fixes #6732 (#11255)

compiler/sigmatch.nim

@@ -124,6 +124,12 @@ proc initCandidate*(ctx: PContext, c: var TCandidate, callee: PType) =
   initIdTable(c.bindings)
 
 proc put(c: var TCandidate, key, val: PType) {.inline.} =
+  when false:
+    let old = PType(idTableGet(c.bindings, key))
+    if old != nil:
+      echo "Putting ", typeToString(key), " ", typeToString(val), " and old is ", typeToString(old)
+      if typeToString(old) == "seq[string]":
+        writeStackTrace()
   idTablePut(c.bindings, key, val.skipIntLit)
 
 proc initCandidate*(ctx: PContext, c: var TCandidate, callee: PSym,
@@ -1520,7 +1526,12 @@ proc typeRelImpl(c: var TCandidate, f, aOrig: PType,
           elif x.kind in {tyGenericInvocation, tyGenericParam}:
             internalError(c.c.graph.config, "wrong instantiated type!")
           else:
-            put(c, f.sons[i], x)
+            let key = f.sons[i]
+            let old = PType(idTableGet(c.bindings, key))
+            if old == nil:
+              put(c, key, x)
+            elif typeRel(c, old, x, flags + {trDontBind}) == isNone:
+              return isNone
 
       if result == isNone:
         # Here object inheriting from generic/specialized generic object

tests/generics/tprevent_double_bind.nim

@@ -0,0 +1,21 @@
+discard """
+  errormsg: "type mismatch: got <TT[seq[string]], proc (v: int){.gcsafe, locks: 0.}>"
+  line: 20
+"""
+
+# bug #6732
+import typetraits
+
+type
+  TT[T] = ref object of RootObj
+    val: T
+  CB[T] = proc (v: T)
+
+proc testGeneric[T](val: TT[T], cb: CB[T]) =
+  echo val.type.name
+  echo $val.val
+
+var tt = new(TT[seq[string]])
+echo tt.type.name
+tt.testGeneric( proc (v: int) =
+    echo $v )