[PATCH] D121368: [pseudo][WIP] Build Ambiguous forest node in the GLR Parser.

[Haojian Wu via Phabricator via cfe-commits]

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Forest node by design is unmutable. To create an ambiguous node, we have
to know all alternatives in advance.

In order to achieve that, we must perform all reductions in a careful
order (see code comments for details), so that we can gather completed
alternatives as a batch, and process them in a single pass.

E.g. considering the following grammar:

  bnf
  TU := stmt
  TU := expr
  stmt := expr
  expr := ID
  stmt := ID
  
  // Ambiguous stmt forest node:
  //     stmt (ambiguous)
  //    /     \
  //   /      stmt
  //   |       |
  //  stmt   expr
  //    \     /
  //       ID

The ambiguous Stmt node is built in a single section where we perform three reductions:

1. expr := ID
2. stmt := ID
3. stmt := expr (enabled after 1 is performed)

We expect to perform them in a batch way with the order {1}, {2, 3}.
When processing the batch {2, 3} where ambiguity happens, we build an
ambiguous node for stmt.

Based on https://reviews.llvm.org/D121150


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D121368

Files:
  clang/include/clang/Tooling/Syntax/Pseudo/Forest.h
  clang/include/clang/Tooling/Syntax/Pseudo/GLRParser.h
  clang/include/clang/Tooling/Syntax/Pseudo/Grammar.h
  clang/lib/Tooling/Syntax/Pseudo/GLRParser.cpp
  clang/lib/Tooling/Syntax/Pseudo/GrammarBNF.cpp
  clang/tools/clang-pseudo/ClangPseudo.cpp

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