fix #25944, improve Future.then inference

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

Index: pkg/analyzer/lib/src/generated/static_type_analyzer.dart
diff --git a/pkg/analyzer/lib/src/generated/static_type_analyzer.dart b/pkg/analyzer/lib/src/generated/static_type_analyzer.dart
index f68147180e5529973e12766669e0348cd963a001..c891d8aa02a2541858ac3816c9ec8e33aacfcfb0 100644
--- a/pkg/analyzer/lib/src/generated/static_type_analyzer.dart
+++ b/pkg/analyzer/lib/src/generated/static_type_analyzer.dart
@@ -505,7 +505,13 @@ class StaticTypeAnalyzer extends SimpleAstVisitor<Object> {
     // * BlockFunctionBody, if we inferred a type from yield/return.
     // * we also normalize bottom to dynamic here.
     if (_strongMode && (computedType.isBottom || computedType.isDynamic)) {
-      computedType = InferenceContext.getType(body) ?? _dynamicType;
+      TypeContext contextType = InferenceContext.getType(body);
+      if (contextType is FutureUnionTypeContext) {
+        // TODO(jmesserly): can we do something better here?
+        computedType = body.isAsynchronous ? contextType.type : _dynamicType;
+      } else {
+        computedType = contextType ?? _dynamicType;
+      }
       recordInference = !computedType.isDynamic;
     }
 
@@ -665,7 +671,7 @@ class StaticTypeAnalyzer extends SimpleAstVisitor<Object> {
     // If there are no type arguments and we are in strong mode, try to infer
     // some arguments.
     if (_strongMode) {
-      DartType contextType = InferenceContext.getType(node);
+      DartType contextType = InferenceContext.getNonFutureType(node);
 
       // If we have a type from the context, use it.
       if (contextType is InterfaceType &&
@@ -748,7 +754,7 @@ class StaticTypeAnalyzer extends SimpleAstVisitor<Object> {
     // If we have no explicit type arguments, and we are in strong mode
     // then try to infer type arguments.
     if (_strongMode) {
-      DartType contextType = InferenceContext.getType(node);
+      DartType contextType = InferenceContext.getNonFutureType(node);
       // If we have a context type, use that for inference.
       if (contextType is InterfaceType &&
           contextType.typeArguments.length == 2 &&
@@ -1120,7 +1126,7 @@ class StaticTypeAnalyzer extends SimpleAstVisitor<Object> {
     }
     if (_strongMode) {
       staticType = _inferGenericInstantiationFromContext(
-          InferenceContext.getType(node), staticType);
+          InferenceContext.getNonFutureType(node), staticType);
     }
     if (!(_strongMode &&
         _inferObjectAccess(node, staticType, prefixedIdentifier))) {
@@ -1253,7 +1259,7 @@ class StaticTypeAnalyzer extends SimpleAstVisitor<Object> {
     }
     if (_strongMode) {
       staticType = _inferGenericInstantiationFromContext(
-          InferenceContext.getType(node), staticType);
+          InferenceContext.getNonFutureType(node), staticType);
     }
     if (!(_strongMode && _inferObjectAccess(node, staticType, propertyName))) {
       _recordStaticType(propertyName, staticType);
@@ -1357,7 +1363,7 @@ class StaticTypeAnalyzer extends SimpleAstVisitor<Object> {
     }
     if (_strongMode) {
       staticType = _inferGenericInstantiationFromContext(
-          InferenceContext.getType(node), staticType);
+          InferenceContext.getNonFutureType(node), staticType);
     }
     _recordStaticType(node, staticType);
     // TODO(brianwilkerson) I think we want to repeat the logic above using the
@@ -1971,8 +1977,17 @@ class StaticTypeAnalyzer extends SimpleAstVisitor<Object> {
         }
       }
 
-      return ts.inferGenericFunctionCall(_typeProvider, fnType, paramTypes,
-          argTypes, InferenceContext.getType(node));
+      TypeContext returnContext = InferenceContext.getType(node);
+      DartType returnType;
+      if (returnContext is FutureUnionTypeContext) {
+        returnType = fnType.returnType.isDartAsyncFuture
+            ? returnContext.futureOfType
+            : returnContext.type;
+      } else {
+        returnType = returnContext as DartType;
+      }
+      return ts.inferGenericFunctionCall(
+          _typeProvider, fnType, paramTypes, argTypes, returnType);
     }
     return null;
   }