patch 8.2.1387: Vim9: cannot assign to single letter variable with type

Problem:    Vim9: cannot assign to single letter variable with type.
Solution:   Exclude the colon from the variable name. (closes #6647)

src/eval.c

@@ -820,7 +820,15 @@ get_lval(
     {
 	lp->ll_name = name;
 
-	if (in_vim9script() && *p == ':')
+	if (in_vim9script())
+	{
+	    // "a: type" is declaring variable "a" with a type, not "a:".
+	    if (p == name + 2 && p[-1] == ':')
+	    {
+		--p;
+		lp->ll_name_end = p;
+	    }
+	    if (*p == ':')
 	    {
 		scriptitem_T *si = SCRIPT_ITEM(current_sctx.sc_sid);
 		char_u	 *tp = skipwhite(p + 1);
@@ -830,6 +838,7 @@ get_lval(
 		lp->ll_name_end = tp;
 	    }
 	}
+    }
 
     // Without [idx] or .key we are done.
     if ((*p != '[' && *p != '.') || lp->ll_name == NULL)

src/testdir/test_vim9_script.vim

@@ -449,6 +449,17 @@ def Test_assignment_vim9script()
       @+ = 'plus'
       assert_equal('plus', @+)
     endif
+
+    let a: number = 123
+    assert_equal(123, a)
+    let s: string = 'yes'
+    assert_equal('yes', s)
+    let b: number = 42
+    assert_equal(42, b)
+    let w: number = 43
+    assert_equal(43, w)
+    let t: number = 44
+    assert_equal(44, t)
   END
   CheckScriptSuccess(lines)
 enddef

src/version.c

@@ -754,6 +754,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1387,
 /**/
     1386,
 /**/