[cfe-dev] Inconsistent mangling of C++ unnamed types compiling source code vs compiling AST files

[Tom Honermann]

According to the Itanium C++ ABI, The unnamed structures nested in 
struct S below should be assigned the names 'N1SUt_' and 'N1SUt0_' 
respectively.  And indeed, Clang versions 3.3 and newer do assign these 
names when compiling the source directly.  However, when the source is 
first emitted to a .ast file and then compiled, both of the unnamed 
structures are assigned the same mangled name (presumably because 
mangling discriminators are not being preserved in the .ast file).  For 
the test case below, this results in both instantiations of the tf() 
template function receiving the same name and one being discarded.

I tested this behavior with Clang versions 3.3 through trunk (pulled 
tonight).  Clang 3.2 produces consistent behavior, though the names 
generated in that release don't conform to the Itanium ABI.

Warnings regarding the use of an unnamed type as a template argument are 
issued when compiling the test case below.  This test case was contrived 
to demonstrate that both unnamed types are being assigned the same name.

$ cat t.cpp
struct S {
   struct {
     int i;
   } s1;
   struct {
     int i;
   } s2;
} s;

template<typename T>
void tf(T t) {}

void f() {
   tf(s.s1);
   tf(s.s2);
}

$ clang -c t.cpp
...

$ nm t.o
0000000000000000 T _Z1fv
0000000000000000 W _Z2tfIN1SUt0_EEvT_
0000000000000000 W _Z2tfIN1SUt_EEvT_
0000000000000000 B s

$ clang -emit-ast t.cpp
...

$ clang -c t.ast

$ nm t.o
0000000000000000 T _Z1fv
0000000000000000 W _Z2tfIN1SUt_EEvT_
0000000000000000 B s

Tom.