[Bug c/83122] New: -Wconversion and shifting bitwise

["lh_mouse at 126 dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83122

            Bug ID: 83122
           Summary: -Wconversion and shifting bitwise
           Product: gcc
           Version: 7.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: lh_mouse at 126 dot com
  Target Milestone: ---

Minimal testcase:

```c
// gcc -c test.c -Wall -Wextra -pedantic -pedantic-errors -std=c99 -Wconversion

typedef __UINT64_TYPE__ uint64_t;

struct foo {
        uint64_t a :  3;
        uint64_t b : 61;
};

void set_a(struct foo *ptr, uint64_t val){
        ptr->b = val >> 3;
}
```


The program above results in a warning about loss of precision:

```
test.c: In function 'set_a':
test.c:11:11: warning: conversion to 'long long unsigned int:61' from 'uint64_t
{aka long long unsigned int}' may alter its value [-Wconversion]
  ptr->b = val >> 3;
           ^~~
```

This is a false positive since the 3 MSBs of `val` are filled with zeroes.

In reality, the warning persists even after I replace `val >> 3` with `val >>
63`.