[Thanatos9404]

Fixes #21789

The generic registration functions (register.Function*, register.DoFn* and register.Combiner*) pass every struct type a function takes or returns to runtime.RegisterType, which panics for unnamed types. So a function with an anonymous struct parameter panicked in init() and couldn't use generic registration at all.

registerMethodTypes now adds a struct type to the runtime registry only when it has a type key, and still adds every struct type to the schema registry, which the worker needs to map a row back to the user's Go type. The change is in register.tmpl, and register.go was regenerated with go generate. An unnamed struct that can't be converted to a schema now fails in beam.Init, like a named one, instead of panicking earlier.

I also enabled the register.Function1x1(castFn) call in the BigQuery xlang integration test that was waiting on this. I only checked that its test binary now initialises.

Tests: three new tests register functions and a DoFn with anonymous struct parameters or return types. All three fail on master with the panic and pass with this change. CHANGES.md is updated under Bugfixes for 2.78.0.

[github-actions]

📋 Review Summary

This PR resolves a panic during generic registration (register.Function*, register.DoFn*, register.Combiner*) when functions or methods accept or return anonymous struct types. By introducing registerStructType, unnamed structs are skipped for runtime.RegisterType (which requires a type key) while still being registered with schema.RegisterType so row schemas and nested named structs are properly mapped on workers.

🔍 General Feedback

• The changes in register.tmpl and the regenerated register.go are clean, minimal, and accurately target the root cause (#21789).
• Re-enabling register.Function1x1(castFn) in bigquery_test.go and the new unit tests in register_test.go thoroughly cover parameter, return, and DoFn registration paths.

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[github-actions]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.41%. Comparing base (a8d6afd) to head (7241c9b).
⚠️ Report is 35 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #40649       +/-   ##
===========================================
- Coverage   80.04%   56.41%   -23.64%     
===========================================
  Files         490      871      +381     
  Lines       82916   154091    +71175     
===========================================
+ Hits        66373    86923    +20550     
- Misses      16543    65314    +48771     
- Partials        0     1854     +1854     

Flag	Coverage Δ
go	28.87% <ø> (?)

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