Summary

• Prevent same-path generation overwriting (struct + union marker).

Problem

• In designs that:
  • put a user type into a shared package via Meta("struct:pkg:path", "types"), and
  • include that user type as a branch of a union, and
  • reference that user type from more than one service,
    generation could produce multiple files targeting the same output path (e.g., gen/types/summary.go). The later render overwrote the earlier one, leaving only the union "marker" method and dropping the struct definition.

When it shows up

• After goa gen, you might see files like gen/types/.go contain only:
  func (*Type) Val() {}
• Build errors such as: undefined: in code that references the struct.

Root cause

• Generator wrote files sequentially without coalescing sections that shared the same path.

Fix

• New merge-by-path step in generator.Generate: for each output path, merge headers (imports) and non-header sections, skipping duplicates, then render once. This ensures the struct and union marker end up in the same file.

Tests

• generate_merge_test.go: minimal same-path merge unit test (struct + method).
• generate_union_merge_integration_test.go: small DSL with user type in types/, union including that type, and two services referencing it; asserts both struct and union marker are present in gen/types/summary.go.
• All repo tests + JSON-RPC integration tests pass.

Notes

• Also simplified CLI usage template to avoid fmt parsing pitfalls when examples contain %.