[cling] Add deprecation warning for declarations without auto (root…

…-project#14645)

* [cling] Add deprecation warning for declarations without `auto`

Declarations without the auto keyword are not part of standard C++. Even
though it is a nice feature to have, it requires a patch on top of clang
and is one of the hurdles preventing us from using the upstream clang.

Implicit auto injection is currently only supported at the prompt (and
only in the top-most function-level scope). So it should ideally not break
other features.

There are a few GitHub and JIRA issues related to this feature that can
also be closed if we completely remove it.

* Set severity for diag::warn_deprecated_message manually

core/metacling/src/TClingCallbacks.cxx

@@ -33,6 +33,7 @@
 #include "clang/Sema/Scope.h"
 #include "clang/Serialization/ASTReader.h"
 #include "clang/Serialization/GlobalModuleIndex.h"
+#include "clang/Basic/DiagnosticSema.h"
 
 #include "llvm/ExecutionEngine/Orc/Core.h"
 
@@ -1004,6 +1005,12 @@ bool TClingCallbacks::tryInjectImplicitAutoKeyword(LookupResult &R, Scope *S) {
    Result->addAttr(AnnotateAttr::CreateImplicit(C, "__Auto"));
 
    R.addDecl(Result);
+
+   // Raise a warning when trying to use implicit auto injection feature.
+   SemaRef.getDiagnostics().setSeverity(diag::warn_deprecated_message, diag::Severity::Warning, SourceLocation());
+   SemaRef.Diag(Loc, diag::warn_deprecated_message)
+      << "declaration without the 'auto' keyword" << DC << Loc << FixItHint::CreateInsertion(Loc, "auto ");
+
    // Say that we can handle the situation. Clang should try to recover
    return true;
 }

core/metacling/test/TClingTests.cxx

@@ -239,6 +239,19 @@ TEST_F(TClingTests, GetSharedLibDeps)
 }
 #endif
 
+// Check that a warning message is generated when using auto-injection.
+TEST_F(TClingTests, WarningAutoInjection)
+{
+   ROOT::TestSupport::CheckDiagsRAII diags;
+   diags.requiredDiag(kWarning, "cling", "declaration without the 'auto' keyword is deprecated",
+                      /*matchFullMessage=*/false);
+
+   gInterpreter->ProcessLine("/* no auto */ t = new int;");
+
+   auto t = gInterpreter->GetDataMember(nullptr, "t");
+   EXPECT_TRUE(t != nullptr);
+}
+
 // Check the interface which interacts with the cling::LookupHelper.
 TEST_F(TClingTests, ClingLookupHelper) {
   // Exception spec evaluation.