C++: Fix missing results for Node.asDefinition

[MathiasVP]

For a dataflow node n one can ask if n is the node corresponding to a StoreInstruction. This is useful when one wants to deal uniformly with all kinds of assignments/initializers/increments/decrements/etc.

The predicate also tells you whether an assignment overwrites an entire buffer or whether it may be a partial write (i.e., node.asDefinition(true) means the write may not overwrite the entire buffer). In order to do that it uses some of the SSA predicates (in particular, SsaImpl::defToNode) to find the corresponding Ssa::Definition belonging to the StoreInstruction.

However, if there's no corresponding SSA information for the StoreInstruction (for instance, if it's a store to a field or to a variable that is not live) then SsaImpl::defToNode won't hold. And so the entire node.asDefinition predicate did not have a result.

This PR fixes this so that, instead of the predicate not having a result, it will have a result with uncertain = true. This means that node.asDefinition can now be used to grab all assignments/initializers/increments/decrements/etc regardless of whether we have SSA information for it.

cc @bdrodes

[Copilot]

Pull request overview

Fixes DataFlow::Node.asDefinition so it returns a result (marked uncertain = true) even when the underlying StoreInstruction has no corresponding SSA definition, preventing “missing result” cases.

Changes:

• Adjusted Node.asDefinition/1 implementation to avoid depending on SsaImpl::defToNode for producing any result.
• Added a new InlineExpectations-based regression test for asDefinition over different store forms (including a field store / dead variable store).

Reviewed changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated 1 comment.

File	Description
cpp/ql/lib/semmle/code/cpp/ir/dataflow/internal/DataFlowUtil.qll	Updates asDefinition uncertainty determination so stores without SSA still yield a result.
cpp/ql/test/library-tests/dataflow/asDefinition/test.ql	Adds an InlineExpectations test query for Node.asDefinition.
cpp/ql/test/library-tests/dataflow/asDefinition/test.cpp	Adds C++ test cases exercising assignments/initializers including a field store.
cpp/ql/test/library-tests/dataflow/asDefinition/test.expected	Empty InlineExpectations expected-output file for the new test.

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