fixes #5638

compiler/semcall.nim

@@ -250,13 +250,11 @@ proc resolveOverloads(c: PContext, n, orig: PNode,
       if result.state in {csEmpty, csNoMatch}:
         tryOp "."
 
-    elif nfDotSetter in n.flags:
-      internalAssert f.kind == nkIdent and n.sonsLen == 3
+    elif nfDotSetter in n.flags and f.kind == nkIdent and n.len == 3:
       let calleeName = newStrNode(nkStrLit,
         f.ident.s[0..f.ident.s.len-2]).withInfo(n.info)
       let callOp = newIdentNode(getIdent".=", n.info)
       n.sons[0..1] = [callOp, n[1], calleeName]
-      #excl(n.flags, nfDotSetter)
       orig.sons[0..1] = [callOp, orig[1], calleeName]
       pickBest(callOp)
 

tests/metatype/tcompilesregression.nim

@@ -0,0 +1,18 @@
+discard """
+  output: '''ok'''
+"""
+
+# bug #5638
+
+type X = object
+  a_impl: int
+
+proc a(x: X): int =
+  x.a_impl
+
+var x: X
+assert(not compiles((block:
+  x.a = 1
+)))
+
+echo "ok"