fix #26992, inference failures are now an error

CHANGELOG.md

 ## 1.20.0
 
 ### Dart VM
 
 * We have improved the way that the VM locates the native code library for a
   native extension (e.g. `dart-ext:` import). We have updated this
   [article on native extensions](https://www.dartlang.org/articles/dart-vm/native-extensions)
   to reflect the VM's improved behavior.
 
 ### Core library changes
 * `dart:core`: Remove deprecated `Resource` class.
   Use the class in `package:resource` instead.
 * `dart:async`
   * `Future.wait` now catches synchronous errors and returns them in the
     returned Future.
 * `dart:io`
   * Added `WebSocket.addUtf8Text` to allow sending a pre-encoded text message
     without a round-trip UTF-8 conversion.
 
+## Strong Mode
+
+* Breaking change - it is an error if a generic type parameter cannot be
+    inferred (SDK issue [26992](https://github.com/dart-lang/sdk/issues/26992)).
+
+    ```dart
+    class Cup<T> {
+      Cup(T t);
+    }
+    main() {
+      // Error because:
+      // - if we choose Cup<num> it is not assignable to `cOfInt`,
+      // - if we choose Cup<int> then `n` is not assignable to int.
+      num n;
+      C<int> cOfInt = new C(n);
+    }
+    ```
+
 ## 1.19.0
 
 ### Language changes
 
 * The language now allows a trailing comma after the last argument of a call and
  the last parameter of a function declaration. This can make long argument or
  parameter lists easier to maintain, as commas can be left as-is when
  reordering lines. For details, see SDK issue
  [26644](https://github.com/dart-lang/sdk/issues/26644).
 
@@ skipping 1185 matching lines @@
     they will keep the Dart process alive until they time out. This fixes the
     handling of persistent connections. Previously, the client would shut down
     immediately after a request.
 
   * **Breaking change:** `HttpServer` no longer compresses all traffic by
     default. The new `autoCompress` property can be set to `true` to re-enable
     compression.
 
 * `dart:isolate`: `Isolate.spawnUri` added the optional `packageRoot` argument,
   which controls how it resolves `package:` URIs.