Fix #5983

compiler/semexprs.nim

@@ -106,7 +106,10 @@ proc checkConvertible(c: PContext, castDest, src: PType): TConvStatus =
       result = convNotNeedeed
     return
   var d = skipTypes(castDest, abstractVar)
-  var s = skipTypes(src, abstractVar-{tyTypeDesc})
+  var s = src
+  if s.kind in tyUserTypeClasses and s.isResolvedUserTypeClass:
+    s = s.lastSon
+  s = skipTypes(s, abstractVar-{tyTypeDesc})
   var pointers = 0
   while (d != nil) and (d.kind in {tyPtr, tyRef}) and (d.kind == s.kind):
     d = d.lastSon

compiler/types.nim

@@ -424,6 +424,12 @@ template bindConcreteTypeToUserTypeClass*(tc, concrete: PType) =
   tc.sons.safeAdd concrete
   tc.flags.incl tfResolved
 
+# TODO: It would be a good idea to kill the special state of a resolved
+# concept by switching to tyAlias within the instantiated procs.
+# Currently, tyAlias is always skipped with lastSon, which means that
+# we can store information about the matched concept in another position.
+# Then builtInFieldAccess can be modified to properly read the derived
+# consts and types stored within the concept.
 template isResolvedUserTypeClass*(t: PType): bool =
   tfResolved in t.flags
 

tests/concepts/t5983.nim

@@ -0,0 +1,22 @@
+discard """
+  output: "20.0 USD"
+"""
+
+import typetraits
+
+const currencies = ["USD", "EUR"] # in real code 120 currencies
+
+type USD* = distinct float # in real code 120 types generates using macro
+type EUR* = distinct float
+
+type CurrencyAmount = concept c
+  type t = c.type
+  const name = c.type.name
+  name in currencies
+
+proc `$`(x: CurrencyAmount): string =
+  $float(x) & " " & x.name
+
+let amount = 20.USD
+echo amount
+