fix: avoid throwing in warn mode when validation internals fail

packages/flow-runtime/src/TypeContext.js

@@ -1027,7 +1027,19 @@ export default class TypeContext {
   }
 
   warn <T, V: T | any> (type: Type<T>, input: V, prefix: string = '', path?: string[]): V {
-    const validation = this.validate(type, input, prefix, path);
+    let validation;
+    try {
+      validation = this.validate(type, input, prefix, path);
+    }
+    catch (error) {
+      if (error instanceof Error) {
+        this.emitWarningMessage(error.message);
+      }
+      else {
+        this.emitWarningMessage(String(error));
+      }
+      return input;
+    }
     const message = makeWarningMessage(validation);
     if (typeof message === 'string') {
       this.emitWarningMessage(message);

packages/flow-runtime/src/warn.test.js

@@ -0,0 +1,26 @@
+/* @flow */
+import {doesNotThrow, ok} from 'assert';
+
+import t from './globalContext';
+
+describe('Warn API', () => {
+  it('should emit a warning instead of throwing when validation internals throw', () => {
+    const shapeOfString = t.$shape(t.string());
+    const messages = [];
+
+    const emitWarningMessage = t.emitWarningMessage;
+    t.emitWarningMessage = (message: string) => {
+      messages.push(message);
+    };
+
+    try {
+      doesNotThrow(() => t.warn(shapeOfString, {name: 'Alice'}));
+    }
+    finally {
+      t.emitWarningMessage = emitWarningMessage;
+    }
+
+    ok(messages.length > 0);
+    ok(/Can only \$Shape<T> object types\./.test(messages[0]));
+  });
+});