segfault in compiler when accessing field on incorrectly derefenced slice

[shawnl]

shawn@shawn-desktop:~/git/zig/std/crypto$ lldb zig test chacha20.zig 
(lldb) target create "zig"
Current executable set to 'zig' (x86_64).
(lldb) settings set -- target.run-args  "test" "chacha20.zig"
(lldb) r
bProcess 1732 launched: '/home/shawn/bin/zig' (x86_64)
bProcess 1732 stopped
* thread #1, name = 'zig', stop reason = signal SIGSEGV: invalid address (fault address: 0x0)
    frame #0: 0x00007ffff37f6ed0 zig`type_is_invalid(TypeTableEntry*)
zig`type_is_invalid:
->  0x7ffff37f6ed0 <+0>: movl   (%rdi), %edx
    0x7ffff37f6ed2 <+2>: cmpl   $0x9, %edx
    0x7ffff37f6ed5 <+5>: je     0x7ffff37f6f08            ; <+56>
    0x7ffff37f6ed7 <+7>: jbe    0x7ffff37f6ef8            ; <+40>
(lldb) bt
* thread #1, name = 'zig', stop reason = signal SIGSEGV: invalid address (fault address: 0x0)
  * frame #0: 0x00007ffff37f6ed0 zig`type_is_invalid(TypeTableEntry*)
    frame #1: 0x00007ffff386713e zig`ir_analyze_instruction_field_ptr(IrAnalyze*, IrInstructionFieldPtr*) + 110
    frame #2: 0x00007ffff386983a zig`ir_analyze_instruction_nocast(IrAnalyze*, IrInstruction*) + 5402
    frame #3: 0x00007ffff386ac3a zig`ir_analyze(CodeGen*, IrExecutable*, IrExecutable*, TypeTableEntry*, AstNode*) + 250
    frame #4: 0x00007ffff3805575 zig`analyze_fn_ir(CodeGen*, FnTableEntry*, AstNode*) + 53
    frame #5: 0x00007ffff3806322 zig`semantic_analyze(CodeGen*) + 482
    frame #6: 0x00007ffff382c1f9 zig`codegen_build(CodeGen*) + 1081
    frame #7: 0x00007ffff376c9d1 zig`main + 7313
    frame #8: 0x00007ffff65d669d zig`__libc_start_main(main=(zig`main), argc=3, argv=0x00007fffffffe098) at __libc_start_main.c:74
    frame #9: 0x00007ffff37f4567 zig`__dls2(base=<unavailable>, sp=<unavailable>) at rcrt1.c:14
    frame #10: 0x00007ffff37f43f5 zig`_start + 22

program: https://gist.github.com/shawnl/8ebb7bab14658cfa4b38b203b9ec618e

[tiehuis]

Reduced test case. The problem here is when referencing a child member of a slice after de-referencing it.

fn add(a: []u8) void {
    const b = a.*.len;
}

pub fn main() void {
    var r: [10]u8 = undefined;
    add(r[0..]);
}

Instead we should emit a compile error as is currently the case when trying to deference the slice without accessing a member.

fn add(a: []u8) void {
    const b = a.*;
}

pub fn main() void {
    var r: [10]u8 = undefined;
    add(r[0..]);
}

results in

/tmp/t.zig:2:16: error: attempt to dereference non-pointer type '[]u8'
    const b = a.*;