checker: allow ++, -- on byteptr, charptr (#7218)

doc/docs.md

@@ -2668,7 +2668,7 @@ Examples of potentially memory-unsafe operations are:
 
 To mark potentially memory-unsafe operations, enclose them in an `unsafe` block:
 
-```v failcompile
+```v wip
 // allocate 2 uninitialized bytes & return a reference to them
 mut p := unsafe { malloc(2) }
 p[0] = `h` // Error: pointer indexing is only allowed in `unsafe` blocks

vlib/v/checker/checker.v

@@ -4135,14 +4135,13 @@ fn (c &Checker) has_return(stmts []ast.Stmt) ?bool {
 pub fn (mut c Checker) postfix_expr(mut node ast.PostfixExpr) table.Type {
 	typ := c.expr(node.expr)
 	typ_sym := c.table.get_type_symbol(typ)
-	// if !typ.is_number() {
-	if !typ_sym.is_number() {
+	if !typ_sym.is_number() && typ_sym.kind !in [.byteptr, .charptr] {
 		c.error('invalid operation: $node.op.str() (non-numeric type `$typ_sym.name`)',
 			node.pos)
 	} else {
 		node.auto_locked, _ = c.fail_if_immutable(node.expr)
 	}
-	if (typ.is_ptr() || typ_sym.is_pointer()) && !c.inside_unsafe {
+	if !c.inside_unsafe && (typ.is_ptr() || typ_sym.is_pointer()) {
 		c.warn('pointer arithmetic is only allowed in `unsafe` blocks', node.pos)
 	}
 	return typ

vlib/v/tests/ptr_arithmetic_test.v

@@ -28,4 +28,7 @@ fn test_ptr_arithmetic_over_byteptr() {
 	assert q == byteptr(9)
 	s := unsafe { q - 1 }
 	assert s == byteptr(8)
+
+	unsafe {q++ q++ q--}
+	assert q == byteptr(10)
 }