3.3.3 Regression: Segfaults using @extend with unknown selector

[irundaia]

There seems to be an issue with unfound selectors when using @extend. A segfault occurs when entering a selector which cannot be found elsewhere in the code. For example:

.z-depth-1 {
  box-shadow: 0 2px 5px 0 rgba(0, 0, 0, 0.16), 0 2px 10px 0 rgba(0, 0, 0, 0.12);
}

.z-depth-1-half {
  box-shadow: 0 5px 11px 0 rgba(0, 0, 0, 0.18), 0 4px 15px 0 rgba(0, 0, 0, 0.15);
}

.btn {
  @extend .z-depth-1;

  &:hover {
    @extend .z-depth-half-1;
  }
}

While I think that it should report an error (even though ruby sass does not) currently, anything using libsass 3.3.3 crashes with a segfault. Using libsass 3.3.2 it will be compiled into:

.z-depth-1, .btn {
    box-shadow: 0 2px 5px 0 rgba(0, 0, 0, 0.16), 0 2px 10px 0 rgba(0, 0, 0, 0.12);
}

.z-depth-1-half {
    box-shadow: 0 5px 11px 0 rgba(0, 0, 0, 0.18), 0 4px 15px 0 rgba(0, 0, 0, 0.15);
}

[saper]

Confirmed using

/home/saper/sw/sassc/bin/sassc --version
sassc: 3.3.0
libsass: 3.3.3-2-g20bfd
sass2scss: 1.0.5

Test added sass/sass-spec#728

[saper]

We actually crash trying to write the error message about it being undefined.

Stacktrace, sel is NULL:

Program received signal SIGSEGV, Segmentation fault.
0x000000000055f7d5 in Sass::Extend::operator() (this=0x7fffffffda90, b=0x8020558c0) at src/extend.cpp:2042
2042            std::string str_sel(sel->to_string({ NESTED, 5 }));
(gdb) bt
#0  0x000000000055f7d5 in Sass::Extend::operator() (this=0x7fffffffda90, b=0x8020558c0)
    at src/extend.cpp:2042
#1  0x00000000004ee79e in Sass::Block::perform (this=0x8020558c0, op=0x7fffffffda90) at src/ast.hpp:425
#2  0x00000000004afc42 in Sass::Context::compile (this=0x802024280) at src/context.cpp:660
#3  0x00000000004ae99e in Sass::File_Context::parse (this=0x802024280) at src/context.cpp:583
#4  0x0000000000422857 in sass_parse_block (compiler=0x80201a060) at src/sass_context.cpp:321
#5  0x0000000000422681 in sass_compiler_parse (compiler=0x80201a060) at src/sass_context.cpp:467
#6  0x0000000000422305 in sass_compile_context (c_ctx=0x80201f100, cpp_ctx=0x802024280)
    at src/sass_context.cpp:356
#7  0x000000000042256c in sass_compile_file_context (file_ctx=0x80201f100) at src/sass_context.cpp:454
#8  0x000000000041b13f in compile_file (options=0x80201c080, input_path=0x7fffffffea7f "input.scss", 
    outfile=0x0) at sassc.c:131
#9  0x000000000041b945 in main (argc=<optimized out>, argv=<optimized out>) at sassc.c:332