test: default value set at copied structure

src/Lean/Elab/Structure.lean

@@ -380,7 +380,7 @@ where
           let existingFieldType ← inferType existingFieldInfo.fvar
           unless (← isDefEq fieldType existingFieldType) do
             throwError "parent field type mismatch, field '{fieldName}' from parent '{parentStructName}' {← mkHasTypeButIsExpectedMsg fieldType existingFieldType}"
-          -- TODO: if new field has a default value, it should probably override the default at `infos` (if it has one)
+          /- Remark: if structure has a default value for this field, it will be set at the `processOveriddenDefaultValues` below. -/
           copy (i+1) (fieldMap.insert fieldName existingFieldInfo.fvar) infos
         | none =>
           let some fieldInfo ← getFieldInfo? (← getEnv) parentStructName fieldName | unreachable!

tests/lean/run/diamond4.lean

@@ -0,0 +1,14 @@
+structure A where
+  a : Nat
+
+structure B where
+  a : Nat := 1
+  b : Nat
+
+structure C extends A, B
+
+def f (b : Nat) : C :=
+  { b }
+
+theorem ex (b : Nat) : (f b).a = 1 :=
+  rfl