Temporarily restore ad hoc Future.then inference

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

Index: pkg/analyzer/lib/src/generated/resolver.dart
diff --git a/pkg/analyzer/lib/src/generated/resolver.dart b/pkg/analyzer/lib/src/generated/resolver.dart
index 61f6a106b1835ee569844af2117f9db7b3e197b1..5d90f3c03d3c59932fa4978a73d9d30a7cf213f9 100644
--- a/pkg/analyzer/lib/src/generated/resolver.dart
+++ b/pkg/analyzer/lib/src/generated/resolver.dart
@@ -5218,8 +5218,26 @@ class ResolverVisitor extends ScopedVisitor {
   }
 
   /**
+   * Returns true if this method is `Future.then` or an override thereof.
+   *
+   * If so we will apply special typing rules in strong mode, to handle the
+   * implicit union of `S | Future<S>`
+   */
+  // TODO(leafp): Eliminate this when code is switched to using FutureOr
+  bool isFutureThen(Element element) {
+    // If we are a method named then
+    if (element is MethodElement && element.name == 'then') {
+      DartType type = element.enclosingElement.type;
+      // On Future or a subtype, then we're good.
+      return (type.isDartAsyncFuture || isSubtypeOfFuture(type));
+    }
+    return false;
+  }
+
+  /**
    * Returns true if this type is any subtype of the built in Future type.
    */
+  // TODO(leafp): Eliminate this when code is switched to using FutureOr
   bool isSubtypeOfFuture(DartType type) =>
       typeSystem.isSubtypeOf(type, typeProvider.futureDynamicType);
 
@@ -6060,8 +6078,27 @@ class ResolverVisitor extends ScopedVisitor {
               matchFunctionTypeParameters(node.typeParameters, functionType);
           if (functionType is FunctionType) {
             _inferFormalParameterList(node.parameters, functionType);
-            InferenceContext.setType(
-                node.body, _computeReturnOrYieldType(functionType.returnType));
+            DartType returnType;
+            ParameterElement parameterElement =
+                resolutionMap.staticParameterElementForExpression(node);
+            if (isFutureThen(parameterElement?.enclosingElement)) {
+              var futureThenType =
+                  InferenceContext.getContext(node.parent) as FunctionType;
+
+              // TODO(leafp): Get rid of this once code has been updated to use
+              // FutureOr
+              // Introduce FutureOr<T> for backwards compatibility if it was
+              // missing in old code.
+              if (futureThenType.parameters.isNotEmpty) {
+                if (!futureThenType.parameters[0].type.isDartAsyncFutureOr) {
+                  var typeParamS =
+                      futureThenType.returnType.flattenFutures(typeSystem);
+                  returnType = _createFutureOr(typeParamS);
+                }
+              }
+            }
+            returnType ??= _computeReturnOrYieldType(functionType.returnType);
+            InferenceContext.setType(node.body, returnType);
           }
         }
         super.visitFunctionExpression(node);