ERL-558 Add the missing function clause for string:prefix (#1702)

OTP-14942

2 files changed, 9 insertions, 3 deletions

diff --git a/lib/stdlib/src/string.erl b/lib/stdlib/src/string.erl
index ab041ff53c..6f5e617230 100644
--- a/lib/stdlib/src/string.erl
+++ b/lib/stdlib/src/string.erl
@@ -411,10 +411,12 @@ to_number(_, Number, Rest, _, Tail) ->
 %% Return the remaining string with prefix removed or else nomatch
 -spec prefix(String::unicode:chardata(), Prefix::unicode:chardata()) ->
                     'nomatch' | unicode:chardata().
-prefix(Str, []) -> Str;
 prefix(Str, Prefix0) ->
-    Prefix = unicode:characters_to_list(Prefix0),
-    case prefix_1(Str, Prefix) of
+    Result = case unicode:characters_to_list(Prefix0) of
+                 [] -> Str;
+                 Prefix -> prefix_1(Str, Prefix)
+             end,
+    case Result of
         [] when is_binary(Str) -> <<>>;
         Res -> Res
     end.
diff --git a/lib/stdlib/test/string_SUITE.erl b/lib/stdlib/test/string_SUITE.erl
index f43bfb4482..17714b8d4d 100644
--- a/lib/stdlib/test/string_SUITE.erl
+++ b/lib/stdlib/test/string_SUITE.erl
@@ -485,6 +485,10 @@ to_float(_) ->
 prefix(_) ->
     ?TEST("", ["a"], nomatch),
     ?TEST("a", [""], "a"),
+    ?TEST("a", [[[]]], "a"),
+    ?TEST("a", [<<>>], "a"),
+    ?TEST("a", [[<<>>]], "a"),
+    ?TEST("a", [[[<<>>]]], "a"),
     ?TEST("b", ["a"], nomatch),
     ?TEST("a", ["a"], ""),
     ?TEST("å", ["a"], nomatch),