diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 9f5673af3eee9..18c1f42f838fb 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -3563,7 +3563,7 @@ namespace ts {
                     context.approximateLength += 6;
                     return createKeywordTypeNode(SyntaxKind.ObjectKeyword);
                 }
-                if (type.flags & TypeFlags.TypeParameter && (type as TypeParameter).isThisType) {
+                if (isThisTypeParameter(type)) {
                     if (context.flags & NodeBuilderFlags.InObjectTypeLiteral) {
                         if (!context.encounteredError && !(context.flags & NodeBuilderFlags.AllowThisInObjectLiteral)) {
                             context.encounteredError = true;
@@ -10193,6 +10193,10 @@ namespace ts {
             return maybeTypeOfKind(type, TypeFlags.InstantiableNonPrimitive | TypeFlags.Index);
         }
 
+        function isThisTypeParameter(type: Type): boolean {
+            return !!(type.flags & TypeFlags.TypeParameter && (<TypeParameter>type).isThisType);
+        }
+
         function getSimplifiedType(type: Type, writing: boolean): Type {
             return type.flags & TypeFlags.IndexedAccess ? getSimplifiedIndexedAccessType(<IndexedAccessType>type, writing) :
                 type.flags & TypeFlags.Conditional ? getSimplifiedConditionalType(<ConditionalType>type, writing) :
@@ -16772,7 +16776,7 @@ namespace ts {
             }
 
             function narrowByInKeyword(type: Type, literal: LiteralExpression, assumeTrue: boolean) {
-                if ((type.flags & (TypeFlags.Union | TypeFlags.Object)) || (type.flags & TypeFlags.TypeParameter && (type as TypeParameter).isThisType)) {
+                if (type.flags & (TypeFlags.Union | TypeFlags.Object) || isThisTypeParameter(type)) {
                     const propName = escapeLeadingUnderscores(literal.text);
                     return filterType(type, t => isTypePresencePossible(t, propName, assumeTrue));
                 }
@@ -20092,7 +20096,7 @@ namespace ts {
                         return anyType;
                     }
                     if (right.escapedText && !checkAndReportErrorForExtendingInterface(node)) {
-                        reportNonexistentProperty(right, leftType.flags & TypeFlags.TypeParameter && (leftType as TypeParameter).isThisType ? apparentType : leftType);
+                        reportNonexistentProperty(right, isThisTypeParameter(leftType) ? apparentType : leftType);
                     }
                     return errorType;
                 }
@@ -20496,7 +20500,7 @@ namespace ts {
 
             const effectiveIndexType = isForInVariableForNumericPropertyNames(indexExpression) ? numberType : indexType;
             const accessFlags = isAssignmentTarget(node) ?
-                AccessFlags.Writing | (isGenericObjectType(objectType) ? AccessFlags.NoIndexSignatures : 0) :
+                AccessFlags.Writing | (isGenericObjectType(objectType) && !isThisTypeParameter(objectType) ? AccessFlags.NoIndexSignatures : 0) :
                 AccessFlags.None;
             const indexedAccessType = getIndexedAccessTypeOrUndefined(objectType, effectiveIndexType, node, accessFlags) || errorType;
             return checkIndexedAccessIndexType(indexedAccessType, node);
diff --git a/tests/baselines/reference/keyofAndIndexedAccess2.errors.txt b/tests/baselines/reference/keyofAndIndexedAccess2.errors.txt
index 5b749df2cab36..9ebf28f1857e9 100644
--- a/tests/baselines/reference/keyofAndIndexedAccess2.errors.txt
+++ b/tests/baselines/reference/keyofAndIndexedAccess2.errors.txt
@@ -219,4 +219,13 @@ tests/cases/conformance/types/keyof/keyofAndIndexedAccess2.ts(108,5): error TS23
         let x: Array<string>[K] = 'abc';
         let y: ReadonlyArray<string>[K] = 'abc';
     }
+    
+    // Repro from #31439
+    
+    export class c {
+      [x: string]: string;
+      constructor() {
+        this["a"] = "b";
+      }
+    }
     
\ No newline at end of file
diff --git a/tests/baselines/reference/keyofAndIndexedAccess2.js b/tests/baselines/reference/keyofAndIndexedAccess2.js
index 7bf53b9fba491..374b13b5a643a 100644
--- a/tests/baselines/reference/keyofAndIndexedAccess2.js
+++ b/tests/baselines/reference/keyofAndIndexedAccess2.js
@@ -136,6 +136,15 @@ function fn4<K extends number>() {
     let x: Array<string>[K] = 'abc';
     let y: ReadonlyArray<string>[K] = 'abc';
 }
+
+// Repro from #31439
+
+export class c {
+  [x: string]: string;
+  constructor() {
+    this["a"] = "b";
+  }
+}
 
 
 //// [keyofAndIndexedAccess2.js]
@@ -226,3 +235,9 @@ function fn4() {
     let x = 'abc';
     let y = 'abc';
 }
+// Repro from #31439
+export class c {
+    constructor() {
+        this["a"] = "b";
+    }
+}
diff --git a/tests/baselines/reference/keyofAndIndexedAccess2.symbols b/tests/baselines/reference/keyofAndIndexedAccess2.symbols
index 2155451ef7497..a3412e8a47d31 100644
--- a/tests/baselines/reference/keyofAndIndexedAccess2.symbols
+++ b/tests/baselines/reference/keyofAndIndexedAccess2.symbols
@@ -503,3 +503,17 @@ function fn4<K extends number>() {
 >K : Symbol(K, Decl(keyofAndIndexedAccess2.ts, 133, 13))
 }
 
+// Repro from #31439
+
+export class c {
+>c : Symbol(c, Decl(keyofAndIndexedAccess2.ts, 136, 1))
+
+  [x: string]: string;
+>x : Symbol(x, Decl(keyofAndIndexedAccess2.ts, 141, 3))
+
+  constructor() {
+    this["a"] = "b";
+>this : Symbol(c, Decl(keyofAndIndexedAccess2.ts, 136, 1))
+  }
+}
+
diff --git a/tests/baselines/reference/keyofAndIndexedAccess2.types b/tests/baselines/reference/keyofAndIndexedAccess2.types
index af43ed0f4eb27..b7b71ba4f9f71 100644
--- a/tests/baselines/reference/keyofAndIndexedAccess2.types
+++ b/tests/baselines/reference/keyofAndIndexedAccess2.types
@@ -499,3 +499,21 @@ function fn4<K extends number>() {
 >'abc' : "abc"
 }
 
+// Repro from #31439
+
+export class c {
+>c : c
+
+  [x: string]: string;
+>x : string
+
+  constructor() {
+    this["a"] = "b";
+>this["a"] = "b" : "b"
+>this["a"] : string
+>this : this
+>"a" : "a"
+>"b" : "b"
+  }
+}
+
diff --git a/tests/cases/conformance/types/keyof/keyofAndIndexedAccess2.ts b/tests/cases/conformance/types/keyof/keyofAndIndexedAccess2.ts
index 5251a83bb0611..a99c2b0a7a106 100644
--- a/tests/cases/conformance/types/keyof/keyofAndIndexedAccess2.ts
+++ b/tests/cases/conformance/types/keyof/keyofAndIndexedAccess2.ts
@@ -138,3 +138,12 @@ function fn4<K extends number>() {
     let x: Array<string>[K] = 'abc';
     let y: ReadonlyArray<string>[K] = 'abc';
 }
+
+// Repro from #31439
+
+export class c {
+  [x: string]: string;
+  constructor() {
+    this["a"] = "b";
+  }
+}