[pseudo] Fix HeadsPartition is not initialized correctly.

The bug was that if we recover from the token 0, we will make the
Heads empty (Line646), which results no recovery being applied.

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D132388

clang-tools-extra/pseudo/lib/GLR.cpp

@@ -31,7 +31,6 @@ Token::Index findRecoveryEndpoint(ExtensionID Strategy, Token::Index Begin,
                                   const TokenStream &Tokens,
                                   const Language &Lang) {
   assert(Strategy != 0);
-  assert(Begin > 0);
   if (auto S = Lang.RecoveryStrategies.lookup(Strategy))
     return S(Begin, Tokens);
   return Token::Invalid;
@@ -614,7 +613,7 @@ const ForestNode &glrParse(const ParseParams &Params, SymbolID StartSymbol,
   // Invariant: Heads is partitioned by source: {shifted | reduced}.
   // HeadsPartition is the index of the first head formed by reduction.
   // We use this to discard and recreate the reduced heads during recovery.
-  unsigned HeadsPartition = 0;
+  unsigned HeadsPartition = Heads.size();
   std::vector<const GSS::Node *> NextHeads;
   auto MaybeGC = [&, Roots(std::vector<const GSS::Node *>{}), I(0u)]() mutable {
     assert(NextHeads.empty() && "Running GC at the wrong time!");

clang-tools-extra/pseudo/unittests/GLRTest.cpp

@@ -652,6 +652,31 @@ TEST_F(GLRTest, RecoverUnrestrictedReduce) {
             "[  1, end) └─word := <opaque>\n");
 }
 
+TEST_F(GLRTest, RecoveryFromStartOfInput) {
+  build(R"bnf(
+    _ := start [recover=Fallback] EOF
+
+    start := IDENTIFIER
+  )bnf");
+  TestLang.Table = LRTable::buildSLR(TestLang.G);
+  bool fallback_recovered = false;
+  auto fallback = [&](Token::Index Start, const TokenStream & Code) {
+    fallback_recovered = true;
+    return Code.tokens().size();
+  };
+  TestLang.RecoveryStrategies.try_emplace(
+      extensionID("Fallback"),
+      fallback);
+  clang::LangOptions LOptions;
+  TokenStream Tokens = cook(lex("?", LOptions), LOptions);
+
+  const ForestNode &Parsed =
+      glrParse({Tokens, Arena, GSStack}, id("start"), TestLang);
+  EXPECT_TRUE(fallback_recovered);
+  EXPECT_EQ(Parsed.dumpRecursive(TestLang.G),
+            "[  0, end) start := <opaque>\n");
+}
+
 TEST_F(GLRTest, RepeatedRecovery) {
   // We require multiple steps of recovery at eof and then a reduction in order
   // to successfully parse.