Qiskit / qiskit / 24255087278

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Fix `TemplateOptimization` dropping the `global_phase` on substitution (#15943)

* fix(transpiler): TemplateOptimization drops circuit global_phase on substitution

circuit_to_dagdependency did not copy global_phase when converting a
template QuantumCircuit to DAGDependency, so template phases were lost
before matching. TemplateSubstitution.run_dag_opt also constructed a
new DAGDependency for the optimised output without inheriting the
original circuit's global_phase, resetting it to zero whenever at
least one substitution was applied.

Fixed by copying global_phase in circuit_to_dagdependency and seeding
dag_dep_opt.global_phase from the circuit DAG at construction time.
Also added the per-match phase subtraction (Fix 3) for when templates
with nonzero global_phase become accepted via the identity check fix
in #14538.

Fixes #14537.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix(transpiler): TemplateOptimization drops circuit global_phase on substitution

circuit_to_dagdependency did not copy global_phase when converting a
template QuantumCircuit to DAGDependency, so template phases were lost
before matching. TemplateSubstitution.run_dag_opt also constructed a
new DAGDependency for the optimised output without inheriting the
original circuit's global_phase, resetting it to zero whenever at
least one substitution was applied. Additionally, when a template
carries a nonzero global_phase phi_T (gate content implements
e^{-i*phi_T} * I while the full operator is I), the per-match phase
contribution was not being subtracted from the output circuit.

Fixes #14537.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* style: black reformat test_template_matching.py

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* docs: use RST hyperlink for issue reference in release note

The :issue: role is not available in this project's Sphinx config.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* test(transpiler): ... (continued)

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3 of 3 new or added lines in 2 files covered. (100.0%)

23 existing lines in 4 files now uncovered.

104552 of 119542 relevant lines covered (87.46%)

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