In _matchInterfaceSubtypeOf, account for mixins having null superclass.

Fixes #34907.

Change-Id: I1ba56bf57d6ca5a7a69f262e12df801dc2cc4a4a
Reviewed-on: https://dart-review.googlesource.com/c/81322
Auto-Submit: Paul Berry <paulberry@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>

pkg/analyzer/lib/src/generated/type_system.dart

@@ -508,13 +508,17 @@ class GenericInferrer {
     //
     // We don't need undo logic here because if the classes don't match, nothing
     // is added to the constraint set.
-    if (guardedInterfaceSubtype(i1.superclass)) return true;
+    var superclass = i1.superclass;
+    if (superclass != null && guardedInterfaceSubtype(superclass)) return true;
     for (final parent in i1.interfaces) {
       if (guardedInterfaceSubtype(parent)) return true;
     }
     for (final parent in i1.mixins) {
       if (guardedInterfaceSubtype(parent)) return true;
     }
+    for (final parent in i1.superclassConstraints) {
+      if (guardedInterfaceSubtype(parent)) return true;
+    }
     return false;
   }
 

pkg/analyzer/test/generated/non_error_resolver_test.dart

@@ -3655,6 +3655,29 @@ enum E {
     verify([source]);
   }
 
+  test_methodCallTypeInference_mixinType() async {
+    Source source = addSource('''
+main() {
+  C<int> c = f();
+}
+
+class C<T> {}
+
+mixin M<T> on C<T> {}
+
+M<T> f<T>() => null;
+''');
+    var result = await computeAnalysisResult(source);
+    assertNoErrors(source);
+    verify([source]);
+    var main = result.unit.declarations[0] as FunctionDeclaration;
+    var body = main.functionExpression.body as BlockFunctionBody;
+    var cDeclaration = body.block.statements[0] as VariableDeclarationStatement;
+    var fInvocation =
+        cDeclaration.variables.variables[0].initializer as MethodInvocation;
+    expect(fInvocation.staticInvokeType.toString(), '() → M<int>');
+  }
+
   test_methodDeclaration_scope_signature() async {
     Source source = addSource(r'''
 const app = 0;

tests/language_2/issue34907_test.dart

@@ -0,0 +1,18 @@
+// Copyright (c) 2018, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import "package:expect/expect.dart";
+
+class C<T> {}
+
+mixin M<T> on C<T> {}
+
+M<T> f<T>() {
+  Expect.equals(T, int);
+  return null;
+}
+
+main() {
+  C<int> c = f();
+}