[clang] [clang-tools-extra] [clang][Tooling] Support 'c++latest' in InterpolatingCompilationDatabase (PR #160030)

[Nathan Ridge via cfe-commits]

https://github.com/HighCommander4 updated https://github.com/llvm/llvm-project/pull/160030

>From 9ed89b70375baf1c89086fe773da1cf24ac5c656 Mon Sep 17 00:00:00 2001
From: Nathan Ridge <zeratul976 at hotmail.com>
Date: Wed, 3 Sep 2025 02:37:53 -0400
Subject: [PATCH 1/2] [clangd] Fix off-by-one error in CommandMangler

SawInput() is intended to be called for every argument after a `--`,
but it was mistakenly being called for the `--` itself.

Partially fixes https://github.com/clangd/clangd/issues/1850
---
 clang-tools-extra/clangd/CompileCommands.cpp           |  3 ++-
 .../clangd/unittests/CompileCommandsTests.cpp          | 10 ++++++++++
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/clang-tools-extra/clangd/CompileCommands.cpp b/clang-tools-extra/clangd/CompileCommands.cpp
index 80391fe8cce25..c9da98e96ccfb 100644
--- a/clang-tools-extra/clangd/CompileCommands.cpp
+++ b/clang-tools-extra/clangd/CompileCommands.cpp
@@ -270,7 +270,8 @@ void CommandMangler::operator()(tooling::CompileCommand &Command,
   if (auto *DashDash =
           ArgList.getLastArgNoClaim(driver::options::OPT__DASH_DASH)) {
     auto DashDashIndex = DashDash->getIndex() + 1; // +1 accounts for Cmd[0]
-    for (unsigned I = DashDashIndex; I < Cmd.size(); ++I)
+    // Another +1 so we don't treat the `--` itself as an input.
+    for (unsigned I = DashDashIndex + 1; I < Cmd.size(); ++I)
       SawInput(Cmd[I]);
     Cmd.resize(DashDashIndex);
   }
diff --git a/clang-tools-extra/clangd/unittests/CompileCommandsTests.cpp b/clang-tools-extra/clangd/unittests/CompileCommandsTests.cpp
index 2ce2975bd962b..e324404e627c2 100644
--- a/clang-tools-extra/clangd/unittests/CompileCommandsTests.cpp
+++ b/clang-tools-extra/clangd/unittests/CompileCommandsTests.cpp
@@ -526,6 +526,16 @@ TEST(CommandMangler, RespectsOriginalSysroot) {
                 Not(HasSubstr(testPath("fake/sysroot"))));
   }
 }
+
+TEST(CommandMangler, StdLatestFlag) {
+  const auto Mangler = CommandMangler::forTests();
+  tooling::CompileCommand Cmd;
+  Cmd.CommandLine = {"clang-cl", "/std:c++latest", "--", "/Users/foo.cc"};
+  Mangler(Cmd, "/Users/foo.cc");
+  // Check that the /std:c++latest flag is not dropped
+  EXPECT_THAT(llvm::join(Cmd.CommandLine, " "), HasSubstr("/std:c++latest"));
+}
+
 } // namespace
 } // namespace clangd
 } // namespace clang

>From 818b056d7b30f2e8283195044e8d2845a4cc96e5 Mon Sep 17 00:00:00 2001
From: Nathan Ridge <zeratul976 at hotmail.com>
Date: Fri, 19 Sep 2025 00:51:40 -0400
Subject: [PATCH 2/2] [clang][Tooling] Support 'c++latest' in
 InterpolatingCompilationDatabase

Fixes https://github.com/clangd/clangd/issues/527
Fixes https://github.com/clangd/clangd/issues/1850
---
 .../clangd/unittests/CompileCommandsTests.cpp |  9 ++++++
 .../InterpolatingCompilationDatabase.cpp      | 30 ++++++++++++++++---
 2 files changed, 35 insertions(+), 4 deletions(-)

diff --git a/clang-tools-extra/clangd/unittests/CompileCommandsTests.cpp b/clang-tools-extra/clangd/unittests/CompileCommandsTests.cpp
index e324404e627c2..660540afd2320 100644
--- a/clang-tools-extra/clangd/unittests/CompileCommandsTests.cpp
+++ b/clang-tools-extra/clangd/unittests/CompileCommandsTests.cpp
@@ -536,6 +536,15 @@ TEST(CommandMangler, StdLatestFlag) {
   EXPECT_THAT(llvm::join(Cmd.CommandLine, " "), HasSubstr("/std:c++latest"));
 }
 
+TEST(CommandMangler, StdLatestFlag_Inference) {
+  const auto Mangler = CommandMangler::forTests();
+  tooling::CompileCommand Cmd;
+  Cmd.CommandLine = {"clang-cl", "/std:c++latest", "--", "/Users/foo.cc"};
+  Mangler(Cmd, "/Users/foo.hpp");
+  // Check that the /std:c++latest flag is not dropped during inference
+  EXPECT_THAT(llvm::join(Cmd.CommandLine, " "), HasSubstr("/std:c++latest"));
+}
+
 } // namespace
 } // namespace clangd
 } // namespace clang
diff --git a/clang/lib/Tooling/InterpolatingCompilationDatabase.cpp b/clang/lib/Tooling/InterpolatingCompilationDatabase.cpp
index 995019ca5a4d4..28568426a6c48 100644
--- a/clang/lib/Tooling/InterpolatingCompilationDatabase.cpp
+++ b/clang/lib/Tooling/InterpolatingCompilationDatabase.cpp
@@ -123,6 +123,15 @@ static types::ID foldType(types::ID Lang) {
   }
 }
 
+// Return the language standard that's activated by the /std:c++latest
+// flag in clang-CL mode.
+static LangStandard::Kind latestLangStandard() {
+  // FIXME: Have a single source of truth for the mapping from
+  // c++latest --> c++26 that's shared by the driver code
+  // (clang/lib/Driver/ToolChains/Clang.cpp) and this file.
+  return LangStandard::lang_cxx26;
+}
+
 // A CompileCommand that can be applied to another file.
 struct TransferableCommand {
   // Flags that should not apply to all files are stripped from CommandLine.
@@ -237,9 +246,16 @@ struct TransferableCommand {
     // --std flag may only be transferred if the language is the same.
     // We may consider "translating" these, e.g. c++11 -> c11.
     if (Std != LangStandard::lang_unspecified && foldType(TargetType) == Type) {
-      Result.CommandLine.emplace_back((
-          llvm::Twine(ClangCLMode ? "/std:" : "-std=") +
-          LangStandard::getLangStandardForKind(Std).getName()).str());
+      const char *Spelling =
+          LangStandard::getLangStandardForKind(Std).getName();
+      // In clang-cl mode, the latest standard is spelled 'c++latest' rather
+      // than e.g. 'c++26', and the driver does not accept the latter, so emit
+      // the spelling that the driver does accept.
+      if (ClangCLMode && Std == latestLangStandard()) {
+        Spelling = "c++latest";
+      }
+      Result.CommandLine.emplace_back(
+          (llvm::Twine(ClangCLMode ? "/std:" : "-std=") + Spelling).str());
     }
     Result.CommandLine.push_back("--");
     Result.CommandLine.push_back(std::string(Filename));
@@ -296,8 +312,14 @@ struct TransferableCommand {
   // Try to interpret the argument as '-std='.
   std::optional<LangStandard::Kind> tryParseStdArg(const llvm::opt::Arg &Arg) {
     using namespace driver::options;
-    if (Arg.getOption().matches(ClangCLMode ? OPT__SLASH_std : OPT_std_EQ))
+    if (Arg.getOption().matches(ClangCLMode ? OPT__SLASH_std : OPT_std_EQ)) {
+      // "c++latest" is not a recognized LangStandard, but it's accepted by
+      // the clang driver in CL mode.
+      if (ClangCLMode && StringRef(Arg.getValue()) == "c++latest") {
+        return latestLangStandard();
+      }
       return LangStandard::getLangKind(Arg.getValue());
+    }
     return std::nullopt;
   }
 };