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The re-normalization performed from diagnose_nested_requirement doesn't always work because we may have already lost the necessary template context that determines the set of in-scope template parameters used by the nested-requirement. This leads to normalization producing atoms that have incomplete/bogus parameter mappings, which breaks satisfaction. To fix this, we could just use the normal form that we previously computed at parse time, but this normal form lacks the diagnostic information that leads to good error messages. Instead, this patch makes diagnose_nested_requirement normalize twice at parse time -- once without diagnostic information and once with -- so that routines can use the "regular" normal form when performing satisfaction quietly and the "diagnostic" normal form when performing satisfaction noisily. Moreover, this patch makes tsubst_nested_requirement always first perform satisfaction quietly so that the satisfaction cache can get consistently utilized. This patch also adds some sanity checks to build_parameter_mapping that would have caught the underlying bug sooner (and deterministically). gcc/cp/ChangeLog: PR c++/97093 * constraint.cc (parameter_mapping_equivalent_p): Add some sanity checks. Clarify comment. (tsubst_nested_requirement): Always perform satisfaction quietly first. If that yields an erroneous result, emit a context message and replay satisfaction noisily with the diagnostic normal form. (finish_nested_requirement): Normalize the constraint-expression twice, once with diagnostic information and once without. Store them in a TREE_LIST within the TREE_TYPE. (diagnose_nested_requirement): When replaying satisfaction, use the diagnostic normal form instead of renormalizing on the spot. gcc/testsuite/ChangeLog: PR c++/97093 * g++.dg/cpp2a/concepts-requires22.C: New test.
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