Handle type predicates in api check. (#2201)

Author: sobolk

scripts/components/api-changes-validator/api_usage_statements_generators.ts

@@ -566,7 +566,18 @@ export class CallableUsageStatementsGenerator
       ).generate().usageStatement ?? '';
     let returnValueAssignmentTarget = '';
     if (this.functionType.type.kind !== ts.SyntaxKind.VoidKeyword) {
-      returnValueAssignmentTarget = `const returnValue: ${this.functionType.type.getText()} = `;
+      let returnType;
+      if (this.functionType.type.kind === ts.SyntaxKind.TypePredicate) {
+        // Example type predicate looks like this
+        // '(input: unknown) => input is SampleType;'
+        // It's a special syntax that tells compiler that it's safe to assume
+        // type after invoking the check.
+        // But when it comes to value assignment this is treated as boolean.
+        returnType = 'boolean';
+      } else {
+        returnType = this.functionType.type.getText();
+      }
+      returnValueAssignmentTarget = `const returnValue: ${returnType} = `;
     }
     const minParameterUsage =
       new CallableParameterUsageStatementsGenerator(

scripts/components/api-changes-validator/test-resources/test-projects/without-breaks/project-without-breaks/API.md

@@ -84,4 +84,10 @@ export type SampleTypeThatReferencesFunction<T extends typeof someFunction1> = {
 export type SampleIgnoredType = {
   someProperty: string;
 };
+
+export const sampleTypePredicate: (input: unknown) => input is SampleType;
+
+export class SampleClassWithTypePredicate {
+  static sampleTypePredicate: (input: unknown) => input is SampleType;
+}
 ```

scripts/components/api-changes-validator/test-resources/test-projects/without-breaks/project-without-breaks/src/index.ts

@@ -115,3 +115,13 @@ export type SampleTypeThatReferencesFunction<T extends typeof someFunction1> = {
 export type SampleIgnoredType = {
   someProperty: number;
 };
+
+export const sampleTypePredicate = (input: unknown): input is SampleType => {
+  throw new Error();
+};
+
+export class SampleClassWithTypePredicate {
+  static sampleTypePredicate = (input: unknown): input is SampleType => {
+    throw new Error();
+  };
+}