[cling] TransactionUnloader: ensure function instantiations are proce…

…ssed only once

Implicit instantiation of a function template calls
`DeclCollector::HandleCXXImplicitFunctionInstantiation()`, which appends the
FunctionDecl to the transaction.  According to clang documentation, the body of
the function has not yet been instantiated. `HandleTopLevelDecl()` will be
called again for this decl at the end of the TU, which will append it
again to the transaction - same `Decl *`, different ConsumerCallInfo.

This is by design. However, unloading of decls in the transaction should
not process the same `Decl *` twice. In particular, entries with ConsumerCallInfo
== `kCCIHandleCXXImplicitFunctionInstantiation` will omitted.

Fixes issue #9850.

interpreter/cling/lib/Interpreter/TransactionUnloader.cpp

@@ -35,13 +35,23 @@ namespace cling {
 
       if (Call == Transaction::kCCIHandleVTable)
         continue;
+
+      // The documentation for `ASTConsumer::HandleCXXImplicitFunctionInstantiation()`
+      // states that implicit function instantiations come through both
+      // `HandleCXXImplicitFunctionInstantiation` (before the body is intantiated)
+      // and `HandleTopLevelDecl` (after).
+      // Therefore, the same decl is duplicated in the transaction differing in the ConsumerCallInfo. `UnloadDecl()` should be called only once.
+      if (Call == Transaction::kCCIHandleCXXImplicitFunctionInstantiation)
+        continue;
+
       // The non templated classes come through HandleTopLevelDecl and
       // HandleTagDeclDefinition, this is why we need to filter.
-      if (Call == Transaction::kCCIHandleTagDeclDefinition)
-      if (const CXXRecordDecl* D
-        = dyn_cast<CXXRecordDecl>(DGR.getSingleDecl()))
-      if (D->getTemplateSpecializationKind() == TSK_Undeclared)
-        continue;
+      if (Call == Transaction::kCCIHandleTagDeclDefinition) {
+        if (const CXXRecordDecl* D
+            = dyn_cast<CXXRecordDecl>(DGR.getSingleDecl()))
+          if (D->getTemplateSpecializationKind() == TSK_Undeclared)
+            continue;
+      }
 
       if (Call == Transaction::kCCINone)
         m_Interp->unload(*(*T->rnested_begin()));

interpreter/cling/test/CodeUnloading/RereadFile.C

@@ -28,3 +28,8 @@ macro() // CHECK: 2.version 2
 //CHECK: 13
 .x unnamedns.h
 //CHECK-NEXT: 13
+
+.x templatedfunc.h
+//CHECK: 4
+.x templatedfunc.h
+//CHECK-NEXT: 4

interpreter/cling/test/CodeUnloading/templatedfunc.h

@@ -0,0 +1,10 @@
+template <typename T> T square(T x) {
+  // This unused lambda caused a crash if the file is unloaded; see
+  // https://github.com/root-project/root/issues/9850
+  auto lambda = [](double x) { return x; };
+  return x * x;
+}
+
+void templatedfunc() {
+  printf("%d\n", square(2));
+}