Fix nested_format_specs buffer context construction - #4963
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…mat_specs Signed-off-by: Tyagiquamar <mohdquamartyagi@gmail.com>
vitaut
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Oct 3, 2026
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This seems to move the mismatch from the output iterator to the argument pack. ctx.args() has type basic_format_args<FormatContext>, while the new buffer context expects basic_format_args<basic_format_context<basic_appender<Char>, Char>>, and there is no conversion between those when FormatContext is a generic_context.
Could you add the exact #4949 reproducer to compile-test.cc? I believe this still fails to instantiate.
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Problem
In
nested_format_specs::write,buffer_ctxwas constructed directly asFormatContext(basic_appender<Char>(buf), ctx.args(), ctx.locale()). WhenFormatContextis not parameterized withbasic_appender<Char>(such as when custom contexts or different output iterators are passed, e.g. during compilation/custom formatters with nested formatted ranges likestd::arrayor tuples), attempting to constructFormatContextdirectly withbasic_appender<Char>leads to template instantiation and iterator mismatch errors (#4949).Solution
basic_format_context<basic_appender<Char>, Char>for the temporary buffer context innested_format_specs::write().Validation
ranges-test(42/42 tests) andcompile-test(19/19 tests).