patch 8.2.3475: expression register set by not executed put command

Problem:    Expression register set by not executed put command.
Solution:   Do not set the register if the command is skipped. (closes #8909)

src/ex_docmd.c

@@ -2382,9 +2382,12 @@ do_one_cmd(
 	    // for '=' register: accept the rest of the line as an expression
 	    if (ea.arg[-1] == '=' && ea.arg[0] != NUL)
 	    {
-		set_expr_line(vim_strsave(ea.arg), &ea);
+		if (!ea.skip)
+		{
+		    set_expr_line(vim_strsave(ea.arg), &ea);
+		    did_set_expr_line = TRUE;
+		}
 		ea.arg += STRLEN(ea.arg);
-		did_set_expr_line = TRUE;
 	    }
 #endif
 	    ea.arg = skipwhite(ea.arg);

src/testdir/test_excmd.vim

@@ -647,4 +647,12 @@ func Test_command_not_implemented_E319()
   endif
 endfunc
 
+func Test_not_break_expression_register()
+  call setreg('=', '1+1')
+  if 0
+    put =1
+  endif
+  call assert_equal('1+1', getreg('=', 1))
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab

src/version.c

@@ -757,6 +757,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3475,
 /**/
     3474,
 /**/