =? 400 4 The problem is that in module a, `foo!(a.A)(a.A)` is mangled the same as module b's `foo!(b.A)(b.A)`, because the extern(C++) function name mangler does not use module name prefix for D types. That is, the mangler is mangling `foo!(A)(A)` instead of `foo!(a.A)(a.A)` and `foo!(b.A)(b.A)`. The two function symbols are merged by the linker (instead of erroring on multiple definition), because the symbols come from templates and merging is required behavior. --> only one of the two _different_ definitions survive, and hence miscompilation results. The fix: use the full D type for mangling.
> The fix: use the full D type for mangling. Assuming you suggest mangling `module mod; extern(C++) class C {};` as `mod::C`, that would prevent all interop with external C++ symbols in the root namespace, as there is no root namespace in D. Likewise, you'd have to have a huge `std.d` file for the std::* symbols etc.
> Assuming you suggest mangling `module mod; extern(C++) class C {};` as `mod::C` No, of course not. I suggest mangling `module mod; class C {};` as `mod.C`. Currently we mangle extern(D) types as if they are extern(C++) types for extern(C++) templated functions. In the original example, we currently mangle `foo!(a.A)(a.A)` as `foo!(A)(A)`. Hence the miscompilation / clash with mangling of `foo!(b.A)(b.A)`.
(In reply to johanengelen from comment #2) > Currently we mangle extern(D) types as if they are extern(C++) types for > extern(C++) templated functions. Ah now I get it, I overlooked that the 2 structs are extern(D). [The function doesn't need to be templated.] Not sure if a dot can be used in typenames for C++ mangling. Also not sure how much code that would break (structs needing `extern(C++)` for C++ interop, whereas that's optional now).
> Not sure if a dot can be used in typenames for C++ mangling. Good point. `a.A` should not be mangled as `a::A` (using the C++ namespace divider), because that then would clash with C++ types. I think it is OK if the mangle can never be expressed in C++ when there are D types involved, because C++ cannot express the D types anyway. What I mean is, `foo!A` should be expressible in C++: ``` extern(C++) struct A {} extern(C++) int foo(T)(T t) {} ``` but doesn't have to in this code: ``` struct A {} extern(C++) int foo(T)(T t) {} ``` So C++ doesn't have to be able to express the mangled name, but it would definitely be nice if C++ demanglers can demangle it into something sensible. A dot is actually allowed in symbol names, so we can use it as part of the name? (i.e. pretend that the class name of A is "a.A".)
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