clang's -Wunreachable-code does not fire if the first unreachable line is a macro

[nico]

This warns (as it should):

Code:

[[noreturn]] void notreached();
void g();

void f() {
  notreached();
  g();
}

Output:

% clang -c notreach.cc -Wunreachable-code
notreach.cc:6:3: warning: code will never be executed [-Wunreachable-code]
    6 |   g();
      |   ^
1 warning generated.

This doesn't:

Code:

[[noreturn]] void notreached();
void g();

#define MY_MACRO(x) (void)(x)

void f() {
  notreached();
  MY_MACRO(5);
  g();
}

Output:

% clang -c notreach2.cc -Wunreachable-code
# no output

Not emitting unreachable code warnings for the macro itself probably makes sense, but the diagnostic disappearing completely seems surprising.

(reported downstream at https://crbug.com/379845253)

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Nico Weber (nico)

This warns (as it should):

thakis@<!-- -->Mac src % cat notreach.cc                        
[[noreturn]] void notreached();
void g();

void f() {
  notreached();
  g();
}

% clang -c notreach.cc -Wunreachable-code
notreach.cc:6:3: warning: code will never be executed [-Wunreachable-code]
    6 |   g();
      |   ^
1 warning generated.

This doesn't:

% cat notreach2.cc                        
[[noreturn]] void notreached();
void g();

#define MY_MACRO(x) (void)(x)

void f() {
  notreached();
  MY_MACRO(5);
  g();
}
% clang -c notreach2.cc -Wunreachable-code
# no output

Not emitting unreachable code warnings for the macro itself probably makes sense, but the diagnostic disappearing completely seems surprising.

[shafik]

In the future please put the code into a separate block from output.

[nico]

@yronglin is doing somewhat similar things in #117437