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[cling] Fix infinite recursion when printing self-referencing containers #21072

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Merged
dpiparo merged 1 commit into from
Jan 30, 2026
Merged

[cling] Fix infinite recursion when printing self-referencing containers #21072

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14 changes: 12 additions & 2 deletions interpreter/cling/include/cling/Interpreter/RuntimePrintValue.h
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Original file line number Diff line number Diff line change
Expand Up @@ -210,10 +210,20 @@ namespace cling {
const void* M = TypeTest::isMap(obj);

std::string str("{ ");
str += printValue(&(*iter), M);

// If the dereferenced iterator points to the container itself, we have
// infinite recursion. This occurs with scalar values in nlohmann::json
Comment on lines +214 to +215
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@pcanal pcanal Jan 29, 2026

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I am confused. In the example, the 'points to the container itself' happens only if the container is empty and thus this line should not be reached (see line 208).

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@pcanal pcanal Jan 29, 2026

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This is probably related to the odd implementation mentioned at https://github.com/root-project/root/pull/21072/changes#r2742975180

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@devajithvs devajithvs Jan 30, 2026 •
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This is a problem in nlohmann::json, scalar values are iterable and can dereference to themselves.

root [0] nlohmann::json j;
root [1] j[0] = 1;
root [2] auto it = j.begin();
root [3] bool(it == j.end())
(bool) false
root [4] auto& elem = *it;
root [5] auto elem_it = elem.begin();
root [6] bool(elem_it == elem.end()) // line 208 check
(bool) false
root [7] bool(&elem == &(*elem_it)) // iterator dereferences to itself and infinite recursion
(bool) true

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@devajithvs devajithvs Jan 30, 2026

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I was trying to somewhat mimic what scalar values does in nlohmann::json, I have simplified the test case to avoid this confusion.

auto printWithRecursionGuard = [obj](const auto* ptr,
const void* M) -> std::string {
if (static_cast<const void*>(ptr) == static_cast<const void*>(obj))
return "<recursion detected>";
return printValue(ptr, M);
};

str += printWithRecursionGuard(&(*iter), M);
while (++iter != iterEnd) {
str += ", ";
str += printValue(&(*iter), M);
str += printWithRecursionGuard(&(*iter), M);
}
return str + " }";
}
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30 changes: 30 additions & 0 deletions interpreter/cling/test/Prompt/ValuePrinter/Recursion.C
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@@ -0,0 +1,30 @@
//------------------------------------------------------------------------------
// CLING - the C++ LLVM-based InterpreterG :)
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//
// This file is dual-licensed: you can choose to license it under the University
// of Illinois Open Source License or the GNU Lesser General Public License. See
// LICENSE.TXT for details.

//------------------------------------------------------------------------------

// RUN: cat %s | %cling -Xclang -verify 2>&1 | FileCheck %s

.rawInput 1
// When begin() != end() but *begin() points to the container itself
// (in nlohmann::json), printValue_impl infinitely recurses without the
// self-reference check.

class RecursionTest {
public:
RecursionTest() = default;
auto begin() const { return this; } // iterate over self
auto end() const { return this + 1; } // just to make sure begin() != end()
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@pcanal pcanal Jan 30, 2026

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Suggested change
auto end() const { return this + 1; } // just to make sure begin() != end()
auto end() const { return this + 1; } // Hard code the size of the collection to be exactly one and it 'just' contains this object.

};
.rawInput 0

RecursionTest j;
j
// CHECK: (RecursionTest &) { <recursion detected> }

// expected-no-diagnostics
.q
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