bug ? : -Wpedantic -Wconversion 'short a=1; a-=1;' complaint

[Jason Vas Dias]

I really do not think a '-Wpedantic -Wconversion' warning should
be generated for the following code, but it is
(with GCC 6.4.1 and 7.3.1 on RHEL-7.5 Linux) :

 $ echo '
 typedef unsigned short U16_t;
 static void f(void)
 { U16_t a = 1;
   a-=1;
 }' > t.C;

 $ g++ -std=c++14 -Wall -Wextra -pedantic -Wc++11-compat \
   -Wconversion -Wcast-align -Wcast-qual -Wfloat-equal \
   -Wmissing-declarations -Wlogical-op -Wpacked -Wundef \
   -Wuninitialized -Wvariadic-macros -Wwrite-strings \
   -c t.C -o /dev/null
 t.C:4:8: conversion to 'U16_t' {aka short unsigned int} from 'int' may \
          alter its value.

I don't control the warning flags, as shown above,
that my code must compile against without warnings.

But I think the warning issued above is a GCC bug -
when I look at the code generated, (compile with -S -o t.S),
I see it actually does generate a 16-bit subtraction, which
is what I wanted:

 	.file	"t.C"
	.text
	.globl	_Z1fv
	.type	_Z1fv, @function
_Z1fv:
.LFB0:
	.cfi_startproc
	pushq	%rbp
	.cfi_def_cfa_offset 16
	.cfi_offset 6, -16
	movq	%rsp, %rbp
	.cfi_def_cfa_register 6
	movw	$1, -2(%rbp)
	subw	$1, -2(%rbp)
#       ^^^^ this is a two-byte word subtract
	nop
	popq	%rbp
	.cfi_def_cfa 7, 8
	ret
	.cfi_endproc
.LFE0:
	.size	_Z1fv, .-_Z1fv
	.ident	"GCC: (GNU) 6.4.1 20180321"
	.section	.note.GNU-stack,"",@progbits



So why is it generating a warning about conversion from 'int' ?

I don't see any conversion going on in assembler output -
(cast 'a' to a temporary int, do 32-bit integer subtraction of  '1'
 from it, and store sign-extended low 16 bits in 'a' - this is NOT
 what is going on here ) .

I'd like to remove either '-pedantic' or '-Wconversion' from the
warning flags, but this is not an option .

Please can GCC fix this warning bug eventually - I have to wade
through code that generates thousands of them per compilation.

Thanks & Best Regards,
Jason