Fix #5888

compiler/semexprs.nim

@@ -1129,9 +1129,9 @@ proc builtinFieldAccess(c: PContext, n: PNode, flags: TExprFlags): PNode =
     # reset to prevent 'nil' bug: see "tests/reject/tenumitems.nim":
     ty = n.sons[0].typ
     return nil
-  ty = skipTypes(ty, {tyGenericInst, tyVar, tyPtr, tyRef, tyAlias})
   if ty.kind in tyUserTypeClasses and ty.isResolvedUserTypeClass:
     ty = ty.lastSon
+  ty = skipTypes(ty, {tyGenericInst, tyVar, tyPtr, tyRef, tyAlias})
   while tfBorrowDot in ty.flags: ty = ty.skipTypes({tyDistinct})
   var check: PNode = nil
   if ty.kind == tyObject:

compiler/semstmts.nim

@@ -467,6 +467,8 @@ proc hasEmpty(typ: PType): bool =
 
 proc makeDeref(n: PNode): PNode =
   var t = skipTypes(n.typ, {tyGenericInst, tyAlias})
+  if t.kind in tyUserTypeClasses and t.isResolvedUserTypeClass:
+    t = t.lastSon
   result = n
   if t.kind == tyVar:
     result = newNodeIT(nkHiddenDeref, n.info, t.sons[0])

compiler/semtypes.nim

@@ -1415,7 +1415,10 @@ proc semTypeNode(c: PContext, n: PNode, prev: PType): PType =
       result = errorType(c)
     else:
       result = typeExpr.typ.base
-      if result.isMetaType:
+      if result.isMetaType and
+         result.kind != tyUserTypeClass:
+           # the dot expression may refer to a concept type in
+           # a different module. allow a normal alias then.
         let preprocessed = semGenericStmt(c, n)
         result = makeTypeFromExpr(c, preprocessed.copyTree)
       else:

tests/concepts/t5888.nim

@@ -0,0 +1,26 @@
+discard """
+output: '''
+true
+true
+true
+f
+0
+'''
+"""
+
+import t5888lib/ca, t5888lib/opt
+
+type LocalCA = ca.CA
+
+proc f(c: CA) =
+  echo "f"
+  echo c.x
+
+var o = new(Opt)
+
+echo o is CA
+echo o is LocalCA
+echo o is ca.CA
+
+o.f()
+

tests/concepts/t5888lib/ca.nim

@@ -0,0 +1,4 @@
+type
+  CA* = concept c
+    c.x is int
+

tests/concepts/t5888lib/opt.nim

@@ -0,0 +1,6 @@
+import ca
+
+type
+  Opt* = object
+    x*: int
+