[clang-tools-extra] [clang-tidy] Add support for lambda-expression in `use-trailing-return-type` check (PR #135383)

[Baranov Victor via cfe-commits]

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@@ -494,4 +554,75 @@ void UseTrailingReturnTypeCheck::check(const MatchFinder::MatchResult &Result) {
       << FixItHint::CreateInsertion(InsertionLoc, " -> " + ReturnType);
 }
 
+void UseTrailingReturnTypeCheck::diagOnLambda(
+    const LambdaExpr *Lambda,
+    const ast_matchers::MatchFinder::MatchResult &Result) {
+
+  const CXXMethodDecl *Method = Lambda->getCallOperator();
+  if (!Method || Lambda->hasExplicitResultType())
+    return;
+
+  const QualType ReturnType = Method->getReturnType();
+  if (ReturnType->isUndeducedAutoType() &&
+      TransformLambdas == TransformLambda::AllExceptAuto)
+    return;
+
+  const SourceLocation TrailingReturnInsertLoc =
+      findLambdaTrailingReturnInsertLoc(Method, *Result.SourceManager,
+                                        getLangOpts(), *Result.Context);
+
+  if (TrailingReturnInsertLoc.isValid())
+    diag(Lambda->getBeginLoc(), "use a trailing return type for this lambda")
+        << FixItHint::CreateInsertion(
+               TrailingReturnInsertLoc,
+               " -> " +
+                   ReturnType.getAsString(Result.Context->getPrintingPolicy()));
+  else
+    diag(Lambda->getBeginLoc(), "use a trailing return type for this lambda");
+}
+
+SourceLocation UseTrailingReturnTypeCheck::findLambdaTrailingReturnInsertLoc(
+    const CXXMethodDecl *Method, const SourceManager &SM,
+    const LangOptions &LangOpts, const ASTContext &Ctx) {
+  // 'requires' keyword is present in lambda declaration
+  if (Method->getTrailingRequiresClause()) {
+    SourceLocation ParamEndLoc;
+    if (Method->param_empty()) {
+      ParamEndLoc = Method->getBeginLoc();
+    } else {
+      ParamEndLoc = Method->getParametersSourceRange().getEnd();
+    }
+
+    std::pair<FileID, unsigned> ParamEndLocInfo =
----------------
vbvictor wrote:

This code is very alike to `findTrailingReturnTypeSourceLocation` function. Finding the position of `requires` keyword.

https://github.com/llvm/llvm-project/pull/135383