patch 9.1.0642: Check that mapping rhs starts with lhs fails if not simplified

Problem:  Check that mapping rhs starts with lhs doesn't work if lhs is
          not simplified.
Solution: Keep track of the mapblock containing the alternative lhs and
          also compare with it (zeertzjq).

fixes: #15376
closes: #15384

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/getchar.c b/src/getchar.c
index 4af1769..54222ec 100644
--- a/src/getchar.c
+++ b/src/getchar.c
@@ -3156,6 +3156,7 @@
 	int	save_m_noremap;
 	int	save_m_silent;
 	char_u	*save_m_keys;
+	char_u	*save_alt_m_keys;
 #else
 # define save_m_noremap mp->m_noremap
 # define save_m_silent mp->m_silent
@@ -3204,6 +3205,7 @@
 	save_m_noremap = mp->m_noremap;
 	save_m_silent = mp->m_silent;
 	save_m_keys = NULL;  // only saved when needed
+	save_alt_m_keys = NULL;  // only saved when needed
 
 	/*
 	 * Handle ":map <expr>": evaluate the {rhs} as an expression.  Also
@@ -3221,6 +3223,8 @@
 	    may_garbage_collect = FALSE;
 
 	    save_m_keys = vim_strsave(mp->m_keys);
+	    save_alt_m_keys = mp->m_alt != NULL
+				    ? vim_strsave(mp->m_alt->m_keys) : NULL;
 	    map_str = eval_map_expr(mp, NUL);
 
 	    // The mapping may do anything, but we expect it to take care of
@@ -3278,15 +3282,20 @@
 		noremap = save_m_noremap;
 	    else if (
 #ifdef FEAT_EVAL
-		STRNCMP(map_str, save_m_keys != NULL ? save_m_keys : mp->m_keys,
-								(size_t)keylen)
-#else
-		STRNCMP(map_str, mp->m_keys, (size_t)keylen)
+		save_m_expr ?
+		(save_m_keys != NULL
+			&& STRNCMP(map_str, save_m_keys, (size_t)keylen) == 0)
+		|| (save_alt_m_keys != NULL
+			&& STRNCMP(map_str, save_alt_m_keys,
+					    STRLEN(save_alt_m_keys)) == 0) :
 #endif
-		   != 0)
-		noremap = REMAP_YES;
-	    else
+		STRNCMP(map_str, mp->m_keys, (size_t)keylen) == 0
+		|| (mp->m_alt != NULL
+			&& STRNCMP(map_str, mp->m_alt->m_keys,
+					    STRLEN(mp->m_alt->m_keys)) == 0))
 		noremap = REMAP_SKIP;
+	    else
+		noremap = REMAP_YES;
 	    i = ins_typebuf(map_str, noremap,
 					 0, TRUE, cmd_silent || save_m_silent);
 #ifdef FEAT_EVAL
@@ -3296,6 +3305,7 @@
 	}
 #ifdef FEAT_EVAL
 	vim_free(save_m_keys);
+	vim_free(save_alt_m_keys);
 #endif
 	*keylenp = keylen;
 	if (i == FAIL)