Allow exporting a binding before its declaration

src/compiler/checker.ts

@@ -742,6 +742,13 @@ namespace ts {
             // declaration is after usage, but it can still be legal if usage is deferred:
             // 1. inside a function
             // 2. inside an instance property initializer, a reference to a non-instance property
+            // 3. inside an export specifier
+
+            if (usage.parent.kind === SyntaxKind.ExportSpecifier) {
+                // export specifiers do not use the variable, they only make it available for use
+                return true;
+            }
+
             const container = getEnclosingBlockScopeContainer(declaration);
             const isInstanceProperty = declaration.kind === SyntaxKind.PropertyDeclaration && !(getModifierFlags(declaration) & ModifierFlags.Static);
             return isUsedInFunctionOrInstanceProperty(usage, isInstanceProperty, container);

tests/baselines/reference/exportBinding.js

@@ -0,0 +1,19 @@
+//// [exportBinding.ts]
+export { x }
+const x = 'x'
+
+export { Y as Z }
+class Y {}
+
+
+//// [exportBinding.js]
+"use strict";
+exports.__esModule = true;
+var x = 'x';
+exports.x = x;
+var Y = (function () {
+    function Y() {
+    }
+    return Y;
+}());
+exports.Z = Y;

tests/baselines/reference/exportBinding.symbols

@@ -0,0 +1,14 @@
+=== tests/cases/conformance/es6/modules/exportBinding.ts ===
+export { x }
+>x : Symbol(x, Decl(exportBinding.ts, 0, 8))
+
+const x = 'x'
+>x : Symbol(x, Decl(exportBinding.ts, 1, 5))
+
+export { Y as Z }
+>Y : Symbol(Z, Decl(exportBinding.ts, 3, 8))
+>Z : Symbol(Z, Decl(exportBinding.ts, 3, 8))
+
+class Y {}
+>Y : Symbol(Y, Decl(exportBinding.ts, 3, 17))
+

tests/baselines/reference/exportBinding.types

@@ -0,0 +1,15 @@
+=== tests/cases/conformance/es6/modules/exportBinding.ts ===
+export { x }
+>x : "x"
+
+const x = 'x'
+>x : "x"
+>'x' : "x"
+
+export { Y as Z }
+>Y : typeof Y
+>Z : typeof Y
+
+class Y {}
+>Y : Y
+

tests/cases/conformance/es6/modules/exportBinding.ts

@@ -0,0 +1,5 @@
+export { x }
+const x = 'x'
+
+export { Y as Z }
+class Y {}