cgen: resolve generic map types in concrete contexts

[guweigang]

Summary

Fix C generation for generic map key/value types when a generic struct method is instantiated with multiple concrete types.

The bug showed up when a generic struct owns a map whose value type depends on T, for example map[string]StoredValue[T]. The C backend could reuse stale map key/value types from a previous concrete instantiation, producing mixed C types for different Ts.

Changes

• Resolve map key/value types from the current concrete map type before generating map literals and map index access.
• Reuse the same map type resolution path for map index expressions.
• Resolve if value := map[key] guard variables from the concrete map value type.
• Use the expected struct field map type when generating map literals inside struct initialization.
• Add a regression test covering:
  • map[string]Wrapped[T]
  • map[string]T
  • map[int]Wrapped[T]
  • map[int]T
  • empty and seeded map literals
  • map set/get and if-guard access

Validation

• ./v test vlib/v/tests/generics
• ./v test vlib/v/tests/options/option_map_struct_field_init_test.v
• ./v test vlib/v/tests/concurrency/shared_elem_test.v
• ./v vlib/v/generics/new_generics_regression_test.v
• git diff --check

[chatgpt-codex-connector]

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Codex Review: Didn't find any major issues. You're on a roll.

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