KITFORMA · EDITORIAL ANSWER
Why does a template need typename before T::value_type?
KitForma editorial guide. In a dependent context, the parser may not know that a qualified name denotes a type. The disambiguator states that intended interpretation.
Step-by-step answer
In a dependent context, the parser may not know that a qualified name denotes a type. The disambiguator states that intended interpretation.
For a declaration such as
typename T::value_type item{};, use typename where the language requires it and ensure the eventual T really supplies that type. Prefer a constrained interface when only containers with that member make sense. Do not add typename to unrelated expressions indiscriminately.
Instantiate with a valid container and a type lacking value_type to distinguish parsing from substitution errors. The need can vary by syntactic context and language version; read the first diagnostic. The keyword clarifies parsing, not the existence or semantics of the nested type.Sources and verification
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