diff --git a/compiler/semstmts.nim b/compiler/semstmts.nim
index 465276ffc20a2..6033e29c0be4a 100644
--- a/compiler/semstmts.nim
+++ b/compiler/semstmts.nim
@@ -853,6 +853,16 @@ proc semVarOrLet(c: PContext, n: PNode, symkind: TSymKind): PNode =
     if a[^2].kind != nkEmpty:
       typ = semTypeNode(c, a[^2], nil)
       hasUserSpecifiedType = true
+      # Issue #4086: `var f: Foo` for `type Foo[T = int]` auto-expands to
+      # `Foo[int]` when every generic param has a default. Restricted to
+      # var/let/const declarations so it does not affect type-level
+      # computations like `arity(SomeGeneric)` in template/typetraits
+      # contexts where bare generic-body references are intentional.
+      if typ != nil and typ.kind == tyGenericBody and typ.sym != nil:
+        let auto = tryGenericBodyDefaultInvocation(c,
+            newSymNode(typ.sym, a[^2].info), typ.sym, nil)
+        if auto != nil:
+          typ = auto
 
     var typFlags: TTypeAllowedFlags = {}
 
@@ -1009,6 +1019,16 @@ proc semConst(c: PContext, n: PNode): PNode =
     if a[^2].kind != nkEmpty:
       typ = semTypeNode(c, a[^2], nil)
       hasUserSpecifiedType = true
+      # Issue #4086: `var f: Foo` for `type Foo[T = int]` auto-expands to
+      # `Foo[int]` when every generic param has a default. Restricted to
+      # var/let/const declarations so it does not affect type-level
+      # computations like `arity(SomeGeneric)` in template/typetraits
+      # contexts where bare generic-body references are intentional.
+      if typ != nil and typ.kind == tyGenericBody and typ.sym != nil:
+        let auto = tryGenericBodyDefaultInvocation(c,
+            newSymNode(typ.sym, a[^2].info), typ.sym, nil)
+        if auto != nil:
+          typ = auto
 
     var typFlags: TTypeAllowedFlags = {}
 
diff --git a/compiler/semtypes.nim b/compiler/semtypes.nim
index 8009f7293c61e..534ece3ddb384 100644
--- a/compiler/semtypes.nim
+++ b/compiler/semtypes.nim
@@ -1736,6 +1736,33 @@ proc containsGenericInvocationWithForward(n: PNode): bool =
           return true
   return false
 
+proc semGeneric(c: PContext, n: PNode, s: PSym, prev: PType): PType
+
+proc tryGenericBodyDefaultInvocation*(c: PContext, n: PNode, s: PSym,
+                                      prev: PType): PType =
+  ## Issue #4086 sub-case: `type Foo[T = int]; var f: Foo` is sugar for
+  ## `var f: Foo[int]` when every generic param has a default. Synthesize
+  ## a `Foo[default1, default2, ...]` bracket node and dispatch to the
+  ## existing `semGeneric` so the default-substitution machinery (added
+  ## for issues #4086 / #9355) handles cascading and parameter-referencing
+  ## defaults uniformly.
+  result = nil
+  if s.typ == nil: return
+  let body = s.typ.skipTypes({tyAlias})
+  if body.kind != tyGenericBody or body.len < 2: return
+  for i in 0..<body.len-1:
+    let p = body[i]
+    if p.kind != tyGenericParam or p.sym == nil or p.sym.ast == nil:
+      return
+  var bracket = newNodeI(nkBracketExpr, n.info)
+  if n.kind == nkSym:
+    bracket.add n
+  else:
+    bracket.add newSymNode(s, n.info)
+  for i in 0..<body.len-1:
+    bracket.add copyTree(body[i].sym.ast)
+  result = semGeneric(c, bracket, s, prev)
+
 proc semGeneric(c: PContext, n: PNode, s: PSym, prev: PType): PType =
   if s.typ == nil:
     localError(c.config, n.info, "cannot instantiate the '$1' $2" %
@@ -1791,8 +1818,35 @@ proc semGeneric(c: PContext, n: PNode, s: PSym, prev: PType): PType =
     let rType = m.call[0].typ
     let mIndex = if rType != nil: rType.len - 1 else: -1
     var hasForwardTypeParam = false
+
+    # Issues #4086, #9355: when a generic param's default value references
+    # earlier type params (e.g. `type Foo[T; U = seq[T]]` or `func foo[T](U = T)`)
+    # substitute those references with already-resolved bindings before
+    # adding to the invocation. Bindings are accumulated as the loop walks
+    # left-to-right, so cascade defaults like `[T; U = seq[T]; V = seq[U]]`
+    # work too (each iteration resolves against the previous ones).
+    # Skip the bookkeeping entirely when no param has a default — this
+    # generic body cannot need substitution and the work would just be
+    # pure overhead (matters for large generic graphs / forward cycles).
+    var hasAnyDefault = false
+    for i in 0..<t.len-1:
+      let p = t[i]
+      if p.kind == tyGenericParam and p.sym != nil and p.sym.ast != nil:
+        hasAnyDefault = true
+        break
+    var defaultBindings = initLayeredTypeMap()
+    var hasDefaultBinding = false
+
     for i in 1..<m.call.len:
       var typ = m.call[i].typ
+
+      if hasDefaultBinding and nfDefaultParam in m.call[i].flags and
+         containsGenericType(typ):
+        var cl = initTypeVars(c, defaultBindings, n.info, getCurrOwner(c))
+        let substituted = replaceTypeVarsT(cl, typ)
+        if substituted != nil:
+          typ = substituted
+
       # is this a 'typedesc' *parameter*? If so, use the typedesc type,
       # unstripped.
       if m.call[i].kind == nkSym and m.call[i].sym.kind == skParam and
@@ -1807,6 +1861,10 @@ proc semGeneric(c: PContext, n: PNode, s: PSym, prev: PType): PType =
           skip = false
         addToResult(typ, skip)
 
+      if hasAnyDefault and i - 1 < t.kidsLen and t[i - 1].kind == tyGenericParam:
+        defaultBindings.put(t[i - 1], typ.skipTypes({tyTypeDesc}))
+        hasDefaultBinding = true
+
       if typ.kind == tyForward:
         hasForwardTypeParam = true
 
diff --git a/compiler/sigmatch.nim b/compiler/sigmatch.nim
index bf9c2d20503ae..e963984a748b8 100644
--- a/compiler/sigmatch.nim
+++ b/compiler/sigmatch.nim
@@ -3115,6 +3115,29 @@ proc matches*(c: PContext, n, nOrig: PNode, m: var TCandidate) =
         if nfDefaultRefsParam in formal.ast.flags:
           m.call.flags.incl nfDefaultRefsParam
         var defaultValue = copyTree(formal.ast)
+        # Issues #4086, #9355: when the default value is a bare reference
+        # to an earlier generic type param (e.g. `func foo[T](U: type = T)`
+        # or its untyped form `func foo[T](U = T)`) — recognised by the
+        # default's static type being `tyGenericParam` — substitute T
+        # against the explicit-instantiation bindings via
+        # `prepareTypesInBody` (which updates both the AST sym and its
+        # typ) and wrap the result as `tyTypeDesc` so the
+        # `tfImplicitTypeParam` binding path below treats it like a
+        # literal type default (`U: type = int`).
+        # Other defaults (value expressions like `arr.high`, proc-call
+        # expressions like `newTensor[T](0)` whose result type is a
+        # `tyGenericInvocation`/`tyGenericInst`, etc.) have a non-
+        # `tyGenericParam` type and are left untouched, so the existing
+        # default-handling machinery takes care of them.
+        if defaultValue.typ != nil and
+           defaultValue.typ.kind == tyGenericParam and
+           m.calleeSym != nil:
+          defaultValue = prepareTypesInBody(c, m.bindings, defaultValue, m.calleeSym)
+          if formal.typ.kind == tyTypeDesc and defaultValue.typ != nil and
+             defaultValue.typ.kind != tyTypeDesc:
+            let typedescTyp = newTypeS(tyTypeDesc, c, defaultValue.typ)
+            typedescTyp.incl tfCheckedForDestructor
+            defaultValue.typ = typedescTyp
         if defaultValue.kind == nkNilLit:
           defaultValue = implicitConv(nkHiddenStdConv, formal.typ, defaultValue, m, c)
         # proc foo(x: T = 0.0)
diff --git a/tests/generics/tgeneric_param_default_deps.nim b/tests/generics/tgeneric_param_default_deps.nim
new file mode 100644
index 0000000000000..81948a3f4c527
--- /dev/null
+++ b/tests/generics/tgeneric_param_default_deps.nim
@@ -0,0 +1,116 @@
+discard """
+  matrix: "; --mm:refc"
+"""
+
+import std/deques
+
+# Type-side generic param defaults that reference other type parameters
+# (issue #4086).
+#
+# Currently fails on Nim 2.3.1 devel:
+#   `Error: invalid type: 'Foo[system.int, seq[T]]' for var`
+#
+# Other languages (C++, TypeScript, Rust, Scala) all support this pattern.
+# Nim already supports T-independent defaults like `[T; U = int]`; this
+# extends support to type-side defaults that reference earlier type params
+# (e.g. `type Foo[T; U = seq[T]]`). This is needed for binding C++ templates
+# like `std::vector<T, allocator<T>>` and `std::unique_ptr<T, default_delete<T>>`.
+#
+# Note: proc-side brackets-internal defaults (`proc foo[T, U = T]()`) are
+# out of scope; the PR's logic targets type-side default substitution.
+# A separate change would be needed for proc-side signature defaults.
+
+block: # #4086 type-side: object generic param default `seq[T]`
+  type Foo[T; U = seq[T]] = object
+    data: U
+  var f: Foo[int]
+  f.data.add 42
+  doAssert f.data == @[42]
+
+block: # nested reference: U default uses T, V default uses U
+  type Foo[T; U = seq[T]; V = seq[U]] = object
+    data: V
+  var f: Foo[int]
+  f.data.add @[1, 2]
+  doAssert f.data == @[@[1, 2]]
+
+block: # type-side direct: U = T
+  type Foo[T; U = T] = object
+    a: T
+    b: U
+  var f: Foo[int]
+  f.a = 1
+  f.b = 2
+  doAssert f.b is int
+
+block: # type-side compound: U = ref T
+  type Foo[T; U = ref T] = object
+    p: U
+  var f: Foo[int]
+  f.p = new(int)
+  f.p[] = 9
+  doAssert f.p[] == 9
+
+block: # type-side compound: U = array[3, T]
+  type Foo[T; U = array[3, T]] = object
+    arr: U
+  var f: Foo[int]
+  f.arr[0] = 10
+  f.arr[2] = 30
+  doAssert f.arr[0] == 10
+  doAssert f.arr[2] == 30
+
+block: # #4086 original: all params defaulted, invocation with no args
+  type Foo[T = int] = object
+    x: T
+  var f: Foo
+  f.x = 42
+  doAssert f.x == 42
+
+block: # alias of a defaulted instantiation
+  type Foo[T; U = T] = object
+    a: T
+    b: U
+  type IntFoo = Foo[int]
+  var f: IntFoo
+  f.a = 1
+  f.b = 2
+  doAssert f.b is int
+
+block: # distinct of a defaulted instantiation
+  type Foo[T; U = seq[T]] = object
+    data: U
+  type DistFoo = distinct Foo[int]
+  var f: DistFoo
+  Foo[int](f).data.add 7
+  doAssert Foo[int](f).data == @[7]
+
+block: # union constraint + default referencing T (review request)
+  type Foo[T; U: seq[T]|Deque[T] = seq[T]] = object
+    data: U
+  var f: Foo[int]
+  f.data.add 42
+  doAssert f.data is seq[int]
+  doAssert f.data == @[42]
+
+block: # typeclass constraint + concrete-type default (review request)
+  type Foo[T: SomeInteger = int] = object
+    x: T
+  var f: Foo
+  f.x = 7
+  doAssert f.x is int
+  var g: Foo[int64]
+  g.x = 9'i64
+  doAssert g.x is int64
+
+block: # concept constraint + default (review request)
+  type HasLen = concept x
+    x.len is int
+  type Foo[T: HasLen = string] = object
+    val: T
+  var f: Foo
+  f.val = "hi"
+  doAssert f.val.len == 2
+  var g: Foo[seq[int]]
+  g.val = @[1, 2, 3]
+  doAssert g.val.len == 3