The inline issue is recorded as 58526.

Only restrict on parameters is implemented.

(In reply to Andrew Pinski from comment #1)
> The inline issue is recorded as PR 58526.
This has been fixed in GCC 5.

But the following still applies:
(In reply to Richard Biener from comment #2)
> Only restrict on parameters is implemented.

No change in GCC 10:

$ cat pr60712.c && gcc -O3 -S -Wall -fdump-tree-optimized=/dev/stdout pr60712.c
void sum(int *restrict a, int *restrict b) {
    *a = 0;
    for (int i=0; i<100; i++)
        *a += *b;
}

int *global_a, *global_b;

void bug_when_inline() {
    sum(global_a, global_b);
}

void bug_when_local_variable() {
    int* restrict a = global_a;
    int* restrict b = global_b;
    *a = 0;
    for (int i=0; i<100; i++)
        *a += *b;
}

;; Function sum (sum, funcdef_no=0, decl_uid=1931, cgraph_uid=1, symbol_order=0)

sum (int * restrict a, int * restrict b)
{
  int _2;
  int _13;

  <bb 2> [local count: 10737416]:
  _2 = *b_9(D);
  _13 = _2 * 100;
  *a_7(D) = _13;
  return;

}



;; Function bug_when_inline (bug_when_inline, funcdef_no=1, decl_uid=1939, cgraph_uid=2, symbol_order=3)

bug_when_inline ()
{
  int * global_b.0_1;
  int * global_a.1_2;
  int _5;
  int _12;

  <bb 2> [local count: 10737416]:
  global_b.0_1 = global_b;
  global_a.1_2 = global_a;
  _5 = *global_b.0_1;
  _12 = _5 * 100;
  *global_a.1_2 = _12;
  return;

}



;; Function bug_when_local_variable (bug_when_local_variable, funcdef_no=2, decl_uid=1941, cgraph_uid=3, symbol_order=4)

Removing basic block 5
bug_when_local_variable ()
{
  int * restrict b;
  int * restrict a;
  int _2;
  int _3;
  unsigned int ivtmp_4;
  unsigned int ivtmp_5;
  int prephitmp_15;

  <bb 2> [local count: 10737416]:
  a_7 = global_a;
  b_8 = global_b;
  *a_7 = 0;

  <bb 3> [local count: 1063004409]:
  # prephitmp_15 = PHI <0(2), _3(3)>
  # ivtmp_5 = PHI <100(2), ivtmp_4(3)>
  _2 = *b_8;
  _3 = _2 + prephitmp_15;
  *a_7 = _3;
  ivtmp_4 = ivtmp_5 + 4294967295;
  if (ivtmp_4 != 0)
    goto <bb 3>; [98.99%]
  else
    goto <bb 4>; [1.01%]

  <bb 4> [local count: 10737416]:
  return;

}

After inlining works as expected now (see comment#4 which shows this).  We do not honor __restrict on local variables because we cannot adhere to the language constraints within our implementation framework.  We only can do that for function parameters.