Go: cannot assign to struct field in map (m["key"].Field = value)

cannot assign to struct field m["user"].Age in map (compile error: cannot assign to field of map index expression)

Immediate Remediation

bash

cat <<'EOF'
// Before (error)
m := map[string]User{"user": {Name: "Alice"}}
m["user"].Age = 30 // cannot assign

// Fix 1: use a map of pointers
m := map[string]*User{"user": {Name: "Alice"}}
m["user"].Age = 30

// Fix 2: copy, mutate, reassign
u := m["user"]
u.Age = 30
m["user"] = u
EOF

Root Cause Analysis

In Go, map index expressions are not addressable, so you cannot directly assign to a field of a struct stored in a map. The map may rehash and move entries, so the language forbids taking addresses of map elements.

Verification & Guardrails

Using `map[string]*User` allows direct field assignment but requires careful nil handling and pointer semantics.
The copy-and-reassign approach works for any map value type and avoids pointer aliasing.
Run `go vet` or `go build` to confirm the fix compiles.

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