[Option] For typo '-foo', suggest '--foo'

Summary:
https://reviews.llvm.org/rL321877 introduced the `OptTable::findNearest`
method, to find the closest edit distance option for a given string.
However, the implementation contained a bug: for a typo `-foo` with an
edit distance of 1 away from a valid option `--foo`, `findNearest`
would suggest a nearby option of `foo`. That is, the result would not
include the `--` prefix, and so was not a valid option.

Fix the bug by ensuring that the prefix string is initialized to one of
the valid prefixes for the option.

Test Plan: `check-llvm-unit`

Reviewers: v.g.vassilev, teemperor, ruiu, jroelofs, yamaguchi

Reviewed By: jroelofs

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D41873

llvm-svn: 322109

3 files changed, 5 insertions, 2 deletions

diff --git a/llvm/lib/Option/OptTable.cpp b/llvm/lib/Option/OptTable.cpp
index f85b9043091..dcd1cc46d96 100644
--- a/llvm/lib/Option/OptTable.cpp
+++ b/llvm/lib/Option/OptTable.cpp
@@ -277,8 +277,8 @@ unsigned OptTable::findNearest(StringRef Option, std::string &NearestString,
       continue;
     // Find the most appropriate prefix. For example, if a user asks for
     // "--helm", suggest "--help" over "-help".
-    StringRef Prefix;
-    for (int P = 0; CandidateInfo.Prefixes[P]; P++) {
+    StringRef Prefix = CandidateInfo.Prefixes[0];
+    for (int P = 1; CandidateInfo.Prefixes[P]; P++) {
       if (Option.startswith(CandidateInfo.Prefixes[P]))
         Prefix = CandidateInfo.Prefixes[P];
     }
diff --git a/llvm/unittests/Option/OptionParsingTest.cpp b/llvm/unittests/Option/OptionParsingTest.cpp
index 26bbb37602f..eef21ab5120 100644
--- a/llvm/unittests/Option/OptionParsingTest.cpp
+++ b/llvm/unittests/Option/OptionParsingTest.cpp
@@ -283,6 +283,8 @@ TEST(Option, FindNearest) {
   EXPECT_EQ(Nearest, "-blorp");
   EXPECT_EQ(1U, T.findNearest("--blorm", Nearest));
   EXPECT_EQ(Nearest, "--blorp");
+  EXPECT_EQ(1U, T.findNearest("-fjormp", Nearest));
+  EXPECT_EQ(Nearest, "--fjormp");
 
   // The nearest candidate respects the prefix and value delimiter
   // of the original string.
diff --git a/llvm/unittests/Option/Opts.td b/llvm/unittests/Option/Opts.td
index 539850df134..c4544b5b3f9 100644
--- a/llvm/unittests/Option/Opts.td
+++ b/llvm/unittests/Option/Opts.td
@@ -34,4 +34,5 @@ def Cramb : Joined<["/"], "cramb:">, HelpText<"The cramb option">, MetaVarName<"
 def Doopf1 : Flag<["-"], "doopf1">, HelpText<"The doopf1 option">, Flags<[OptFlag1]>;
 def Doopf2 : Flag<["-"], "doopf2">, HelpText<"The doopf2 option">, Flags<[OptFlag2]>;
 def Ermgh : Joined<["--"], "ermgh">, HelpText<"The ermgh option">, MetaVarName<"ERMGH">, Flags<[OptFlag1]>;
+def Fjormp : Flag<["--"], "fjormp">, HelpText<"The fjormp option">, Flags<[OptFlag1]>;
 def DashDash : Option<["--"], "", KIND_REMAINING_ARGS>;