Merge pull request #5949 from Microsoft/typeParametersAsConstraints

Type parameters as constraints

tests/cases/compiler/declarationEmitFBoundedTypeParams.ts

@@ -0,0 +1,8 @@
+// @declaration: true
+
+// Repro from #6040
+
+function append<a, b extends a>(result: a[], value: b): a[] {
+    result.push(value);
+    return result;
+}

tests/cases/compiler/typeInferenceFBoundedTypeParams.ts

@@ -0,0 +1,22 @@
+// Example from #6037
+
+function fold<a, r>(values: a[], result: r, fold: (result: r, value: a) => r): r {
+    for (let value of values) {
+        result = fold(result, value);
+    }
+    return result;
+}
+
+function append<a, b extends a>(values: a[], value: b): a[] {
+    values.push(value);
+    return values;
+}
+
+fold(
+    [1, 2, 3],
+    [] as [string, string][],
+    (result, value) => append(
+        result,
+        ["", ""]
+    )
+);

tests/cases/compiler/typeParameterConstraintInstantiation.ts

@@ -0,0 +1,8 @@
+// Check that type parameter constraints are properly instantiated
+
+interface Mapper<T> {
+    map<U extends T, V extends U[]>(f: (item: T) => U): V;
+}
+
+var m: Mapper<string>;
+var a = m.map((x: string) => x);  // string[]