[PATCH] D151785: [clangd] Desugar dependent type aliases for auto type hints

[Younan Zhang via Phabricator via cfe-commits]

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This patch alleviates https://github.com/clangd/clangd/issues/1298.

Containers in C++ such as `std::vector` or `llvm::SmallVector`,
introduce a series of type aliases to adapt to generic algorithms.

Currently, If we write an declarator involving expressions with
these containers and `auto` placeholder, we probably obtain opaque
type alias like following:

  c++
  std::vector<int> v = {1, 2, 3};
  auto value = v[1]; // hint for `value`: value_type
  auto *ptr = &v[0]; // hint for `ptr`: value_type *

These hints are useless for most of the time. It would be nice if we
desugar the type of `value_type` and print `int`, `int *` respectively
in this situation. However, things are complicated if user introduces type-
alias for brevity: we don't want to make the length of hints be too long!

This patch introduces a heuristic method that displays the desugared type
for type-aliases that depend on some template parameters or simply
reduce to builtin types e.g. int, double.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D151785

Files:
  clang-tools-extra/clangd/InlayHints.cpp
  clang-tools-extra/clangd/unittests/InlayHintTests.cpp

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