diff --git a/src/cmd/compile/internal/gc/reflect.go b/src/cmd/compile/internal/gc/reflect.go
index 2863d4b5d0..7a93ece8b9 100644
--- a/src/cmd/compile/internal/gc/reflect.go
+++ b/src/cmd/compile/internal/gc/reflect.go
@@ -1095,6 +1095,28 @@ func needkeyupdate(t *types.Type) bool {
 	}
 }
 
+// hashMightPanic reports whether the hash of a map key of type t might panic.
+func hashMightPanic(t *types.Type) bool {
+	switch t.Etype {
+	case TINTER:
+		return true
+
+	case TARRAY:
+		return hashMightPanic(t.Elem())
+
+	case TSTRUCT:
+		for _, t1 := range t.Fields().Slice() {
+			if hashMightPanic(t1.Type) {
+				return true
+			}
+		}
+		return false
+
+	default:
+		return false
+	}
+}
+
 // formalType replaces byte and rune aliases with real types.
 // They've been separate internally to make error messages
 // better, but we have to merge them in the reflect tables.
@@ -1257,25 +1279,33 @@ func dtypesym(t *types.Type) *obj.LSym {
 		ot = dsymptr(lsym, ot, s1, 0)
 		ot = dsymptr(lsym, ot, s2, 0)
 		ot = dsymptr(lsym, ot, s3, 0)
+		var flags uint32
+		// Note: flags must match maptype accessors in ../../../../runtime/type.go
+		// and maptype builder in ../../../../reflect/type.go:MapOf.
 		if t.Key().Width > MAXKEYSIZE {
 			ot = duint8(lsym, ot, uint8(Widthptr))
-			ot = duint8(lsym, ot, 1) // indirect
+			flags |= 1 // indirect key
 		} else {
 			ot = duint8(lsym, ot, uint8(t.Key().Width))
-			ot = duint8(lsym, ot, 0) // not indirect
 		}
 
 		if t.Elem().Width > MAXVALSIZE {
 			ot = duint8(lsym, ot, uint8(Widthptr))
-			ot = duint8(lsym, ot, 1) // indirect
+			flags |= 2 // indirect value
 		} else {
 			ot = duint8(lsym, ot, uint8(t.Elem().Width))
-			ot = duint8(lsym, ot, 0) // not indirect
 		}
-
 		ot = duint16(lsym, ot, uint16(bmap(t).Width))
-		ot = duint8(lsym, ot, uint8(obj.Bool2int(isreflexive(t.Key()))))
-		ot = duint8(lsym, ot, uint8(obj.Bool2int(needkeyupdate(t.Key()))))
+		if isreflexive(t.Key()) {
+			flags |= 4 // reflexive key
+		}
+		if needkeyupdate(t.Key()) {
+			flags |= 8 // need key update
+		}
+		if hashMightPanic(t.Key()) {
+			flags |= 16 // hash might panic
+		}
+		ot = duint32(lsym, ot, flags)
 		ot = dextratype(lsym, ot, t, 0)
 
 	case TPTR:
diff --git a/src/reflect/type.go b/src/reflect/type.go
index f48f9cf09d..5ce80c61dc 100644
--- a/src/reflect/type.go
+++ b/src/reflect/type.go
@@ -394,16 +394,13 @@ type interfaceType struct {
 // mapType represents a map type.
 type mapType struct {
 	rtype
-	key           *rtype // map key type
-	elem          *rtype // map element (value) type
-	bucket        *rtype // internal bucket structure
-	keysize       uint8  // size of key slot
-	indirectkey   uint8  // store ptr to key instead of key itself
-	valuesize     uint8  // size of value slot
-	indirectvalue uint8  // store ptr to value instead of value itself
-	bucketsize    uint16 // size of bucket
-	reflexivekey  bool   // true if k==k for all keys
-	needkeyupdate bool   // true if we need to update key on an overwrite
+	key        *rtype // map key type
+	elem       *rtype // map element (value) type
+	bucket     *rtype // internal bucket structure
+	keysize    uint8  // size of key slot
+	valuesize  uint8  // size of value slot
+	bucketsize uint16 // size of bucket
+	flags      uint32
 }
 
 // ptrType represents a pointer type.
@@ -1859,6 +1856,8 @@ func MapOf(key, elem Type) Type {
 	}
 
 	// Make a map type.
+	// Note: flag values must match those used in the TMAP case
+	// in ../cmd/compile/internal/gc/reflect.go:dtypesym.
 	var imap interface{} = (map[unsafe.Pointer]unsafe.Pointer)(nil)
 	mt := **(**mapType)(unsafe.Pointer(&imap))
 	mt.str = resolveReflectName(newName(s, "", false))
@@ -1867,23 +1866,29 @@ func MapOf(key, elem Type) Type {
 	mt.key = ktyp
 	mt.elem = etyp
 	mt.bucket = bucketOf(ktyp, etyp)
+	mt.flags = 0
 	if ktyp.size > maxKeySize {
 		mt.keysize = uint8(ptrSize)
-		mt.indirectkey = 1
+		mt.flags |= 1 // indirect key
 	} else {
 		mt.keysize = uint8(ktyp.size)
-		mt.indirectkey = 0
 	}
 	if etyp.size > maxValSize {
 		mt.valuesize = uint8(ptrSize)
-		mt.indirectvalue = 1
+		mt.flags |= 2 // indirect value
 	} else {
 		mt.valuesize = uint8(etyp.size)
-		mt.indirectvalue = 0
 	}
 	mt.bucketsize = uint16(mt.bucket.size)
-	mt.reflexivekey = isReflexive(ktyp)
-	mt.needkeyupdate = needKeyUpdate(ktyp)
+	if isReflexive(ktyp) {
+		mt.flags |= 4
+	}
+	if needKeyUpdate(ktyp) {
+		mt.flags |= 8
+	}
+	if hashMightPanic(ktyp) {
+		mt.flags |= 16
+	}
 	mt.ptrToThis = 0
 
 	ti, _ := lookupCache.LoadOrStore(ckey, &mt.rtype)
@@ -2122,6 +2127,27 @@ func needKeyUpdate(t *rtype) bool {
 	}
 }
 
+// hashMightPanic reports whether the hash of a map key of type t might panic.
+func hashMightPanic(t *rtype) bool {
+	switch t.Kind() {
+	case Interface:
+		return true
+	case Array:
+		tt := (*arrayType)(unsafe.Pointer(t))
+		return hashMightPanic(tt.elem)
+	case Struct:
+		tt := (*structType)(unsafe.Pointer(t))
+		for _, f := range tt.fields {
+			if hashMightPanic(f.typ) {
+				return true
+			}
+		}
+		return false
+	default:
+		return false
+	}
+}
+
 // Make sure these routines stay in sync with ../../runtime/map.go!
 // These types exist only for GC, so we only fill out GC relevant info.
 // Currently, that's just size and the GC program. We also fill in string
diff --git a/src/runtime/map.go b/src/runtime/map.go
index d835cc831a..9c25b63348 100644
--- a/src/runtime/map.go
+++ b/src/runtime/map.go
@@ -404,6 +404,9 @@ func mapaccess1(t *maptype, h *hmap, key unsafe.Pointer) unsafe.Pointer {
 		msanread(key, t.key.size)
 	}
 	if h == nil || h.count == 0 {
+		if t.hashMightPanic() {
+			t.key.alg.hash(key, 0) // see issue 23734
+		}
 		return unsafe.Pointer(&zeroVal[0])
 	}
 	if h.flags&hashWriting != 0 {
@@ -434,12 +437,12 @@ bucketloop:
 				continue
 			}
 			k := add(unsafe.Pointer(b), dataOffset+i*uintptr(t.keysize))
-			if t.indirectkey {
+			if t.indirectkey() {
 				k = *((*unsafe.Pointer)(k))
 			}
 			if alg.equal(key, k) {
 				v := add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+i*uintptr(t.valuesize))
-				if t.indirectvalue {
+				if t.indirectvalue() {
 					v = *((*unsafe.Pointer)(v))
 				}
 				return v
@@ -460,6 +463,9 @@ func mapaccess2(t *maptype, h *hmap, key unsafe.Pointer) (unsafe.Pointer, bool)
 		msanread(key, t.key.size)
 	}
 	if h == nil || h.count == 0 {
+		if t.hashMightPanic() {
+			t.key.alg.hash(key, 0) // see issue 23734
+		}
 		return unsafe.Pointer(&zeroVal[0]), false
 	}
 	if h.flags&hashWriting != 0 {
@@ -490,12 +496,12 @@ bucketloop:
 				continue
 			}
 			k := add(unsafe.Pointer(b), dataOffset+i*uintptr(t.keysize))
-			if t.indirectkey {
+			if t.indirectkey() {
 				k = *((*unsafe.Pointer)(k))
 			}
 			if alg.equal(key, k) {
 				v := add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+i*uintptr(t.valuesize))
-				if t.indirectvalue {
+				if t.indirectvalue() {
 					v = *((*unsafe.Pointer)(v))
 				}
 				return v, true
@@ -535,12 +541,12 @@ bucketloop:
 				continue
 			}
 			k := add(unsafe.Pointer(b), dataOffset+i*uintptr(t.keysize))
-			if t.indirectkey {
+			if t.indirectkey() {
 				k = *((*unsafe.Pointer)(k))
 			}
 			if alg.equal(key, k) {
 				v := add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+i*uintptr(t.valuesize))
-				if t.indirectvalue {
+				if t.indirectvalue() {
 					v = *((*unsafe.Pointer)(v))
 				}
 				return k, v
@@ -620,14 +626,14 @@ bucketloop:
 				continue
 			}
 			k := add(unsafe.Pointer(b), dataOffset+i*uintptr(t.keysize))
-			if t.indirectkey {
+			if t.indirectkey() {
 				k = *((*unsafe.Pointer)(k))
 			}
 			if !alg.equal(key, k) {
 				continue
 			}
 			// already have a mapping for key. Update it.
-			if t.needkeyupdate {
+			if t.needkeyupdate() {
 				typedmemmove(t.key, k, key)
 			}
 			val = add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+i*uintptr(t.valuesize))
@@ -658,12 +664,12 @@ bucketloop:
 	}
 
 	// store new key/value at insert position
-	if t.indirectkey {
+	if t.indirectkey() {
 		kmem := newobject(t.key)
 		*(*unsafe.Pointer)(insertk) = kmem
 		insertk = kmem
 	}
-	if t.indirectvalue {
+	if t.indirectvalue() {
 		vmem := newobject(t.elem)
 		*(*unsafe.Pointer)(val) = vmem
 	}
@@ -676,7 +682,7 @@ done:
 		throw("concurrent map writes")
 	}
 	h.flags &^= hashWriting
-	if t.indirectvalue {
+	if t.indirectvalue() {
 		val = *((*unsafe.Pointer)(val))
 	}
 	return val
@@ -693,6 +699,9 @@ func mapdelete(t *maptype, h *hmap, key unsafe.Pointer) {
 		msanread(key, t.key.size)
 	}
 	if h == nil || h.count == 0 {
+		if t.hashMightPanic() {
+			t.key.alg.hash(key, 0) // see issue 23734
+		}
 		return
 	}
 	if h.flags&hashWriting != 0 {
@@ -724,20 +733,20 @@ search:
 			}
 			k := add(unsafe.Pointer(b), dataOffset+i*uintptr(t.keysize))
 			k2 := k
-			if t.indirectkey {
+			if t.indirectkey() {
 				k2 = *((*unsafe.Pointer)(k2))
 			}
 			if !alg.equal(key, k2) {
 				continue
 			}
 			// Only clear key if there are pointers in it.
-			if t.indirectkey {
+			if t.indirectkey() {
 				*(*unsafe.Pointer)(k) = nil
 			} else if t.key.kind&kindNoPointers == 0 {
 				memclrHasPointers(k, t.key.size)
 			}
 			v := add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+i*uintptr(t.valuesize))
-			if t.indirectvalue {
+			if t.indirectvalue() {
 				*(*unsafe.Pointer)(v) = nil
 			} else if t.elem.kind&kindNoPointers == 0 {
 				memclrHasPointers(v, t.elem.size)
@@ -897,7 +906,7 @@ next:
 			continue
 		}
 		k := add(unsafe.Pointer(b), dataOffset+uintptr(offi)*uintptr(t.keysize))
-		if t.indirectkey {
+		if t.indirectkey() {
 			k = *((*unsafe.Pointer)(k))
 		}
 		v := add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+uintptr(offi)*uintptr(t.valuesize))
@@ -909,7 +918,7 @@ next:
 			// through the oldbucket, skipping any keys that will go
 			// to the other new bucket (each oldbucket expands to two
 			// buckets during a grow).
-			if t.reflexivekey || alg.equal(k, k) {
+			if t.reflexivekey() || alg.equal(k, k) {
 				// If the item in the oldbucket is not destined for
 				// the current new bucket in the iteration, skip it.
 				hash := alg.hash(k, uintptr(h.hash0))
@@ -930,13 +939,13 @@ next:
 			}
 		}
 		if (b.tophash[offi] != evacuatedX && b.tophash[offi] != evacuatedY) ||
-			!(t.reflexivekey || alg.equal(k, k)) {
+			!(t.reflexivekey() || alg.equal(k, k)) {
 			// This is the golden data, we can return it.
 			// OR
 			// key!=key, so the entry can't be deleted or updated, so we can just return it.
 			// That's lucky for us because when key!=key we can't look it up successfully.
 			it.key = k
-			if t.indirectvalue {
+			if t.indirectvalue() {
 				v = *((*unsafe.Pointer)(v))
 			}
 			it.value = v
@@ -1160,7 +1169,7 @@ func evacuate(t *maptype, h *hmap, oldbucket uintptr) {
 					throw("bad map state")
 				}
 				k2 := k
-				if t.indirectkey {
+				if t.indirectkey() {
 					k2 = *((*unsafe.Pointer)(k2))
 				}
 				var useY uint8
@@ -1168,7 +1177,7 @@ func evacuate(t *maptype, h *hmap, oldbucket uintptr) {
 					// Compute hash to make our evacuation decision (whether we need
 					// to send this key/value to bucket x or bucket y).
 					hash := t.key.alg.hash(k2, uintptr(h.hash0))
-					if h.flags&iterator != 0 && !t.reflexivekey && !t.key.alg.equal(k2, k2) {
+					if h.flags&iterator != 0 && !t.reflexivekey() && !t.key.alg.equal(k2, k2) {
 						// If key != key (NaNs), then the hash could be (and probably
 						// will be) entirely different from the old hash. Moreover,
 						// it isn't reproducible. Reproducibility is required in the
@@ -1203,12 +1212,12 @@ func evacuate(t *maptype, h *hmap, oldbucket uintptr) {
 					dst.v = add(dst.k, bucketCnt*uintptr(t.keysize))
 				}
 				dst.b.tophash[dst.i&(bucketCnt-1)] = top // mask dst.i as an optimization, to avoid a bounds check
-				if t.indirectkey {
+				if t.indirectkey() {
 					*(*unsafe.Pointer)(dst.k) = k2 // copy pointer
 				} else {
 					typedmemmove(t.key, dst.k, k) // copy value
 				}
-				if t.indirectvalue {
+				if t.indirectvalue() {
 					*(*unsafe.Pointer)(dst.v) = *(*unsafe.Pointer)(v)
 				} else {
 					typedmemmove(t.elem, dst.v, v)
@@ -1274,12 +1283,12 @@ func reflect_makemap(t *maptype, cap int) *hmap {
 	if !ismapkey(t.key) {
 		throw("runtime.reflect_makemap: unsupported map key type")
 	}
-	if t.key.size > maxKeySize && (!t.indirectkey || t.keysize != uint8(sys.PtrSize)) ||
-		t.key.size <= maxKeySize && (t.indirectkey || t.keysize != uint8(t.key.size)) {
+	if t.key.size > maxKeySize && (!t.indirectkey() || t.keysize != uint8(sys.PtrSize)) ||
+		t.key.size <= maxKeySize && (t.indirectkey() || t.keysize != uint8(t.key.size)) {
 		throw("key size wrong")
 	}
-	if t.elem.size > maxValueSize && (!t.indirectvalue || t.valuesize != uint8(sys.PtrSize)) ||
-		t.elem.size <= maxValueSize && (t.indirectvalue || t.valuesize != uint8(t.elem.size)) {
+	if t.elem.size > maxValueSize && (!t.indirectvalue() || t.valuesize != uint8(sys.PtrSize)) ||
+		t.elem.size <= maxValueSize && (t.indirectvalue() || t.valuesize != uint8(t.elem.size)) {
 		throw("value size wrong")
 	}
 	if t.key.align > bucketCnt {
diff --git a/src/runtime/type.go b/src/runtime/type.go
index 88a44a37ed..f7f99924ea 100644
--- a/src/runtime/type.go
+++ b/src/runtime/type.go
@@ -361,17 +361,32 @@ type interfacetype struct {
 }
 
 type maptype struct {
-	typ           _type
-	key           *_type
-	elem          *_type
-	bucket        *_type // internal type representing a hash bucket
-	keysize       uint8  // size of key slot
-	indirectkey   bool   // store ptr to key instead of key itself
-	valuesize     uint8  // size of value slot
-	indirectvalue bool   // store ptr to value instead of value itself
-	bucketsize    uint16 // size of bucket
-	reflexivekey  bool   // true if k==k for all keys
-	needkeyupdate bool   // true if we need to update key on an overwrite
+	typ        _type
+	key        *_type
+	elem       *_type
+	bucket     *_type // internal type representing a hash bucket
+	keysize    uint8  // size of key slot
+	valuesize  uint8  // size of value slot
+	bucketsize uint16 // size of bucket
+	flags      uint32
+}
+
+// Note: flag values must match those used in the TMAP case
+// in ../cmd/compile/internal/gc/reflect.go:dtypesym.
+func (mt *maptype) indirectkey() bool { // store ptr to key instead of key itself
+	return mt.flags&1 != 0
+}
+func (mt *maptype) indirectvalue() bool { // store ptr to value instead of value itself
+	return mt.flags&2 != 0
+}
+func (mt *maptype) reflexivekey() bool { // true if k==k for all keys
+	return mt.flags&4 != 0
+}
+func (mt *maptype) needkeyupdate() bool { // true if we need to update key on an overwrite
+	return mt.flags&8 != 0
+}
+func (mt *maptype) hashMightPanic() bool { // true if hash function might panic
+	return mt.flags&16 != 0
 }
 
 type arraytype struct {
diff --git a/test/fixedbugs/issue23734.go b/test/fixedbugs/issue23734.go
new file mode 100644
index 0000000000..dd5869b8f0
--- /dev/null
+++ b/test/fixedbugs/issue23734.go
@@ -0,0 +1,32 @@
+// run
+
+// Copyright 2018 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.
+
+package main
+
+func main() {
+	m := map[interface{}]int{}
+	k := []int{}
+
+	mustPanic(func() {
+		_ = m[k]
+	})
+	mustPanic(func() {
+		_, _ = m[k]
+	})
+	mustPanic(func() {
+		delete(m, k)
+	})
+}
+
+func mustPanic(f func()) {
+	defer func() {
+		r := recover()
+		if r == nil {
+			panic("didn't panic")
+		}
+	}()
+	f()
+}