diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 83fea11c993..57905315d73 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -1056,9 +1056,9 @@ namespace ts {
                 // block-scoped variable and namespace module. However, only when we
                 // try to resolve name in /*1*/ which is used in variable position,
                 // we want to check for block-scoped
-                if (meaning & SymbolFlags.BlockScopedVariable) {
+                if (meaning & SymbolFlags.BlockScopedVariable || (meaning & SymbolFlags.Class && (meaning & SymbolFlags.Value) === SymbolFlags.Value)) {
                     const exportOrLocalSymbol = getExportSymbolOfValueSymbolIfExported(result);
-                    if (exportOrLocalSymbol.flags & SymbolFlags.BlockScopedVariable) {
+                    if (exportOrLocalSymbol.flags & SymbolFlags.BlockScopedVariable || exportOrLocalSymbol.flags & SymbolFlags.Class) {
                         checkResolvedBlockScopedVariable(exportOrLocalSymbol, errorLocation);
                     }
                 }
@@ -1171,14 +1171,19 @@ namespace ts {
         }
 
         function checkResolvedBlockScopedVariable(result: Symbol, errorLocation: Node): void {
-            Debug.assert((result.flags & SymbolFlags.BlockScopedVariable) !== 0);
+            Debug.assert(!!(result.flags & SymbolFlags.BlockScopedVariable || result.flags & SymbolFlags.Class));
             // Block-scoped variables cannot be used before their definition
-            const declaration = forEach(result.declarations, d => isBlockOrCatchScoped(d) ? d : undefined);
+            const declaration = forEach(result.declarations, d => isBlockOrCatchScoped(d) || isClassLike(d) ? d : undefined);
 
             Debug.assert(declaration !== undefined, "Block-scoped variable declaration is undefined");
 
             if (!isInAmbientContext(declaration) && !isBlockScopedNameDeclaredBeforeUse(declaration, errorLocation)) {
-                error(errorLocation, Diagnostics.Block_scoped_variable_0_used_before_its_declaration, declarationNameToString(declaration.name));
+                if (result.flags & SymbolFlags.BlockScopedVariable) {
+                    error(errorLocation, Diagnostics.Block_scoped_variable_0_used_before_its_declaration, declarationNameToString(declaration.name));
+                }
+                else {
+                    error(errorLocation, Diagnostics.Class_0_used_before_its_declaration, declarationNameToString(declaration.name));
+                }
             }
         }
 
@@ -13171,10 +13176,17 @@ namespace ts {
                 }
                 return unknownType;
             }
-            if (prop.valueDeclaration &&
-                isInPropertyInitializer(node) &&
-                !isBlockScopedNameDeclaredBeforeUse(prop.valueDeclaration, right)) {
-                error(right, Diagnostics.Block_scoped_variable_0_used_before_its_declaration, right.text);
+            if (prop.valueDeclaration) {
+                if (isInPropertyInitializer(node) &&
+                    !isBlockScopedNameDeclaredBeforeUse(prop.valueDeclaration, right)) {
+                    error(right, Diagnostics.Block_scoped_variable_0_used_before_its_declaration, right.text);
+                }
+                if (prop.valueDeclaration.kind === SyntaxKind.ClassDeclaration &&
+                    node.parent && node.parent.kind !== SyntaxKind.TypeReference &&
+                    !isInAmbientContext(prop.valueDeclaration) &&
+                    !isBlockScopedNameDeclaredBeforeUse(prop.valueDeclaration, right)) {
+                    error(right, Diagnostics.Class_0_used_before_its_declaration, right.text);
+                }
             }
 
             markPropertyAsReferenced(prop);
@@ -19421,14 +19433,6 @@ namespace ts {
                         error(node.name || node, Diagnostics.A_mixin_class_must_have_a_constructor_with_a_single_rest_parameter_of_type_any);
                     }
 
-                    if (baseType.symbol && baseType.symbol.valueDeclaration &&
-                        !isInAmbientContext(baseType.symbol.valueDeclaration) &&
-                        baseType.symbol.valueDeclaration.kind === SyntaxKind.ClassDeclaration) {
-                        if (!isBlockScopedNameDeclaredBeforeUse(baseType.symbol.valueDeclaration, node)) {
-                            error(baseTypeNode, Diagnostics.A_class_must_be_declared_after_its_base_class);
-                        }
-                    }
-
                     if (!(staticBaseType.symbol && staticBaseType.symbol.flags & SymbolFlags.Class) && !(baseConstructorType.flags & TypeFlags.TypeVariable)) {
                         // When the static base type is a "class-like" constructor function (but not actually a class), we verify
                         // that all instantiated base constructor signatures return the same type. We can simply compare the type
diff --git a/src/compiler/diagnosticMessages.json b/src/compiler/diagnosticMessages.json
index b9572b2b0f4..691f8e8d31b 100644
--- a/src/compiler/diagnosticMessages.json
+++ b/src/compiler/diagnosticMessages.json
@@ -1435,6 +1435,10 @@
         "category": "Error",
         "code": 2448
     },
+    "Class '{0}' used before its declaration.": {
+        "category": "Error",
+        "code": 2449
+    },
     "Cannot redeclare block-scoped variable '{0}'.": {
         "category": "Error",
         "code": 2451
@@ -2019,10 +2023,6 @@
         "category": "Error",
         "code": 2689
     },
-    "A class must be declared after its base class.": {
-        "category": "Error",
-        "code": 2690
-    },
     "An import path cannot end with a '{0}' extension. Consider importing '{1}' instead.": {
         "category": "Error",
         "code": 2691