fix #5654

compiler/ccgexprs.nim

@@ -267,7 +267,7 @@ proc genAssignment(p: BProc, dest, src: TLoc, flags: TAssignmentFlags) =
     # little HACK to support the new 'var T' as return type:
     linefmt(p, cpsStmts, "$1 = $2;$n", rdLoc(dest), rdLoc(src))
     return
-  let ty = skipTypes(dest.t, abstractRange)
+  let ty = skipTypes(dest.t, abstractRange + tyUserTypeClasses)
   case ty.kind
   of tyRef:
     genRefAssign(p, dest, src, flags)

compiler/ccgtypes.nim

@@ -164,7 +164,7 @@ proc mapType(typ: PType): TCTypeKind =
   of tySet: result = mapSetType(typ)
   of tyOpenArray, tyArray, tyVarargs: result = ctArray
   of tyObject, tyTuple: result = ctStruct
-  of tyUserTypeClass, tyUserTypeClassInst:
+  of tyUserTypeClasses:
     internalAssert typ.isResolvedUserTypeClass
     return mapType(typ.lastSon)
   of tyGenericBody, tyGenericInst, tyGenericParam, tyDistinct, tyOrdinal,
@@ -1131,6 +1131,9 @@ proc genTypeInfo(m: BModule, t: PType): Rope =
   of tyStatic:
     if t.n != nil: result = genTypeInfo(m, lastSon t)
     else: internalError("genTypeInfo(" & $t.kind & ')')
+  of tyUserTypeClasses:
+    internalAssert t.isResolvedUserTypeClass
+    return genTypeInfo(m, t.lastSon)
   of tyProc:
     if t.callConv != ccClosure:
       genTypeInfoAuxBase(m, t, t, result, rope"0")

tests/concepts/trandom_vars.nim

@@ -0,0 +1,42 @@
+discard """
+output: "11.0"
+"""
+
+type
+  # A random number generator
+  Random = object
+    random: proc(): float
+  # A generic typeclass for a random var
+  RandomVar[A] = concept x
+    var rng: Random
+    rng.sample(x) is A
+  # A few concrete instances
+  Uniform = object
+    a, b: float
+  ClosureVar[A] = object
+    f: proc(rng: var Random): A
+
+# How to sample from various concrete instances
+proc sample(rng: var Random, u: Uniform): float = u.a + (u.b - u.a) * rng.random()
+
+proc sample[A](rng: var Random, c: ClosureVar[A]): A = c.f(rng)
+
+proc uniform(a, b: float): Uniform = Uniform(a: a, b: b)
+
+# How to lift a function on values to a function on random variables
+proc map[A, B](x: RandomVar[A], f: proc(a: A): B): ClosureVar[B] =
+  proc inner(rng: var Random): B =
+    f(rng.sample(x))
+
+  result.f = inner
+
+import future
+
+proc fakeRandom(): Random =
+  result.random = () => 0.5
+
+let x = uniform(1, 10).map((x: float) => 2 * x)
+
+var rng = fakeRandom()
+
+echo rng.sample(x)