Try lowercase username on login

Fixes https://github.com/vector-im/riot-web/issues/5446

src/Login.js

@@ -143,17 +143,8 @@ export default class Login {
         Object.assign(loginParams, legacyParams);
 
         const client = this._createTemporaryClient();
-        return client.login('m.login.password', loginParams).then(function(data) {
+
-            return Promise.resolve({
+        const tryFallbackHs = (originalError) => {
-                homeserverUrl: self._hsUrl,
-                identityServerUrl: self._isUrl,
-                userId: data.user_id,
-                deviceId: data.device_id,
-                accessToken: data.access_token,
-            });
-        }, function(error) {
-            if (error.httpStatus === 403) {
-                if (self._fallbackHsUrl) {
             const fbClient = Matrix.createClient({
                 baseUrl: self._fallbackHsUrl,
                 idBaseUrl: this._isUrl,
@@ -169,11 +160,56 @@ export default class Login {
                 });
             }, function(fallback_error) {
                 // throw the original error
-                        throw error;
+                throw originalError;
             });
+        };
+        const tryLowercaseUsername = (originalError) => {
+            const loginParamsLowercase = Object.assign({}, loginParams, {
+                user: username.toLowerCase(),
+                identifier: {
+                    user: username.toLowerCase(),
+                },
+            });
+            return client.login('m.login.password', loginParamsLowercase).then(function(data) {
+                return Promise.resolve({
+                    homeserverUrl: self._fallbackHsUrl,
+                    identityServerUrl: self._isUrl,
+                    userId: data.user_id,
+                    deviceId: data.device_id,
+                    accessToken: data.access_token,
+                });
+            }, function(fallback_error) {
+                // throw the original error
+                throw originalError;
+            });
+        };
+
+        let originalLoginError = null;
+        return client.login('m.login.password', loginParams).then(function(data) {
+            return Promise.resolve({
+                homeserverUrl: self._hsUrl,
+                identityServerUrl: self._isUrl,
+                userId: data.user_id,
+                deviceId: data.device_id,
+                accessToken: data.access_token,
+            });
+        }).catch((error) => {
+            originalLoginError = error;
+            if (error.httpStatus === 403) {
+                if (self._fallbackHsUrl) {
+                    return tryFallbackHs(originalLoginError);
                 }
             }
-            throw error;
+            throw originalLoginError;
+        }).catch((error) => {
+            if (
+                error.httpStatus === 403 &&
+                loginParams.identifier.type === 'm.id.user' &&
+                username.search(/[A-Z]/) > -1
+            ) {
+                return tryLowercaseUsername(originalLoginError);
+            }
+            throw originalLoginError;
         });
     }