[clang] [clang-tools-extra] [clang-tidy] Add readability-avoid-default-lambda-capture (PR #160150)

[JJ Marr via cfe-commits]

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@@ -0,0 +1,67 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "AvoidDefaultLambdaCaptureCheck.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+#include "clang/Basic/Lambda.h"
+#include "clang/Lex/Lexer.h"
+
+using namespace clang::ast_matchers;
+
+namespace clang::tidy::readability {
+
+static std::string generateCaptureText(const LambdaCapture &Capture) {
+  if (Capture.capturesThis())
+    return Capture.getCaptureKind() == LCK_StarThis ? "*this" : "this";
+
+  std::string Result;
+  if (Capture.getCaptureKind() == LCK_ByRef)
+    Result += "&";
+
+  Result += Capture.getCapturedVar()->getName().str();
+  return Result;
+}
+
+void AvoidDefaultLambdaCaptureCheck::registerMatchers(MatchFinder *Finder) {
+  Finder->addMatcher(lambdaExpr(hasDefaultCapture()).bind("lambda"), this);
+}
+
+void AvoidDefaultLambdaCaptureCheck::check(
+    const MatchFinder::MatchResult &Result) {
+  const auto *Lambda = Result.Nodes.getNodeAs<LambdaExpr>("lambda");
+  assert(Lambda);
+
+  const SourceLocation DefaultCaptureLoc = Lambda->getCaptureDefaultLoc();
+  if (DefaultCaptureLoc.isInvalid())
+    return;
+
+  std::vector<std::string> ImplicitCaptures;
+  for (const LambdaCapture &Capture : Lambda->implicit_captures()) {
+    // It is impossible to explicitly capture a VLA in C++, since VLAs don't
+    // exist in ISO C++ and so the syntax was never created to capture them.
+    if (Capture.getCaptureKind() == LCK_VLAType)
+      return;
+    ImplicitCaptures.push_back(generateCaptureText(Capture));
+  }
+
+  auto Diag = diag(DefaultCaptureLoc,
+                   "lambda uses default capture mode; explicitly capture "
+                   "variables instead");
+
+  // For template-dependent lambdas, the list of captures hasn't been created
+  // yet, so the list of implicit captures is empty.
+  if (ImplicitCaptures.empty() && Lambda->isGenericLambda())
----------------
jjmarr-amd wrote:

I don't really understand this comment. My understanding of lambdas is that it creates an object of a unique class with overloaded `operator()` and member variables for the captures.

If the captures are template-dependent, then the list of implicit captures isn't real because the type hasn't been fully resolved yet.

But wouldn't that also mean the `operator()` isn't resolved yet either? Why would the deduction not occur for member variables but already occur for member functions?

https://github.com/llvm/llvm-project/pull/160150