[FileCheck] Fix @LINE value after match failure

[thopre]

Summary

The value of the FileCheckNumericVariable class instance representing
the @LINE numeric variable is set and cleared respectively before and
after substitutions are made, if any. However, when a substitution
fails, the value is not cleared. This causes the next substitution of
@LINE later on to give the wrong value since setValue is a nop if the
value is already set. This is what caused failures after commit r365249.

[jhenderson]

LGTM.

llvm/trunk/lib/Support/FileCheck.cpp

if (!Substitutions.empty()) {
 
     size_t InsertOffset = 0;
     // Substitute all string variables and expressions whose values are only
     // now known. Use of string variables defined on the same line are handled
     // by back-references.
     for (const auto &Substitution : Substitutions) {
       // Substitute and check for failure (e.g. use of undefined variable).
       Expected<std::string> Value = Substitution->getResult();
-      if (!Value)
+      if (!Value) {
+        Context->LineVariable->clearValue();
         return Value.takeError();
+      }
 
       // Plop it into the regex at the adjusted offset.
       TmpStr.insert(TmpStr.begin() + Substitution->getIndex() + InsertOffset,
                     Value->begin(), Value->end());
       InsertOffset += Value->size();
     }
 
     // Match the newly constructed regex.

llvm/trunk/unittests/Support/FileCheckTest.cpp

TEST_F(FileCheckTest, Match) {
   EXPECT_FALSE(Tester.matchExpect("18"));
 
   // Check matching the variable defined matches the correct number only
   Tester.initNextPattern();
   Tester.parsePatternExpect("[[#NUMVAR]] [[#NUMVAR+2]]");
   EXPECT_TRUE(Tester.matchExpect("19 21"));
   EXPECT_TRUE(Tester.matchExpect("18 21"));
   EXPECT_FALSE(Tester.matchExpect("18 20"));
+
+  // Check matching a numeric expression using @LINE after match failure uses
+  // the correct value for @LINE.
+  Tester.initNextPattern();
+  EXPECT_FALSE(Tester.parsePatternExpect("[[#@LINE]]"));
+  // Ok, @LINE is 4 now.
+  EXPECT_FALSE(Tester.matchExpect("4"));
+  Tester.initNextPattern();
+  // @LINE is now 5, match with substitution failure.
+  EXPECT_FALSE(Tester.parsePatternExpect("[[#UNKNOWN]]"));
+  EXPECT_TRUE(Tester.matchExpect("FOO"));
+  Tester.initNextPattern();
+  // Check that @LINE is 6 as expected.
+  EXPECT_FALSE(Tester.parsePatternExpect("[[#@LINE]]"));
+  EXPECT_FALSE(Tester.matchExpect("6"));
 }
 
 TEST_F(FileCheckTest, Substitution) {
   SourceMgr SM;
   FileCheckPatternContext Context;
   std::vector<std::string> GlobalDefines;
   GlobalDefines.emplace_back(std::string("FOO=BAR"));
   EXPECT_FALSE(errorToBool(Context.defineCmdlineVariables(GlobalDefines, SM)));