Created attachment 44786 [details]
Full stderr of compiler

Created attachment 44787 [details]
Code after preprocessing

Created attachment 44788 [details]
Temp compiler file

Sorry, hit the Submit button too early by accident.

Source code and compiler output are attached.

Bug is -Wsign-conversion warning triggerring:

bug.cpp:5:11: error: conversion to ‘long unsigned int’ from ‘int32_t {aka int}’ may change the sign of the result [-Werror=sign-conversion]
   r = r + static_cast<uint64_t>(q);
           ^~~~~~~~~~~~~~~~~~~~~~~~

in function 

void f(uint64_t a, int32_t q) {
  auto& r = a;
  r = r + static_cast<uint64_t>(q);
}

If we replace 'auto& r = a' with 'uint64_t& r = a' warning with disappear.

Same code at Godbolt.org: https://gcc.godbolt.org/

https://gcc.godbolt.org/z/hOpMyc

Nothing to do with deduced types:

typedef unsigned long int uint64_t ;

void f(unsigned long int a, int q) {
  a = a + static_cast<uint64_t>(q);
}

Another test case:

#include <cstddef>
int main() {
    int i = 42;
    size_t s0 = sizeof (int) + (size_t)i;
    size_t s1 = sizeof (int) + static_cast<size_t>(i);
}

https://stackoverflow.com/q/57403497/827263
https://stackoverflow.com/a/57404123/827263

I think I have a fix.  Will test tomorrow morning.

Tested fine:
https://gcc.gnu.org/ml/gcc-patches/2019-08/msg00553.html

Author: mpolacek
Date: Thu Aug  8 15:37:46 2019
New Revision: 274211

URL: https://gcc.gnu.org/viewcvs?rev=274211&root=gcc&view=rev
Log:
	PR c++/87519 - bogus warning with -Wsign-conversion.
	* typeck.c (cp_build_binary_op): Use same_type_p instead of comparing
	the types directly.

	* g++.dg/warn/Wsign-conversion-5.C: New test.

Added:
    trunk/gcc/testsuite/g++.dg/warn/Wsign-conversion-5.C
Modified:
    trunk/gcc/cp/ChangeLog
    trunk/gcc/cp/typeck.c
    trunk/gcc/testsuite/ChangeLog

Fixed on trunk, will backport to 9.3 later.

Author: mpolacek
Date: Thu Aug 15 18:32:33 2019
New Revision: 274545

URL: https://gcc.gnu.org/viewcvs?rev=274545&root=gcc&view=rev
Log:
	PR c++/87519 - bogus warning with -Wsign-conversion.
	* typeck.c (cp_build_binary_op): Use same_type_p instead of comparing
	the types directly.

	* g++.dg/warn/Wsign-conversion-5.C: New test.

Added:
    branches/gcc-9-branch/gcc/testsuite/g++.dg/warn/Wsign-conversion-5.C
Modified:
    branches/gcc-9-branch/gcc/cp/ChangeLog
    branches/gcc-9-branch/gcc/cp/typeck.c

Fixed in 9.3+.