diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index c9eb0176c45e0..10cfe40a70ecb 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -5938,6 +5938,12 @@ namespace ts {
                 && isTypeUsableAsLateBoundName(checkComputedPropertyName(node));
         }
 
+        function isLateBoundName(name: __String): boolean {
+            return (name as string).charCodeAt(0) === CharacterCodes._ &&
+                (name as string).charCodeAt(1) === CharacterCodes._ &&
+                (name as string).charCodeAt(2) === CharacterCodes.at;
+        }
+
         /**
          * Indicates whether a declaration has a late-bindable dynamic name.
          */
@@ -7024,6 +7030,7 @@ namespace ts {
 
         function createUnionOrIntersectionProperty(containingType: UnionOrIntersectionType, name: __String): Symbol | undefined {
             let props: Symbol[] | undefined;
+            let indexTypes: Type[] | undefined;
             const isUnion = containingType.flags & TypeFlags.Union;
             const excludeModifiers = isUnion ? ModifierFlags.NonPublicAccessibilityModifier : 0;
             // Flags we want to propagate to the result if they exist in all source symbols
@@ -7048,14 +7055,21 @@ namespace ts {
                         }
                     }
                     else if (isUnion) {
-                        checkFlags |= CheckFlags.Partial;
+                        const index = !isLateBoundName(name) && ((isNumericLiteralName(name) && getIndexInfoOfType(type, IndexKind.Number)) || getIndexInfoOfType(type, IndexKind.String));
+                        if (index) {
+                            checkFlags |= index.isReadonly ? CheckFlags.Readonly : 0;
+                            indexTypes = append(indexTypes, index.type);
+                        }
+                        else {
+                            checkFlags |= CheckFlags.Partial;
+                        }
                     }
                 }
             }
             if (!props) {
                 return undefined;
             }
-            if (props.length === 1 && !(checkFlags & CheckFlags.Partial)) {
+            if (props.length === 1 && !(checkFlags & CheckFlags.Partial) && !indexTypes) {
                 return props[0];
             }
             let declarations: Declaration[] | undefined;
@@ -7086,6 +7100,7 @@ namespace ts {
                 }
                 propTypes.push(type);
             }
+            addRange(propTypes, indexTypes);
             const result = createSymbol(SymbolFlags.Property | commonFlags, name, syntheticFlag | checkFlags);
             result.containingType = containingType;
             if (!hasNonUniformValueDeclaration && commonValueDeclaration) {
diff --git a/tests/baselines/reference/typedefCrossModule2.symbols b/tests/baselines/reference/typedefCrossModule2.symbols
index 79fbb1ab509a2..b2143e14aa4fb 100644
--- a/tests/baselines/reference/typedefCrossModule2.symbols
+++ b/tests/baselines/reference/typedefCrossModule2.symbols
@@ -14,7 +14,9 @@ var bb;
 
 var bbb = new mod.Baz();
 >bbb : Symbol(bbb, Decl(use.js, 5, 3))
+>mod.Baz : Symbol(Baz)
 >mod : Symbol(mod, Decl(use.js, 0, 3))
+>Baz : Symbol(Baz)
 
 === tests/cases/conformance/jsdoc/mod1.js ===
 // error
diff --git a/tests/baselines/reference/unionTypeWithIndexSignature.errors.txt b/tests/baselines/reference/unionTypeWithIndexSignature.errors.txt
new file mode 100644
index 0000000000000..e95a184d15a5b
--- /dev/null
+++ b/tests/baselines/reference/unionTypeWithIndexSignature.errors.txt
@@ -0,0 +1,47 @@
+tests/cases/conformance/types/union/unionTypeWithIndexSignature.ts(11,3): error TS2339: Property 'bar' does not exist on type 'Missing'.
+  Property 'bar' does not exist on type '{ [s: string]: string; }'.
+tests/cases/conformance/types/union/unionTypeWithIndexSignature.ts(14,4): error TS2540: Cannot assign to 'foo' because it is a constant or a read-only property.
+tests/cases/conformance/types/union/unionTypeWithIndexSignature.ts(24,1): error TS7017: Element implicitly has an 'any' type because type 'Both' has no index signature.
+tests/cases/conformance/types/union/unionTypeWithIndexSignature.ts(25,1): error TS2322: Type '"not ok"' is not assignable to type 'number'.
+tests/cases/conformance/types/union/unionTypeWithIndexSignature.ts(26,1): error TS7017: Element implicitly has an 'any' type because type 'Both' has no index signature.
+
+
+==== tests/cases/conformance/types/union/unionTypeWithIndexSignature.ts (5 errors) ====
+    type Two = { foo: { bar: true }, baz: true } | { [s: string]: string };
+    declare var u: Two
+    u.foo = 'bye'
+    u.baz = 'hi'
+    type Three = { foo: number } | { [s: string]: string } | { [s: string]: boolean };
+    declare var v: Three
+    v.foo = false
+    type Missing = { foo: number, bar: true } | { [s: string]: string } | { foo: boolean }
+    declare var m: Missing
+    m.foo = 'hi'
+    m.bar
+      ~~~
+!!! error TS2339: Property 'bar' does not exist on type 'Missing'.
+!!! error TS2339:   Property 'bar' does not exist on type '{ [s: string]: string; }'.
+    type RO = { foo: number } | { readonly [s: string]: string }
+    declare var ro: RO
+    ro.foo = 'not allowed'
+       ~~~
+!!! error TS2540: Cannot assign to 'foo' because it is a constant or a read-only property.
+    type Num = { '0': string } | { [n: number]: number }
+    declare var num: Num
+    num[0] = 1
+    num['0'] = 'ok'
+    const sym = Symbol()
+    type Both = { s: number, '0': number, [sym]: boolean } | { [n: number]: number, [s: string]: string | number }
+    declare var both: Both
+    both['s'] = 'ok'
+    both[0] = 1
+    both[1] = 0 // not ok
+    ~~~~~~~
+!!! error TS7017: Element implicitly has an 'any' type because type 'Both' has no index signature.
+    both[0] = 'not ok'
+    ~~~~~~~
+!!! error TS2322: Type '"not ok"' is not assignable to type 'number'.
+    both[sym] = 'not ok'
+    ~~~~~~~~~
+!!! error TS7017: Element implicitly has an 'any' type because type 'Both' has no index signature.
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/unionTypeWithIndexSignature.js b/tests/baselines/reference/unionTypeWithIndexSignature.js
new file mode 100644
index 0000000000000..53cf98c612000
--- /dev/null
+++ b/tests/baselines/reference/unionTypeWithIndexSignature.js
@@ -0,0 +1,45 @@
+//// [unionTypeWithIndexSignature.ts]
+type Two = { foo: { bar: true }, baz: true } | { [s: string]: string };
+declare var u: Two
+u.foo = 'bye'
+u.baz = 'hi'
+type Three = { foo: number } | { [s: string]: string } | { [s: string]: boolean };
+declare var v: Three
+v.foo = false
+type Missing = { foo: number, bar: true } | { [s: string]: string } | { foo: boolean }
+declare var m: Missing
+m.foo = 'hi'
+m.bar
+type RO = { foo: number } | { readonly [s: string]: string }
+declare var ro: RO
+ro.foo = 'not allowed'
+type Num = { '0': string } | { [n: number]: number }
+declare var num: Num
+num[0] = 1
+num['0'] = 'ok'
+const sym = Symbol()
+type Both = { s: number, '0': number, [sym]: boolean } | { [n: number]: number, [s: string]: string | number }
+declare var both: Both
+both['s'] = 'ok'
+both[0] = 1
+both[1] = 0 // not ok
+both[0] = 'not ok'
+both[sym] = 'not ok'
+
+
+//// [unionTypeWithIndexSignature.js]
+"use strict";
+u.foo = 'bye';
+u.baz = 'hi';
+v.foo = false;
+m.foo = 'hi';
+m.bar;
+ro.foo = 'not allowed';
+num[0] = 1;
+num['0'] = 'ok';
+const sym = Symbol();
+both['s'] = 'ok';
+both[0] = 1;
+both[1] = 0; // not ok
+both[0] = 'not ok';
+both[sym] = 'not ok';
diff --git a/tests/baselines/reference/unionTypeWithIndexSignature.symbols b/tests/baselines/reference/unionTypeWithIndexSignature.symbols
new file mode 100644
index 0000000000000..b207423057b8b
--- /dev/null
+++ b/tests/baselines/reference/unionTypeWithIndexSignature.symbols
@@ -0,0 +1,123 @@
+=== tests/cases/conformance/types/union/unionTypeWithIndexSignature.ts ===
+type Two = { foo: { bar: true }, baz: true } | { [s: string]: string };
+>Two : Symbol(Two, Decl(unionTypeWithIndexSignature.ts, 0, 0))
+>foo : Symbol(foo, Decl(unionTypeWithIndexSignature.ts, 0, 12))
+>bar : Symbol(bar, Decl(unionTypeWithIndexSignature.ts, 0, 19))
+>baz : Symbol(baz, Decl(unionTypeWithIndexSignature.ts, 0, 32))
+>s : Symbol(s, Decl(unionTypeWithIndexSignature.ts, 0, 50))
+
+declare var u: Two
+>u : Symbol(u, Decl(unionTypeWithIndexSignature.ts, 1, 11))
+>Two : Symbol(Two, Decl(unionTypeWithIndexSignature.ts, 0, 0))
+
+u.foo = 'bye'
+>u.foo : Symbol(foo, Decl(unionTypeWithIndexSignature.ts, 0, 12))
+>u : Symbol(u, Decl(unionTypeWithIndexSignature.ts, 1, 11))
+>foo : Symbol(foo, Decl(unionTypeWithIndexSignature.ts, 0, 12))
+
+u.baz = 'hi'
+>u.baz : Symbol(baz, Decl(unionTypeWithIndexSignature.ts, 0, 32))
+>u : Symbol(u, Decl(unionTypeWithIndexSignature.ts, 1, 11))
+>baz : Symbol(baz, Decl(unionTypeWithIndexSignature.ts, 0, 32))
+
+type Three = { foo: number } | { [s: string]: string } | { [s: string]: boolean };
+>Three : Symbol(Three, Decl(unionTypeWithIndexSignature.ts, 3, 12))
+>foo : Symbol(foo, Decl(unionTypeWithIndexSignature.ts, 4, 14))
+>s : Symbol(s, Decl(unionTypeWithIndexSignature.ts, 4, 34))
+>s : Symbol(s, Decl(unionTypeWithIndexSignature.ts, 4, 60))
+
+declare var v: Three
+>v : Symbol(v, Decl(unionTypeWithIndexSignature.ts, 5, 11))
+>Three : Symbol(Three, Decl(unionTypeWithIndexSignature.ts, 3, 12))
+
+v.foo = false
+>v.foo : Symbol(foo, Decl(unionTypeWithIndexSignature.ts, 4, 14))
+>v : Symbol(v, Decl(unionTypeWithIndexSignature.ts, 5, 11))
+>foo : Symbol(foo, Decl(unionTypeWithIndexSignature.ts, 4, 14))
+
+type Missing = { foo: number, bar: true } | { [s: string]: string } | { foo: boolean }
+>Missing : Symbol(Missing, Decl(unionTypeWithIndexSignature.ts, 6, 13))
+>foo : Symbol(foo, Decl(unionTypeWithIndexSignature.ts, 7, 16))
+>bar : Symbol(bar, Decl(unionTypeWithIndexSignature.ts, 7, 29))
+>s : Symbol(s, Decl(unionTypeWithIndexSignature.ts, 7, 47))
+>foo : Symbol(foo, Decl(unionTypeWithIndexSignature.ts, 7, 71))
+
+declare var m: Missing
+>m : Symbol(m, Decl(unionTypeWithIndexSignature.ts, 8, 11))
+>Missing : Symbol(Missing, Decl(unionTypeWithIndexSignature.ts, 6, 13))
+
+m.foo = 'hi'
+>m.foo : Symbol(foo, Decl(unionTypeWithIndexSignature.ts, 7, 16), Decl(unionTypeWithIndexSignature.ts, 7, 71))
+>m : Symbol(m, Decl(unionTypeWithIndexSignature.ts, 8, 11))
+>foo : Symbol(foo, Decl(unionTypeWithIndexSignature.ts, 7, 16), Decl(unionTypeWithIndexSignature.ts, 7, 71))
+
+m.bar
+>m : Symbol(m, Decl(unionTypeWithIndexSignature.ts, 8, 11))
+
+type RO = { foo: number } | { readonly [s: string]: string }
+>RO : Symbol(RO, Decl(unionTypeWithIndexSignature.ts, 10, 5))
+>foo : Symbol(foo, Decl(unionTypeWithIndexSignature.ts, 11, 11))
+>s : Symbol(s, Decl(unionTypeWithIndexSignature.ts, 11, 40))
+
+declare var ro: RO
+>ro : Symbol(ro, Decl(unionTypeWithIndexSignature.ts, 12, 11))
+>RO : Symbol(RO, Decl(unionTypeWithIndexSignature.ts, 10, 5))
+
+ro.foo = 'not allowed'
+>ro.foo : Symbol(foo, Decl(unionTypeWithIndexSignature.ts, 11, 11))
+>ro : Symbol(ro, Decl(unionTypeWithIndexSignature.ts, 12, 11))
+>foo : Symbol(foo, Decl(unionTypeWithIndexSignature.ts, 11, 11))
+
+type Num = { '0': string } | { [n: number]: number }
+>Num : Symbol(Num, Decl(unionTypeWithIndexSignature.ts, 13, 22))
+>'0' : Symbol('0', Decl(unionTypeWithIndexSignature.ts, 14, 12))
+>n : Symbol(n, Decl(unionTypeWithIndexSignature.ts, 14, 32))
+
+declare var num: Num
+>num : Symbol(num, Decl(unionTypeWithIndexSignature.ts, 15, 11))
+>Num : Symbol(Num, Decl(unionTypeWithIndexSignature.ts, 13, 22))
+
+num[0] = 1
+>num : Symbol(num, Decl(unionTypeWithIndexSignature.ts, 15, 11))
+>0 : Symbol('0', Decl(unionTypeWithIndexSignature.ts, 14, 12))
+
+num['0'] = 'ok'
+>num : Symbol(num, Decl(unionTypeWithIndexSignature.ts, 15, 11))
+>'0' : Symbol('0', Decl(unionTypeWithIndexSignature.ts, 14, 12))
+
+const sym = Symbol()
+>sym : Symbol(sym, Decl(unionTypeWithIndexSignature.ts, 18, 5))
+>Symbol : Symbol(Symbol, Decl(lib.es5.d.ts, --, --), Decl(lib.es2015.symbol.d.ts, --, --), Decl(lib.es2015.symbol.wellknown.d.ts, --, --), Decl(lib.esnext.symbol.d.ts, --, --))
+
+type Both = { s: number, '0': number, [sym]: boolean } | { [n: number]: number, [s: string]: string | number }
+>Both : Symbol(Both, Decl(unionTypeWithIndexSignature.ts, 18, 20))
+>s : Symbol(s, Decl(unionTypeWithIndexSignature.ts, 19, 13))
+>'0' : Symbol('0', Decl(unionTypeWithIndexSignature.ts, 19, 24))
+>[sym] : Symbol([sym], Decl(unionTypeWithIndexSignature.ts, 19, 37))
+>sym : Symbol(sym, Decl(unionTypeWithIndexSignature.ts, 18, 5))
+>n : Symbol(n, Decl(unionTypeWithIndexSignature.ts, 19, 60))
+>s : Symbol(s, Decl(unionTypeWithIndexSignature.ts, 19, 81))
+
+declare var both: Both
+>both : Symbol(both, Decl(unionTypeWithIndexSignature.ts, 20, 11))
+>Both : Symbol(Both, Decl(unionTypeWithIndexSignature.ts, 18, 20))
+
+both['s'] = 'ok'
+>both : Symbol(both, Decl(unionTypeWithIndexSignature.ts, 20, 11))
+>'s' : Symbol(s, Decl(unionTypeWithIndexSignature.ts, 19, 13))
+
+both[0] = 1
+>both : Symbol(both, Decl(unionTypeWithIndexSignature.ts, 20, 11))
+>0 : Symbol('0', Decl(unionTypeWithIndexSignature.ts, 19, 24))
+
+both[1] = 0 // not ok
+>both : Symbol(both, Decl(unionTypeWithIndexSignature.ts, 20, 11))
+
+both[0] = 'not ok'
+>both : Symbol(both, Decl(unionTypeWithIndexSignature.ts, 20, 11))
+>0 : Symbol('0', Decl(unionTypeWithIndexSignature.ts, 19, 24))
+
+both[sym] = 'not ok'
+>both : Symbol(both, Decl(unionTypeWithIndexSignature.ts, 20, 11))
+>sym : Symbol(sym, Decl(unionTypeWithIndexSignature.ts, 18, 5))
+
diff --git a/tests/baselines/reference/unionTypeWithIndexSignature.types b/tests/baselines/reference/unionTypeWithIndexSignature.types
new file mode 100644
index 0000000000000..b8d12eb63d90c
--- /dev/null
+++ b/tests/baselines/reference/unionTypeWithIndexSignature.types
@@ -0,0 +1,161 @@
+=== tests/cases/conformance/types/union/unionTypeWithIndexSignature.ts ===
+type Two = { foo: { bar: true }, baz: true } | { [s: string]: string };
+>Two : Two
+>foo : { bar: true; }
+>bar : true
+>true : true
+>baz : true
+>true : true
+>s : string
+
+declare var u: Two
+>u : Two
+>Two : Two
+
+u.foo = 'bye'
+>u.foo = 'bye' : "bye"
+>u.foo : string | { bar: true; }
+>u : Two
+>foo : string | { bar: true; }
+>'bye' : "bye"
+
+u.baz = 'hi'
+>u.baz = 'hi' : "hi"
+>u.baz : string | true
+>u : Two
+>baz : string | true
+>'hi' : "hi"
+
+type Three = { foo: number } | { [s: string]: string } | { [s: string]: boolean };
+>Three : Three
+>foo : number
+>s : string
+>s : string
+
+declare var v: Three
+>v : Three
+>Three : Three
+
+v.foo = false
+>v.foo = false : false
+>v.foo : string | number | boolean
+>v : Three
+>foo : string | number | boolean
+>false : false
+
+type Missing = { foo: number, bar: true } | { [s: string]: string } | { foo: boolean }
+>Missing : Missing
+>foo : number
+>bar : true
+>true : true
+>s : string
+>foo : boolean
+
+declare var m: Missing
+>m : Missing
+>Missing : Missing
+
+m.foo = 'hi'
+>m.foo = 'hi' : "hi"
+>m.foo : string | number | boolean
+>m : Missing
+>foo : string | number | boolean
+>'hi' : "hi"
+
+m.bar
+>m.bar : any
+>m : Missing
+>bar : any
+
+type RO = { foo: number } | { readonly [s: string]: string }
+>RO : RO
+>foo : number
+>s : string
+
+declare var ro: RO
+>ro : RO
+>RO : RO
+
+ro.foo = 'not allowed'
+>ro.foo = 'not allowed' : "not allowed"
+>ro.foo : any
+>ro : RO
+>foo : any
+>'not allowed' : "not allowed"
+
+type Num = { '0': string } | { [n: number]: number }
+>Num : Num
+>'0' : string
+>n : number
+
+declare var num: Num
+>num : Num
+>Num : Num
+
+num[0] = 1
+>num[0] = 1 : 1
+>num[0] : string | number
+>num : Num
+>0 : 0
+>1 : 1
+
+num['0'] = 'ok'
+>num['0'] = 'ok' : "ok"
+>num['0'] : string | number
+>num : Num
+>'0' : "0"
+>'ok' : "ok"
+
+const sym = Symbol()
+>sym : unique symbol
+>Symbol() : unique symbol
+>Symbol : SymbolConstructor
+
+type Both = { s: number, '0': number, [sym]: boolean } | { [n: number]: number, [s: string]: string | number }
+>Both : Both
+>s : number
+>'0' : number
+>[sym] : boolean
+>sym : unique symbol
+>n : number
+>s : string
+
+declare var both: Both
+>both : Both
+>Both : Both
+
+both['s'] = 'ok'
+>both['s'] = 'ok' : "ok"
+>both['s'] : string | number
+>both : Both
+>'s' : "s"
+>'ok' : "ok"
+
+both[0] = 1
+>both[0] = 1 : 1
+>both[0] : number
+>both : Both
+>0 : 0
+>1 : 1
+
+both[1] = 0 // not ok
+>both[1] = 0 : 0
+>both[1] : any
+>both : Both
+>1 : 1
+>0 : 0
+
+both[0] = 'not ok'
+>both[0] = 'not ok' : "not ok"
+>both[0] : number
+>both : Both
+>0 : 0
+>'not ok' : "not ok"
+
+both[sym] = 'not ok'
+>both[sym] = 'not ok' : "not ok"
+>both[sym] : any
+>both : Both
+>sym : unique symbol
+>'not ok' : "not ok"
+
diff --git a/tests/cases/conformance/types/union/unionTypeWithIndexSignature.ts b/tests/cases/conformance/types/union/unionTypeWithIndexSignature.ts
new file mode 100644
index 0000000000000..cda563811d7aa
--- /dev/null
+++ b/tests/cases/conformance/types/union/unionTypeWithIndexSignature.ts
@@ -0,0 +1,28 @@
+// @target: esnext
+// @strict: true
+type Two = { foo: { bar: true }, baz: true } | { [s: string]: string };
+declare var u: Two
+u.foo = 'bye'
+u.baz = 'hi'
+type Three = { foo: number } | { [s: string]: string } | { [s: string]: boolean };
+declare var v: Three
+v.foo = false
+type Missing = { foo: number, bar: true } | { [s: string]: string } | { foo: boolean }
+declare var m: Missing
+m.foo = 'hi'
+m.bar
+type RO = { foo: number } | { readonly [s: string]: string }
+declare var ro: RO
+ro.foo = 'not allowed'
+type Num = { '0': string } | { [n: number]: number }
+declare var num: Num
+num[0] = 1
+num['0'] = 'ok'
+const sym = Symbol()
+type Both = { s: number, '0': number, [sym]: boolean } | { [n: number]: number, [s: string]: string | number }
+declare var both: Both
+both['s'] = 'ok'
+both[0] = 1
+both[1] = 0 // not ok
+both[0] = 'not ok'
+both[sym] = 'not ok'