diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 1c41be84ba741..3c1ed9a94921e 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -1016,7 +1016,18 @@ namespace ts {
                             }
                         }
                         break;
-
+                    case SyntaxKind.ExpressionWithTypeArguments:
+                        // The type parameters of a class are not in scope in the base class expression.
+                        if (lastLocation === (<ExpressionWithTypeArguments>location).expression && (<HeritageClause>location.parent).token === SyntaxKind.ExtendsKeyword) {
+                            const container = location.parent.parent;
+                            if (isClassLike(container) && (result = lookup(getSymbolOfNode(container).members, name, meaning & SymbolFlags.Type))) {
+                                if (nameNotFoundMessage) {
+                                    error(errorLocation, Diagnostics.Base_class_expressions_cannot_reference_class_type_parameters);
+                                }
+                                return undefined;
+                            }
+                        }
+                        break;
                     // It is not legal to reference a class's own type parameters from a computed property name that
                     // belongs to the class. For example:
                     //
diff --git a/src/compiler/diagnosticMessages.json b/src/compiler/diagnosticMessages.json
index 77e7f7e7b6246..049483fdb8f3f 100644
--- a/src/compiler/diagnosticMessages.json
+++ b/src/compiler/diagnosticMessages.json
@@ -1912,6 +1912,10 @@
         "category": "Error",
         "code": 2560
     },
+    "Base class expressions cannot reference class type parameters.": {
+        "category": "Error",
+        "code": 2561
+    },
     "JSX element attributes type '{0}' may not be a union type.": {
         "category": "Error",
         "code": 2600
diff --git a/tests/baselines/reference/baseExpressionTypeParameters.errors.txt b/tests/baselines/reference/baseExpressionTypeParameters.errors.txt
new file mode 100644
index 0000000000000..4fa8f2f6d3dbb
--- /dev/null
+++ b/tests/baselines/reference/baseExpressionTypeParameters.errors.txt
@@ -0,0 +1,19 @@
+tests/cases/compiler/baseExpressionTypeParameters.ts(10,27): error TS2561: Base class expressions cannot reference class type parameters.
+
+
+==== tests/cases/compiler/baseExpressionTypeParameters.ts (1 errors) ====
+    // Repro from #17829
+    
+    function base<T>() {
+        class Base {
+            static prop: T;
+        }
+        return Base;
+    }
+    
+    class Gen<T> extends base<T>() {}  // Error, T not in scope
+                              ~
+!!! error TS2561: Base class expressions cannot reference class type parameters.
+    class Spec extends Gen<string> {}
+    
+    <string>Spec.prop;
\ No newline at end of file
diff --git a/tests/baselines/reference/baseExpressionTypeParameters.js b/tests/baselines/reference/baseExpressionTypeParameters.js
new file mode 100644
index 0000000000000..a3a77cbc6e347
--- /dev/null
+++ b/tests/baselines/reference/baseExpressionTypeParameters.js
@@ -0,0 +1,50 @@
+//// [baseExpressionTypeParameters.ts]
+// Repro from #17829
+
+function base<T>() {
+    class Base {
+        static prop: T;
+    }
+    return Base;
+}
+
+class Gen<T> extends base<T>() {}  // Error, T not in scope
+class Spec extends Gen<string> {}
+
+<string>Spec.prop;
+
+//// [baseExpressionTypeParameters.js]
+// Repro from #17829
+var __extends = (this && this.__extends) || (function () {
+    var extendStatics = Object.setPrototypeOf ||
+        ({ __proto__: [] } instanceof Array && function (d, b) { d.__proto__ = b; }) ||
+        function (d, b) { for (var p in b) if (b.hasOwnProperty(p)) d[p] = b[p]; };
+    return function (d, b) {
+        extendStatics(d, b);
+        function __() { this.constructor = d; }
+        d.prototype = b === null ? Object.create(b) : (__.prototype = b.prototype, new __());
+    };
+})();
+function base() {
+    var Base = /** @class */ (function () {
+        function Base() {
+        }
+        return Base;
+    }());
+    return Base;
+}
+var Gen = /** @class */ (function (_super) {
+    __extends(Gen, _super);
+    function Gen() {
+        return _super !== null && _super.apply(this, arguments) || this;
+    }
+    return Gen;
+}(base())); // Error, T not in scope
+var Spec = /** @class */ (function (_super) {
+    __extends(Spec, _super);
+    function Spec() {
+        return _super !== null && _super.apply(this, arguments) || this;
+    }
+    return Spec;
+}(Gen));
+Spec.prop;
diff --git a/tests/baselines/reference/inheritFromGenericTypeParameter.errors.txt b/tests/baselines/reference/inheritFromGenericTypeParameter.errors.txt
index 08d138f2e5ddc..c043e37575c9a 100644
--- a/tests/baselines/reference/inheritFromGenericTypeParameter.errors.txt
+++ b/tests/baselines/reference/inheritFromGenericTypeParameter.errors.txt
@@ -1,11 +1,11 @@
-tests/cases/compiler/inheritFromGenericTypeParameter.ts(1,20): error TS2693: 'T' only refers to a type, but is being used as a value here.
+tests/cases/compiler/inheritFromGenericTypeParameter.ts(1,20): error TS2304: Cannot find name 'T'.
 tests/cases/compiler/inheritFromGenericTypeParameter.ts(2,24): error TS2312: An interface may only extend a class or another interface.
 
 
 ==== tests/cases/compiler/inheritFromGenericTypeParameter.ts (2 errors) ====
     class C<T> extends T { }
                        ~
-!!! error TS2693: 'T' only refers to a type, but is being used as a value here.
+!!! error TS2304: Cannot find name 'T'.
     interface I<T> extends T { }
                            ~
 !!! error TS2312: An interface may only extend a class or another interface.
\ No newline at end of file
diff --git a/tests/baselines/reference/typeParameterAsBaseClass.errors.txt b/tests/baselines/reference/typeParameterAsBaseClass.errors.txt
index e633a6dd39669..dec7f7f2a4a24 100644
--- a/tests/baselines/reference/typeParameterAsBaseClass.errors.txt
+++ b/tests/baselines/reference/typeParameterAsBaseClass.errors.txt
@@ -1,11 +1,11 @@
-tests/cases/compiler/typeParameterAsBaseClass.ts(1,20): error TS2693: 'T' only refers to a type, but is being used as a value here.
+tests/cases/compiler/typeParameterAsBaseClass.ts(1,20): error TS2304: Cannot find name 'T'.
 tests/cases/compiler/typeParameterAsBaseClass.ts(2,24): error TS2422: A class may only implement another class or interface.
 
 
 ==== tests/cases/compiler/typeParameterAsBaseClass.ts (2 errors) ====
     class C<T> extends T {}
                        ~
-!!! error TS2693: 'T' only refers to a type, but is being used as a value here.
+!!! error TS2304: Cannot find name 'T'.
     class C2<T> implements T {}
                            ~
 !!! error TS2422: A class may only implement another class or interface.
\ No newline at end of file
diff --git a/tests/baselines/reference/typeParameterAsBaseType.errors.txt b/tests/baselines/reference/typeParameterAsBaseType.errors.txt
index c16b16d0b1a45..835b236994867 100644
--- a/tests/baselines/reference/typeParameterAsBaseType.errors.txt
+++ b/tests/baselines/reference/typeParameterAsBaseType.errors.txt
@@ -1,5 +1,5 @@
-tests/cases/conformance/types/typeParameters/typeParameterAsBaseType.ts(4,20): error TS2693: 'T' only refers to a type, but is being used as a value here.
-tests/cases/conformance/types/typeParameters/typeParameterAsBaseType.ts(5,24): error TS2693: 'U' only refers to a type, but is being used as a value here.
+tests/cases/conformance/types/typeParameters/typeParameterAsBaseType.ts(4,20): error TS2304: Cannot find name 'T'.
+tests/cases/conformance/types/typeParameters/typeParameterAsBaseType.ts(5,24): error TS2304: Cannot find name 'U'.
 tests/cases/conformance/types/typeParameters/typeParameterAsBaseType.ts(7,24): error TS2312: An interface may only extend a class or another interface.
 tests/cases/conformance/types/typeParameters/typeParameterAsBaseType.ts(8,28): error TS2312: An interface may only extend a class or another interface.
 
@@ -10,10 +10,10 @@ tests/cases/conformance/types/typeParameters/typeParameterAsBaseType.ts(8,28): e
     
     class C<T> extends T { }
                        ~
-!!! error TS2693: 'T' only refers to a type, but is being used as a value here.
+!!! error TS2304: Cannot find name 'T'.
     class C2<T, U> extends U { }
                            ~
-!!! error TS2693: 'U' only refers to a type, but is being used as a value here.
+!!! error TS2304: Cannot find name 'U'.
     
     interface I<T> extends T { }
                            ~
diff --git a/tests/cases/compiler/baseExpressionTypeParameters.ts b/tests/cases/compiler/baseExpressionTypeParameters.ts
new file mode 100644
index 0000000000000..ca864f9a7f00f
--- /dev/null
+++ b/tests/cases/compiler/baseExpressionTypeParameters.ts
@@ -0,0 +1,13 @@
+// Repro from #17829
+
+function base<T>() {
+    class Base {
+        static prop: T;
+    }
+    return Base;
+}
+
+class Gen<T> extends base<T>() {}  // Error, T not in scope
+class Spec extends Gen<string> {}
+
+<string>Spec.prop;
\ No newline at end of file