diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 3ed6768cb3d72..f2a34ae9a1a8e 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -6444,7 +6444,8 @@ namespace ts {
                 for (const declaration of symbol.declarations) {
                     if (declaration.kind === SyntaxKind.EnumDeclaration) {
                         for (const member of (<EnumDeclaration>declaration).members) {
-                            const memberType = getFreshTypeOfLiteralType(getLiteralType(getEnumMemberValue(member)!, enumCount, getSymbolOfNode(member))); // TODO: GH#18217
+                            const value = getEnumMemberValue(member);
+                            const memberType = getFreshTypeOfLiteralType(getLiteralType(value !== undefined ? value : 0, enumCount, getSymbolOfNode(member)));
                             getSymbolLinks(getSymbolOfNode(member)).declaredType = memberType;
                             memberTypeList.push(getRegularTypeOfLiteralType(memberType));
                         }
@@ -7453,8 +7454,9 @@ namespace ts {
                 else if (t.flags & (TypeFlags.Any | TypeFlags.String)) {
                     stringIndexInfo = createIndexInfo(propType, !!(templateModifiers & MappedTypeModifiers.IncludeReadonly));
                 }
-                else if (t.flags & TypeFlags.Number) {
-                    numberIndexInfo = createIndexInfo(propType, !!(templateModifiers & MappedTypeModifiers.IncludeReadonly));
+                else if (t.flags & (TypeFlags.Number | TypeFlags.Enum)) {
+                    numberIndexInfo = createIndexInfo(numberIndexInfo ? getUnionType([numberIndexInfo.type, propType]) : propType,
+                        !!(templateModifiers & MappedTypeModifiers.IncludeReadonly));
                 }
             }
         }
@@ -28440,9 +28442,9 @@ namespace ts {
             if (member.initializer) {
                 return computeConstantValue(member);
             }
-            // In ambient enum declarations that specify no const modifier, enum member declarations that omit
-            // a value are considered computed members (as opposed to having auto-incremented values).
-            if (member.parent.flags & NodeFlags.Ambient && !isEnumConst(member.parent)) {
+            // In ambient non-const numeric enum declarations, enum members without initializers are
+            // considered computed members (as opposed to having auto-incremented values).
+            if (member.parent.flags & NodeFlags.Ambient && !isEnumConst(member.parent) && getEnumKind(getSymbolOfNode(member.parent)) === EnumKind.Numeric) {
                 return undefined;
             }
             // If the member declaration specifies no value, the member is considered a constant enum member.
diff --git a/tests/baselines/reference/numericEnumMappedType.js b/tests/baselines/reference/numericEnumMappedType.js
new file mode 100644
index 0000000000000..78a13e443ff42
--- /dev/null
+++ b/tests/baselines/reference/numericEnumMappedType.js
@@ -0,0 +1,110 @@
+//// [numericEnumMappedType.ts]
+// Repro from #31771
+
+enum E1 { ONE, TWO, THREE }
+declare enum E2 { ONE, TWO, THREE }
+
+type Bins1 = { [k in E1]?: string; }
+type Bins2 = { [k in E2]?: string; }
+
+const b1: Bins1 = {};
+const b2: Bins2 = {};
+
+const e1: E1 = E1.ONE;
+const e2: E2 = E2.ONE;
+
+b1[1] = "a";
+b1[e1] = "b";
+
+b2[1] = "a";
+b2[e2] = "b";
+
+// Multiple numeric enum types accrue to the same numeric index signature in a mapped type
+
+declare function val(): number;
+
+enum N1 { A = val(), B = val() }
+enum N2 { C = val(), D = val() }
+
+type T1 = { [K in N1 | N2]: K };
+
+// Enum types with string valued members are always literal enum types and therefore
+// ONE and TWO below are not computed members but rather just numerically valued members
+// with auto-incremented values.
+
+declare enum E { ONE, TWO, THREE = 'x' }
+const e: E = E.ONE;
+const x: E.ONE = e;
+
+
+//// [numericEnumMappedType.js]
+"use strict";
+// Repro from #31771
+var E1;
+(function (E1) {
+    E1[E1["ONE"] = 0] = "ONE";
+    E1[E1["TWO"] = 1] = "TWO";
+    E1[E1["THREE"] = 2] = "THREE";
+})(E1 || (E1 = {}));
+var b1 = {};
+var b2 = {};
+var e1 = E1.ONE;
+var e2 = E2.ONE;
+b1[1] = "a";
+b1[e1] = "b";
+b2[1] = "a";
+b2[e2] = "b";
+var N1;
+(function (N1) {
+    N1[N1["A"] = val()] = "A";
+    N1[N1["B"] = val()] = "B";
+})(N1 || (N1 = {}));
+var N2;
+(function (N2) {
+    N2[N2["C"] = val()] = "C";
+    N2[N2["D"] = val()] = "D";
+})(N2 || (N2 = {}));
+var e = E.ONE;
+var x = e;
+
+
+//// [numericEnumMappedType.d.ts]
+declare enum E1 {
+    ONE = 0,
+    TWO = 1,
+    THREE = 2
+}
+declare enum E2 {
+    ONE,
+    TWO,
+    THREE
+}
+declare type Bins1 = {
+    [k in E1]?: string;
+};
+declare type Bins2 = {
+    [k in E2]?: string;
+};
+declare const b1: Bins1;
+declare const b2: Bins2;
+declare const e1: E1;
+declare const e2: E2;
+declare function val(): number;
+declare enum N1 {
+    A,
+    B
+}
+declare enum N2 {
+    C,
+    D
+}
+declare type T1 = {
+    [K in N1 | N2]: K;
+};
+declare enum E {
+    ONE = 0,
+    TWO = 1,
+    THREE = "x"
+}
+declare const e: E;
+declare const x: E.ONE;
diff --git a/tests/baselines/reference/numericEnumMappedType.symbols b/tests/baselines/reference/numericEnumMappedType.symbols
new file mode 100644
index 0000000000000..08979b3bed100
--- /dev/null
+++ b/tests/baselines/reference/numericEnumMappedType.symbols
@@ -0,0 +1,111 @@
+=== tests/cases/compiler/numericEnumMappedType.ts ===
+// Repro from #31771
+
+enum E1 { ONE, TWO, THREE }
+>E1 : Symbol(E1, Decl(numericEnumMappedType.ts, 0, 0))
+>ONE : Symbol(E1.ONE, Decl(numericEnumMappedType.ts, 2, 9))
+>TWO : Symbol(E1.TWO, Decl(numericEnumMappedType.ts, 2, 14))
+>THREE : Symbol(E1.THREE, Decl(numericEnumMappedType.ts, 2, 19))
+
+declare enum E2 { ONE, TWO, THREE }
+>E2 : Symbol(E2, Decl(numericEnumMappedType.ts, 2, 27))
+>ONE : Symbol(E2.ONE, Decl(numericEnumMappedType.ts, 3, 17))
+>TWO : Symbol(E2.TWO, Decl(numericEnumMappedType.ts, 3, 22))
+>THREE : Symbol(E2.THREE, Decl(numericEnumMappedType.ts, 3, 27))
+
+type Bins1 = { [k in E1]?: string; }
+>Bins1 : Symbol(Bins1, Decl(numericEnumMappedType.ts, 3, 35))
+>k : Symbol(k, Decl(numericEnumMappedType.ts, 5, 16))
+>E1 : Symbol(E1, Decl(numericEnumMappedType.ts, 0, 0))
+
+type Bins2 = { [k in E2]?: string; }
+>Bins2 : Symbol(Bins2, Decl(numericEnumMappedType.ts, 5, 36))
+>k : Symbol(k, Decl(numericEnumMappedType.ts, 6, 16))
+>E2 : Symbol(E2, Decl(numericEnumMappedType.ts, 2, 27))
+
+const b1: Bins1 = {};
+>b1 : Symbol(b1, Decl(numericEnumMappedType.ts, 8, 5))
+>Bins1 : Symbol(Bins1, Decl(numericEnumMappedType.ts, 3, 35))
+
+const b2: Bins2 = {};
+>b2 : Symbol(b2, Decl(numericEnumMappedType.ts, 9, 5))
+>Bins2 : Symbol(Bins2, Decl(numericEnumMappedType.ts, 5, 36))
+
+const e1: E1 = E1.ONE;
+>e1 : Symbol(e1, Decl(numericEnumMappedType.ts, 11, 5))
+>E1 : Symbol(E1, Decl(numericEnumMappedType.ts, 0, 0))
+>E1.ONE : Symbol(E1.ONE, Decl(numericEnumMappedType.ts, 2, 9))
+>E1 : Symbol(E1, Decl(numericEnumMappedType.ts, 0, 0))
+>ONE : Symbol(E1.ONE, Decl(numericEnumMappedType.ts, 2, 9))
+
+const e2: E2 = E2.ONE;
+>e2 : Symbol(e2, Decl(numericEnumMappedType.ts, 12, 5))
+>E2 : Symbol(E2, Decl(numericEnumMappedType.ts, 2, 27))
+>E2.ONE : Symbol(E2.ONE, Decl(numericEnumMappedType.ts, 3, 17))
+>E2 : Symbol(E2, Decl(numericEnumMappedType.ts, 2, 27))
+>ONE : Symbol(E2.ONE, Decl(numericEnumMappedType.ts, 3, 17))
+
+b1[1] = "a";
+>b1 : Symbol(b1, Decl(numericEnumMappedType.ts, 8, 5))
+>1 : Symbol(1)
+
+b1[e1] = "b";
+>b1 : Symbol(b1, Decl(numericEnumMappedType.ts, 8, 5))
+>e1 : Symbol(e1, Decl(numericEnumMappedType.ts, 11, 5))
+
+b2[1] = "a";
+>b2 : Symbol(b2, Decl(numericEnumMappedType.ts, 9, 5))
+
+b2[e2] = "b";
+>b2 : Symbol(b2, Decl(numericEnumMappedType.ts, 9, 5))
+>e2 : Symbol(e2, Decl(numericEnumMappedType.ts, 12, 5))
+
+// Multiple numeric enum types accrue to the same numeric index signature in a mapped type
+
+declare function val(): number;
+>val : Symbol(val, Decl(numericEnumMappedType.ts, 18, 13))
+
+enum N1 { A = val(), B = val() }
+>N1 : Symbol(N1, Decl(numericEnumMappedType.ts, 22, 31))
+>A : Symbol(N1.A, Decl(numericEnumMappedType.ts, 24, 9))
+>val : Symbol(val, Decl(numericEnumMappedType.ts, 18, 13))
+>B : Symbol(N1.B, Decl(numericEnumMappedType.ts, 24, 20))
+>val : Symbol(val, Decl(numericEnumMappedType.ts, 18, 13))
+
+enum N2 { C = val(), D = val() }
+>N2 : Symbol(N2, Decl(numericEnumMappedType.ts, 24, 32))
+>C : Symbol(N2.C, Decl(numericEnumMappedType.ts, 25, 9))
+>val : Symbol(val, Decl(numericEnumMappedType.ts, 18, 13))
+>D : Symbol(N2.D, Decl(numericEnumMappedType.ts, 25, 20))
+>val : Symbol(val, Decl(numericEnumMappedType.ts, 18, 13))
+
+type T1 = { [K in N1 | N2]: K };
+>T1 : Symbol(T1, Decl(numericEnumMappedType.ts, 25, 32))
+>K : Symbol(K, Decl(numericEnumMappedType.ts, 27, 13))
+>N1 : Symbol(N1, Decl(numericEnumMappedType.ts, 22, 31))
+>N2 : Symbol(N2, Decl(numericEnumMappedType.ts, 24, 32))
+>K : Symbol(K, Decl(numericEnumMappedType.ts, 27, 13))
+
+// Enum types with string valued members are always literal enum types and therefore
+// ONE and TWO below are not computed members but rather just numerically valued members
+// with auto-incremented values.
+
+declare enum E { ONE, TWO, THREE = 'x' }
+>E : Symbol(E, Decl(numericEnumMappedType.ts, 27, 32))
+>ONE : Symbol(E.ONE, Decl(numericEnumMappedType.ts, 33, 16))
+>TWO : Symbol(E.TWO, Decl(numericEnumMappedType.ts, 33, 21))
+>THREE : Symbol(E.THREE, Decl(numericEnumMappedType.ts, 33, 26))
+
+const e: E = E.ONE;
+>e : Symbol(e, Decl(numericEnumMappedType.ts, 34, 5))
+>E : Symbol(E, Decl(numericEnumMappedType.ts, 27, 32))
+>E.ONE : Symbol(E.ONE, Decl(numericEnumMappedType.ts, 33, 16))
+>E : Symbol(E, Decl(numericEnumMappedType.ts, 27, 32))
+>ONE : Symbol(E.ONE, Decl(numericEnumMappedType.ts, 33, 16))
+
+const x: E.ONE = e;
+>x : Symbol(x, Decl(numericEnumMappedType.ts, 35, 5))
+>E : Symbol(E, Decl(numericEnumMappedType.ts, 27, 32))
+>ONE : Symbol(E.ONE, Decl(numericEnumMappedType.ts, 33, 16))
+>e : Symbol(e, Decl(numericEnumMappedType.ts, 34, 5))
+
diff --git a/tests/baselines/reference/numericEnumMappedType.types b/tests/baselines/reference/numericEnumMappedType.types
new file mode 100644
index 0000000000000..bffec2ed8ca18
--- /dev/null
+++ b/tests/baselines/reference/numericEnumMappedType.types
@@ -0,0 +1,117 @@
+=== tests/cases/compiler/numericEnumMappedType.ts ===
+// Repro from #31771
+
+enum E1 { ONE, TWO, THREE }
+>E1 : E1
+>ONE : E1.ONE
+>TWO : E1.TWO
+>THREE : E1.THREE
+
+declare enum E2 { ONE, TWO, THREE }
+>E2 : E2
+>ONE : E2
+>TWO : E2
+>THREE : E2
+
+type Bins1 = { [k in E1]?: string; }
+>Bins1 : Bins1
+
+type Bins2 = { [k in E2]?: string; }
+>Bins2 : Bins2
+
+const b1: Bins1 = {};
+>b1 : Bins1
+>{} : {}
+
+const b2: Bins2 = {};
+>b2 : Bins2
+>{} : {}
+
+const e1: E1 = E1.ONE;
+>e1 : E1
+>E1.ONE : E1.ONE
+>E1 : typeof E1
+>ONE : E1.ONE
+
+const e2: E2 = E2.ONE;
+>e2 : E2
+>E2.ONE : E2
+>E2 : typeof E2
+>ONE : E2
+
+b1[1] = "a";
+>b1[1] = "a" : "a"
+>b1[1] : string | undefined
+>b1 : Bins1
+>1 : 1
+>"a" : "a"
+
+b1[e1] = "b";
+>b1[e1] = "b" : "b"
+>b1[e1] : string | undefined
+>b1 : Bins1
+>e1 : E1.ONE
+>"b" : "b"
+
+b2[1] = "a";
+>b2[1] = "a" : "a"
+>b2[1] : string | undefined
+>b2 : Bins2
+>1 : 1
+>"a" : "a"
+
+b2[e2] = "b";
+>b2[e2] = "b" : "b"
+>b2[e2] : string | undefined
+>b2 : Bins2
+>e2 : E2
+>"b" : "b"
+
+// Multiple numeric enum types accrue to the same numeric index signature in a mapped type
+
+declare function val(): number;
+>val : () => number
+
+enum N1 { A = val(), B = val() }
+>N1 : N1
+>A : N1
+>val() : number
+>val : () => number
+>B : N1
+>val() : number
+>val : () => number
+
+enum N2 { C = val(), D = val() }
+>N2 : N2
+>C : N2
+>val() : number
+>val : () => number
+>D : N2
+>val() : number
+>val : () => number
+
+type T1 = { [K in N1 | N2]: K };
+>T1 : T1
+
+// Enum types with string valued members are always literal enum types and therefore
+// ONE and TWO below are not computed members but rather just numerically valued members
+// with auto-incremented values.
+
+declare enum E { ONE, TWO, THREE = 'x' }
+>E : E
+>ONE : E.ONE
+>TWO : E.TWO
+>THREE : E.THREE
+>'x' : "x"
+
+const e: E = E.ONE;
+>e : E
+>E.ONE : E.ONE
+>E : typeof E
+>ONE : E.ONE
+
+const x: E.ONE = e;
+>x : E.ONE
+>E : any
+>e : E.ONE
+
diff --git a/tests/cases/compiler/numericEnumMappedType.ts b/tests/cases/compiler/numericEnumMappedType.ts
new file mode 100644
index 0000000000000..7c03991e6a206
--- /dev/null
+++ b/tests/cases/compiler/numericEnumMappedType.ts
@@ -0,0 +1,39 @@
+// @strict: true
+// @declaration: true
+
+// Repro from #31771
+
+enum E1 { ONE, TWO, THREE }
+declare enum E2 { ONE, TWO, THREE }
+
+type Bins1 = { [k in E1]?: string; }
+type Bins2 = { [k in E2]?: string; }
+
+const b1: Bins1 = {};
+const b2: Bins2 = {};
+
+const e1: E1 = E1.ONE;
+const e2: E2 = E2.ONE;
+
+b1[1] = "a";
+b1[e1] = "b";
+
+b2[1] = "a";
+b2[e2] = "b";
+
+// Multiple numeric enum types accrue to the same numeric index signature in a mapped type
+
+declare function val(): number;
+
+enum N1 { A = val(), B = val() }
+enum N2 { C = val(), D = val() }
+
+type T1 = { [K in N1 | N2]: K };
+
+// Enum types with string valued members are always literal enum types and therefore
+// ONE and TWO below are not computed members but rather just numerically valued members
+// with auto-incremented values.
+
+declare enum E { ONE, TWO, THREE = 'x' }
+const e: E = E.ONE;
+const x: E.ONE = e;