I cannot do anything with this without a reproducible example.

But I suggest that to work around, split your large function into a couple smaller ones.

Created attachment 312 [details]
Reduced test case.

Attached is a reduced test case (can't spend time on it now but wanted to attach this.)

Compile with:

dmd -O -inline -release -c 2773_reduced.d

Bug occurs in DMD 1.041 and DMD 2.026, most likely in 2.027 as well.

-[Unknown]

The error is different in DMD 2.027.

dmd -O -inline -release -c 2773_reduced.d
Internal error: ..\ztc\go.c 243

Created attachment 313 [details]
Workaround patch: make endless iteration not an error.

Please note: this patch does not fix this bug.

I suggest dropping the assert for iter, and instead treating it the same as the clock timeout.  While this makes this class of bug less discoverable, I propose that it's better that it compiles - at least some functions will be optimized.

Regarding this bug - it keeps moving an equation to optimize it, so each time the loop runs it's got more changes.  Unfortunately, the innards of the backend are still a bit beyond me, so I can't see why.

Note that the second call to pointer.clear() can be a call to another method, on the same struct, as long as that method does something (anything.)

Also, reducing the size of the static array below 3 (or making it dynamic) solves it, but a bigger one still dies.  Changing the code within clear() to any other operation also solves it.  And the struct has to be within a class, returned from a method.

-[Unknown]

Here's a slightly reduced test case, which only requires -O (doesn't need -inline -release), and ICEs on both D1 and D2. On D2, you still get an ICE if you remove all references to the class, and just set S2773* pointer = &dat.

struct S2773{
  int[4] data;	
}

S2773 dat;

class C2773 {
  S2773* get()	{  return &dat;  }
}

void main()
{
  C2773 inst = null;
  S2773* pointer = inst.get();
  pointer.data[] = 0;
  pointer.data[] = 0;
}

This isn't a regression. It failed on DMD1.022 as well, which was released in mid-2007, almost 2 years before this bug report.

Even simpler test case, this one fails on both D1.033 and 1.048.
int[4]* get()	{  return null; }

void main()
{
  int[4]* p = get();
  (*p)[] = 0;
  (*p)[] = 0;
}

An even simpler test case shows something interesting: it happens only when there's an array assignment of 0, followed by another use of the same variable.
An array assignment to 0 is an OPmemset operation in the backend.

int* get()  {  return null; }
void main(){
  int* p = get();
  p[0..3] = 0; // must be = 0!! Doesn't happen with any other number.
  p[0] = 7;
}

ANALYSIS:
This is an OPmemset fight! In the optimisation loop, there's a localize step which rearranges the assignment, and there's a canonicalize step which sets it back the way it was before....

cgelem.c,  elmemxxx() line 702 replaces ((e op= v) op e2) with ((e op=v), (e op e2))
ie,  (p = get()), p) memset 0.   ---> ((p = get()), p memset 0.

glocal.c,  local_exp() replaces
p = get(); p memset 0; ---> (p = get(), p) memset 0

So it just keeps going round in circles.
The fight can be broken up by changing cgelem.c elmemxxx() line 701 
to avoid doing the first replacement:

-			if (e1->Eoper == OPcomma || OTassign(e1->Eoper))
+			if (e1->Eoper == OPcomma)

This probably isn't correct, there may be somewhere this particular canonicalisation is important. But without the DMC test suite, I can't tell.
(Note that the comments in the code only refer to the OPcomma transformation, not the assignment one, so I presume the assignment was a later addition).

Fixed dmd 1.049 and 2.034