patch 8.2.5004: right shift on negative number does not work as documented

Problem:    Right shift on negative number does not work as documented.
Solution:   Use a uvarnumber_T type cast.

runtime/doc/eval.txt

@@ -1138,9 +1138,10 @@ expr6 >> expr6	bitwise right shift				*expr->>*
 							*E1282* *E1283*
 The "<<" and ">>" operators can be used to perform bitwise left or right shift
 of the left operand by the number of bits specified by the right operand.  The
-operands must be positive numbers.  The topmost bit (sign bit) is always
-cleared for ">>".  If the right operand (shift amount) is more than the
-maximum number of bits in a number (|v:numbersize|) the result is zero.
+operands are used as positive numbers.  When shifting right with ">>" the
+topmost bit (somtimes called the sign bit) is cleared.  If the right operand
+(shift amount) is more than the maximum number of bits in a number
+(|v:numbersize|) the result is zero.
 
 
 expr6 and expr7				*expr6* *expr7* *E1036* *E1051*
@@ -1417,6 +1418,10 @@ number			number constant			*expr-number*
 Decimal, Hexadecimal (starting with 0x or 0X), Binary (starting with 0b or 0B)
 and Octal (starting with 0, 0o or 0O).
 
+Assuming 64 bit numbers are used (see |v:numbersize|) an unsigned number is
+truncated to 0x7fffffffffffffff or 9223372036854775807.  You can use -1 to get
+0xffffffffffffffff.
+
 						*floating-point-format*
 Floating point numbers can be written in two forms:
 

src/charset.c

@@ -2002,6 +2002,7 @@ vim_str2nr(
 	}
 	else
 	{
+	    // prevent a larg unsigned number to become negative
 	    if (un > VARNUM_MAX)
 		un = VARNUM_MAX;
 	    *nptr = (varnumber_T)un;

src/eval.c

@@ -3091,12 +3091,8 @@ eval5(char_u **arg, typval_T *rettv, evalarg_T *evalarg)
 		rettv->vval.v_number =
 				    rettv->vval.v_number << var2.vval.v_number;
 	    else
-	    {
 		rettv->vval.v_number =
-				    rettv->vval.v_number >> var2.vval.v_number;
-		// clear the topmost sign bit
-		rettv->vval.v_number &= ~((uvarnumber_T)1 << MAX_LSHIFT_BITS);
-	    }
+		      (uvarnumber_T)rettv->vval.v_number >> var2.vval.v_number;
 	}
 
 	clear_tv(&var2);

src/testdir/test_expr.vim

@@ -958,6 +958,8 @@ func Test_bitwise_shift()
     call assert_equal(0, 0 >> 4)
     call assert_equal(0, 999999 >> 100)
     call assert_equal(0, 999999 << 100)
+    call assert_equal(-1, -1 >> 0)
+    call assert_equal(-1, -1 << 0)
     VAR a = 8
     VAR b = 2
     call assert_equal(2, a >> b)
@@ -976,6 +978,15 @@ func Test_bitwise_shift()
     for i in range(0, v:numbersize - 2)
         LET val = and(val, invert(1 << i))
     endfor
+    #" -1 has all the bits set
+    call assert_equal(-2, -1 << 1)
+    call assert_equal(-4, -1 << 2)
+    call assert_equal(-8, -1 << 3)
+    if v:numbersize == 64
+      call assert_equal(0x7fffffffffffffff, -1 >> 1)
+      call assert_equal(0x3fffffffffffffff, -1 >> 2)
+      call assert_equal(0x1fffffffffffffff, -1 >> 3)
+    endif
     call assert_equal(0, val)
     #" multiple operators
     call assert_equal(16, 1 << 2 << 2)

src/version.c

@@ -734,6 +734,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    5004,
 /**/
     5003,
 /**/

src/vim9execute.c

@@ -4096,12 +4096,7 @@ exec_instructions(ectx_T *ectx)
 			case EXPR_RSHIFT: if (arg2 > MAX_LSHIFT_BITS)
 					      res = 0;
 					  else
-					  {
-					      res = arg1 >> arg2;
-					      // clear the topmost sign bit
-					      res &= ~((uvarnumber_T)1
-							   << MAX_LSHIFT_BITS);
-					  }
+					      res = (uvarnumber_T)arg1 >> arg2;
 					  break;
 			default: break;
 		    }

src/vim9expr.c

@@ -2719,11 +2719,8 @@ compile_expr5(char_u **arg, cctx_T *cctx, ppconst_T *ppconst)
 	    else if (type == EXPR_LSHIFT)
 		tv1->vval.v_number = tv1->vval.v_number << tv2->vval.v_number;
 	    else
-	    {
-		tv1->vval.v_number = tv1->vval.v_number >> tv2->vval.v_number;
-		// clear the topmost sign bit
-		tv1->vval.v_number &= ~((uvarnumber_T)1 << MAX_LSHIFT_BITS);
-	    }
+		tv1->vval.v_number =
+			(uvarnumber_T)tv1->vval.v_number >> tv2->vval.v_number;
 	    clear_tv(tv2);
 	    --ppconst->pp_used;
 	}