unconditional pointer member access is generated in C for inheritance

[alaviss]

Example

type
  O = ref object of RootObj

proc foo(o: ref RootObj) =
  echo "some side-effect"

var o: O = nil
foo(o)

Actual Output

$ nim r -d:release -d:useMalloc --passC:-fno-omit-frame-pointer --passC:-mno-omit-leaf-frame-pointer --passC:-fsanitize=undefined --passL:-fsanitize=undefined --debuginfo --linedir test.nim

test.nim:8:5: runtime error: member access within null pointer of type 'struct tyObject_OcolonObjectType___31J4fwfXSYbejl8P6EY8zw'
some side-effect
some side-effect

Expected Output

some side-effect

Possible Solution

Don't generate unconditional access

Additional Information

Generated C code:

	o__test_5 = ((tyobject_ocolonobjecttype___31j4fwfxsybejl8p6ey8zw*) nim_nil);
	_1 = &o__test_5->sup; // Unconditional NULL dereference
	foo__test_3(_1);

While most compiler will generate rather benign code for this (ie. just a pointer offset), this is still undefined behavior.

It's not high priority, but it prevents effective usage of undefined behavior sanitizer with nimskull.

[Saem Ghani]

@alaviss pretty sure this can be closed to of you want to do a quick confirmation?

[alaviss]

Yep, this can be closed as fixed by #1196