"In file included from" diagnostic prelude omitted incorrectly if preceding note from the same file

[smeenai]

a.cpp:

#include "b.h"
#include "c.h"

b.h:

void b1();
template <class T> void b2(T x, T y) { x + y; }

c.h:

void c() {
    b1(0);
    b2("0", "0");
}

Run the following:

$ clang -fsyntax-only a.cpp
In file included from a.cpp:2:
./c.h:2:5: error: no matching function for call to 'b1'
    b1(0);
    ^~
./b.h:1:6: note: candidate function not viable: requires 0 arguments, but 1 was provided
void b1();
     ^
./b.h:2:42: error: invalid operands to binary expression ('const char *' and 'const char *')
template <class T> void b2(T x, T y) { x + y; }
                                       ~ ^ ~
./c.h:3:5: note: in instantiation of function template specialization 'b2<const char *>' requested here
    b2("0", "0");
    ^
2 errors generated.

Note that the ./b.h:2:42: error: line is missing the In file included from prelude indicating the source of the include. This seems to occur when the previously generated note was from the same file the error is being generated from; if I change the b1(0) line to b3() instead, so that the first error doesn't have a note from b.h, then the second error does get the expected prelude.

I discovered this while debugging a script to parse Clang error output. I imagine that SARIF output will be a better option for that sort of work once it's stabilized, but it doesn't currently seem to include the "In file included from" information, which is useful for my purposes.

[shafik]

@AaronBallman do you have a feeling for why the diagnostic is failing in this way?

[AaronBallman]

I believe this behavior is on purpose to reduce the amount of diagnostic noise -- we silence include stacks from notes by default, but we have -fdiagnostics-show-note-include-stack as an option to add this extra information. With that, I get:

F:\source\llvm-project>llvm\out\build\x64-Debug\bin\clang.exe -fsyntax-only -fdiagnostics-show-note-include-stack "C:\Users\aballman\OneDrive - Intel Corporation\Desktop\a.cpp"
In file included from C:\Users\aballman\OneDrive - Intel Corporation\Desktop\a.cpp:2:
C:\Users\aballman\OneDrive - Intel Corporation\Desktop/c.h:2:5: error: no matching function for call to 'b1'
    b1(0);
    ^~
In file included from C:\Users\aballman\OneDrive - Intel Corporation\Desktop\a.cpp:1:
C:\Users\aballman\OneDrive - Intel Corporation\Desktop/b.h:1:6: note: candidate function not viable: requires 0
      arguments, but 1 was provided
void b1();
     ^
C:\Users\aballman\OneDrive - Intel Corporation\Desktop/b.h:2:42: error: invalid operands to binary expression
      ('const char *' and 'const char *')
template <class T> void b2(T x, T y) { x + y; }
                                       ~ ^ ~
In file included from C:\Users\aballman\OneDrive - Intel Corporation\Desktop\a.cpp:2:
C:\Users\aballman\OneDrive - Intel Corporation\Desktop/c.h:3:5: note: in instantiation of function template
      specialization 'b2<const char *>' requested here
    b2("0", "0");
    ^
2 errors generated.

[smeenai]

Thanks, I wasn't aware of that option. In this case though, I want the include stack for the line

C:\Users\aballman\OneDrive - Intel Corporation\Desktop/b.h:2:42: error:

My understanding is that include stacks for errors were only supposed to be suppressed if the prior error was from the same file, but in this case we're suppressing when the prior note was from the same file. (It might still make sense to do that with -fdiagnostics-show-note-include-stack, but without that, we're completely missing the include stack for the error.)

[AaronBallman]

Oh, thank you for the clarification, you're right, the preface is missing from the second error still (my eyes glossed over that). I stepped through in a debugger and we're hitting this early return: https://github.com/llvm/llvm-project/blob/main/clang/lib/Frontend/DiagnosticRenderer.cpp#L171 -- we're storing the last location from the note and comparing that against the location for the subsequent error.