fix #25619 - downward inference on generic function tear-offs

[email protected]

Review URL: https://codereview.chromium.org/1644403005 .

Author: John Messerly

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

@@ -1015,6 +1015,10 @@ class StaticTypeAnalyzer extends SimpleAstVisitor<Object> {
     } else if (staticElement is VariableElement) {
       staticType = staticElement.type;
     }
+    if (_strongMode) {
+      staticType = _inferGenericInstantiationFromContext(
+          InferenceContext.getType(node), staticType);
+    }
     if (!(_strongMode &&
         _inferObjectAccess(node, staticType, prefixedIdentifier))) {
       _recordStaticType(prefixedIdentifier, staticType);
@@ -1144,6 +1148,10 @@ class StaticTypeAnalyzer extends SimpleAstVisitor<Object> {
     } else {
       // TODO(brianwilkerson) Report this internal error.
     }
+    if (_strongMode) {
+      staticType = _inferGenericInstantiationFromContext(
+          InferenceContext.getType(node), staticType);
+    }
     if (!(_strongMode && _inferObjectAccess(node, staticType, propertyName))) {
       _recordStaticType(propertyName, staticType);
       _recordStaticType(node, staticType);
@@ -1244,6 +1252,10 @@ class StaticTypeAnalyzer extends SimpleAstVisitor<Object> {
     } else {
       staticType = _dynamicType;
     }
+    if (_strongMode) {
+      staticType = _inferGenericInstantiationFromContext(
+          InferenceContext.getType(node), staticType);
+    }
     _recordStaticType(node, staticType);
     // TODO(brianwilkerson) I think we want to repeat the logic above using the
     // propagated element to get another candidate for the propagated type.
@@ -1882,6 +1894,21 @@ class StaticTypeAnalyzer extends SimpleAstVisitor<Object> {
     return false;
   }
 
+  /**
+   * Given an uninstantiated generic type, try to infer the instantiated generic
+   * type from the surrounding context.
+   */
+  DartType _inferGenericInstantiationFromContext(
+      DartType context, DartType type) {
+    TypeSystem ts = _typeSystem;
+    if (context is FunctionType &&
+        type is FunctionType &&
+        ts is StrongTypeSystemImpl) {
+      return ts.inferFunctionTypeInstantiation(_typeProvider, context, type);
+    }
+    return type;
+  }
+
   /**
    * Given a method invocation [node], attempt to infer a better
    * type for the result if it is an inline JS invocation