This PR fixes a diamond problem in delta deriving where
instance-implicit class parameters in the derived instance type were
using instances synthesized for the underlying type, not the alias type.
When deriving an instance for a type alias (e.g., `def ENat := WithTop
ℕ`), this caused a diamond when the alias has its own instance for a
dependency class (e.g., `AddMonoidWithOne` from `CommSemiring`) that
differs from the underlying type's instance (e.g.,
`WithTop.addMonoidWithOne`). Instance search would fail because it
expected the alias's instance but the derived instance used the
underlying's.
The fix: after synthesis succeeds, for each instance-implicit class
parameter, re-synthesize for the target type and use that instance if
it's defeq to what we synthesized for the underlying type.
### Example
```lean
class MyBase (α : Type) where value : Nat := 42
class MyHigher (α : Type) [MyBase α] : Prop where prop : True
instance instBaseNat : MyBase Nat := {}
def MyAlias := Nat
instance instBaseMyAlias : MyBase MyAlias := {} -- Different expression, but defeq
instance instHigherNat : MyHigher Nat where prop := trivial
deriving instance MyHigher for MyAlias
```
**Before**: `instMyHigherMyAlias : @MyHigher MyAlias instBaseNat` →
instance search fails
**After**: `instMyHigherMyAlias : @MyHigher MyAlias instBaseMyAlias` →
instance search succeeds
### Motivation
This fixes the `CharZero ℕ∞` diamond in Mathlib under #12179 where the
derived instance was using `WithTop.addMonoidWithOne` instead of the
`AddMonoidWithOne` from `CommSemiring ℕ∞`.
🤖 Prepared with Claude Code
---------
Co-authored-by: Claude <noreply@anthropic.com>
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