diff --git a/src/cmd/compile/internal/gc/subr.go b/src/cmd/compile/internal/gc/subr.go
index 2210c0c762..b4b758e07c 100644
--- a/src/cmd/compile/internal/gc/subr.go
+++ b/src/cmd/compile/internal/gc/subr.go
@@ -743,9 +743,21 @@ func assignop(src *Type, dst *Type, why *string) Op {
 	// and either src or dst is not a named type or
 	// both are empty interface types.
 	// For assignable but different non-empty interface types,
-	// we want to recompute the itab.
-	if eqtype(src.Orig, dst.Orig) && (src.Sym == nil || dst.Sym == nil || src.IsEmptyInterface()) {
-		return OCONVNOP
+	// we want to recompute the itab. Recomputing the itab ensures
+	// that itabs are unique (thus an interface with a compile-time
+	// type I has an itab with interface type I).
+	if eqtype(src.Orig, dst.Orig) {
+		if src.IsEmptyInterface() {
+			// Conversion between two empty interfaces
+			// requires no code.
+			return OCONVNOP
+		}
+		if (src.Sym == nil || dst.Sym == nil) && !src.IsInterface() {
+			// Conversion between two types, at least one unnamed,
+			// needs no conversion. The exception is nonempty interfaces
+			// which need to have their itab updated.
+			return OCONVNOP
+		}
 	}
 
 	// 3. dst is an interface type and src implements dst.
diff --git a/test/fixedbugs/issue18595.go b/test/fixedbugs/issue18595.go
new file mode 100644
index 0000000000..d6f07b30e2
--- /dev/null
+++ b/test/fixedbugs/issue18595.go
@@ -0,0 +1,53 @@
+// run
+
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This test makes sure that itabs are unique.
+// More explicitly, we require that only one itab structure exists for the pair of
+// a given compile-time interface type and underlying concrete type.
+// Ensuring this invariant enables fixes for 18492 (improve type switch code).
+
+package main
+
+type I interface {
+	M()
+}
+type J interface {
+	M()
+}
+
+type T struct{}
+
+func (*T) M() {}
+
+func main() {
+	test1()
+	test2()
+}
+
+func test1() {
+	t := new(T)
+	var i1, i2 I
+	var j interface {
+		M()
+	}
+	i1 = t
+	j = t
+	i2 = j
+	if i1 != i2 {
+		panic("interfaces not equal")
+	}
+}
+
+func test2() {
+	t := new(T)
+	i1 := (I)(t)
+	i2 := (I)((interface {
+		M()
+	})((J)(t)))
+	if i1 != i2 {
+		panic("interfaces not equal")
+	}
+}