Fix #5084

compiler/sigmatch.nim

@@ -628,9 +628,9 @@ proc typeRangeRel(f, a: PType): TTypeRelation {.noinline.} =
   else:
     result = isNone
 
-proc matchUserTypeClass*(c: PContext, m: var TCandidate,
-                         ff, a: PType): PType =
+proc matchUserTypeClass*(m: var TCandidate; ff, a: PType): PType =
   var
+    c = m.c
     typeClass = ff.skipTypes({tyUserTypeClassInst})
     body = typeClass.n[3]
     matchedConceptContext: TMatchedConcept
@@ -661,6 +661,9 @@ proc matchUserTypeClass*(c: PContext, m: var TCandidate,
         typeParamName = ff.base.sons[i-1].sym.name
         typ = ff.sons[i]
         param: PSym
+        alreadyBound = PType(idTableGet(m.bindings, typ))
+
+      if alreadyBound != nil: typ = alreadyBound
 
       template paramSym(kind): untyped =
         newSym(kind, typeParamName, typeClass.sym, typeClass.sym.info)
@@ -1490,7 +1493,7 @@ proc typeRelImpl(c: var TCandidate, f, aOrig: PType,
     else:
       considerPreviousT:
         if aOrig == f: return isEqual
-        var matched = matchUserTypeClass(c.c, c, f, aOrig)
+        var matched = matchUserTypeClass(c, f, aOrig)
         if matched != nil:
           bindConcreteTypeToUserTypeClass(matched, a)
           if doBind: put(c, f, matched)

tests/concepts/trandomvars.nim

@@ -0,0 +1,61 @@
+discard """
+output: '''
+true
+true
+true
+3
+18.0
+324.0
+'''
+"""
+
+type RNG = object
+
+proc random(rng: var RNG): float = 1.0
+
+type
+  RandomVar[A] = concept x
+    var rng: RNG
+    rng.sample(x) is A
+
+  Constant[A] = object
+    value: A
+
+  Uniform = object
+    a, b: float
+
+  ClosureVar[A] = proc(rng: var RNG): A
+
+proc sample[A](rng: var RNG, c: Constant[A]): A = c.value
+
+proc sample(rng: var RNG, u: Uniform): float = u.a + (u.b - u.a) * rng.random()
+
+proc sample[A](rng: var RNG, c: ClosureVar[A]): A = c(rng)
+
+proc constant[A](a: A): Constant[A] = Constant[A](value: a)
+
+proc uniform(a, b: float): Uniform = Uniform(a: a, b: b)
+
+proc lift1[A, B](f: proc(a: A): B, r: RandomVar[A]): ClosureVar[B] =
+  proc inner(rng: var RNG): B = f(rng.sample(r))
+
+  return inner
+
+when isMainModule:
+  proc sq(x: float): float = x * x
+
+  let
+    c = constant(3)
+    u = uniform(2, 18)
+    t = lift1(sq, u)
+
+  var rng: RNG
+
+  echo(c is RandomVar[int])
+  echo(u is RandomVar[float])
+  echo(t is RandomVar[float])
+
+  echo rng.sample(c)
+  echo rng.sample(u)
+  echo rng.sample(t)
+