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fix(time): give ParseError the exported fields of Go's ParseError - #527
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The time override's ParseError had lowercase fields, a positional constructor and no Error method, so Go code that makes one with a composite literal, as encoding/json/v2 does, built a broken value, and parse errors were wrapped as plain JavaScript errors that errors.As could not match to *time.ParseError. Give it Go's exported fields (Layout, Value, LayoutElem, ValueElem, Message), an init-object constructor, Go's Error formatting, and a registered struct type, and return it directly from the parse functions. Add a compliance test for errors.As, the fields, and composite literals. Signed-off-by: Randall C. O'Reilly <oreilly@ucdavis.edu>
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Oct 24, 2026
encoding/json/v2 is compiled from Go source, but it uses the internals of encoding/json/jsontext, whose override implements only Value and the formatting options. jsontext.Internal is null there, so any program that imports encoding/json/v2 panics with "invalid memory address or nil pointer dereference" while the package initializes (`var export = jsontext.Internal.Export(...)` in arshal.go), before main runs. Programs can reach it without importing it themselves, through generated symbol tables such as yaegi's standard library symbols for Go 1.27; that is how a Cogent Core web app hit it. I first tried dropping the jsontext override and compiling jsontext from source (listed as a nativeDependency of encoding/json, so RawMessage keeps working). With #526, #527 and #528, json/v2 then works and matches native Go. But every program that uses encoding/json would compile jsontext and its internal packages: for a small Marshal/Unmarshal program the compiled output grew from 1.26 MB to 1.91 MB, and because jsontext.pointerSuffixError embeds error, its promoted Error method is async, which makes fmt calls with interface arguments async as well. That seemed too high a price for programs that never use json/v2. So this adds an override for encoding/json/v2 whose strict parity ledger blocks every export. A program that uses one now fails to compile with parity-blocked-use, naming the symbol, instead of failing at run time. TestOverrideParityVerifierBlocksJSONV2 compiles a real json/v2 program and checks for that diagnostic (it fails without this change). Two compliance outputs gain an erased type-only import of encoding/json/v2, because encoding/json.Unmarshaler is an alias of jsonv2.Unmarshaler since Go 1.27; they still pass.
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The time override's ParseError had lowercase fields, a positional constructor and no Error method. Go code that makes one with a composite literal, as encoding/json/v2 does (&time.ParseError{Layout: ..., Message: ...}), built a broken value, and the parse functions' errors were wrapped as plain JavaScript errors, so errors.As could not match them to *time.ParseError.
This gives ParseError Go's exported fields (Layout, Value, LayoutElem, ValueElem, Message), an init-object constructor like other override structs, Go's Error formatting, and a registered struct type, and the parse functions return it directly. The error text is unchanged. The new compliance test package_import_time_parse_error checks errors.As, the fields, and composite literals against native Go.