[PATCH] FileCheck: fix a bug with multiple --check-prefix options. Similar to r194565

[Daniel Sanders]

Hi samsonov, arsenm,

Directives are being ignored, when they occur between a partial-word false
match and any match on another prefix.

For example, with FOO and BAR prefixes:
   _FOO
   FOO: foo
   BAR: bar
FileCheck incorrectly matches:
   fog
   bar

This happens because FOO falsely matched as a partial word at '_FOO' and was
ignored while BAR matched at 'BAR:'. The match of BAR is incorrectly returned
as the 'first match' causing the FOO directive to be discarded.

Fixed this the same way as r194565 (D2166) did for a similar test case.
The partial-word false match should be counted as a match for the purposes of
finding the first match of a prefix, but should be returned as a false match
using CheckTy::CheckNone so that it isn't treated as a directive.

Fixes PR17995

http://llvm-reviews.chandlerc.com/D2228

Files:
  test/FileCheck/check-multiple-prefixes-nomatch-2.txt
  utils/FileCheck/FileCheck.cpp

Index: test/FileCheck/check-multiple-prefixes-nomatch-2.txt
===================================================================
--- /dev/null
+++ test/FileCheck/check-multiple-prefixes-nomatch-2.txt
@@ -0,0 +1,10 @@
+; RUN: not FileCheck -input-file %s %s -check-prefix=FOO -check-prefix=BAR 2>&1 | FileCheck %s
+
+fog
+bar
+; _FOO not a valid check-line
+; FOO: fo{{o}}
+; BAR: ba{{r}}
+
+; CHECK: {{error: expected string not found in input}}
+; CHECK-NEXT: {{F}}OO: fo{{[{][{]o[}][}]}}
Index: utils/FileCheck/FileCheck.cpp
===================================================================
--- utils/FileCheck/FileCheck.cpp
+++ utils/FileCheck/FileCheck.cpp
@@ -795,10 +795,11 @@
     // it. This should also prevent matching the wrong prefix when one is a
     // substring of another.
     if (PrefixLoc != 0 && IsPartOfWord(Buffer[PrefixLoc - 1]))
-      continue;
+      FirstTy = Check::CheckNone;
+    else
+      FirstTy = FindCheckType(Rest, Prefix);
 
     FirstLoc = PrefixLoc;
-    FirstTy = FindCheckType(Rest, Prefix);
     FirstPrefix = Prefix;
   }
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