Add an edge origin for edges resulting from default values of optional parameters

Change-Id: I252e2e003dc46b4ddf702bf0c7d16a7611c2476a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/120280
Reviewed-by: Paul Berry <paulberry@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>

Author: bwilkerson

pkg/nnbd_migration/lib/src/edge_builder.dart

@@ -563,7 +563,7 @@ class EdgeBuilder extends GeneralizingAstVisitor<DecoratedType>
     } else {
       _handleAssignment(defaultValue,
           destinationType: getOrComputeElementType(node.declaredElement),
-          canInsertChecks: false);
+          fromDefaultValue: true);
     }
     return null;
   }
@@ -1591,7 +1591,7 @@ class EdgeBuilder extends GeneralizingAstVisitor<DecoratedType>
       Expression destinationExpression,
       AssignmentExpression compoundOperatorInfo,
       Expression questionAssignNode,
-      bool canInsertChecks = true}) {
+      bool fromDefaultValue = false}) {
     assert(
         (destinationExpression == null) != (destinationType == null),
         'Either destinationExpression or destinationType should be supplied, '
@@ -1621,12 +1621,18 @@ class EdgeBuilder extends GeneralizingAstVisitor<DecoratedType>
         throw StateError('No type computed for ${expression.runtimeType} '
             '(${expression.toSource()}) offset=${expression.offset}');
       }
-      ExpressionChecksOrigin expressionChecksOrigin;
-      if (canInsertChecks && !sourceType.type.isDynamic) {
-        expressionChecksOrigin = ExpressionChecksOrigin(
-            source, expression, ExpressionChecks(expression.end));
-        _variables.recordExpressionChecks(
-            source, expression, expressionChecksOrigin);
+      EdgeOrigin edgeOrigin;
+      if (!sourceType.type.isDynamic) {
+        if (fromDefaultValue) {
+          edgeOrigin = DefaultValueOrigin(source, expression);
+        } else {
+          ExpressionChecksOrigin expressionChecksOrigin =
+              ExpressionChecksOrigin(
+                  source, expression, ExpressionChecks(expression.end));
+          _variables.recordExpressionChecks(
+              source, expression, expressionChecksOrigin);
+          edgeOrigin = expressionChecksOrigin;
+        }
       }
       if (compoundOperatorInfo != null) {
         var compoundOperatorMethod = compoundOperatorInfo.staticElement;
@@ -1640,7 +1646,7 @@ class EdgeBuilder extends GeneralizingAstVisitor<DecoratedType>
           DecoratedType compoundOperatorType =
               getOrComputeElementType(compoundOperatorMethod);
           assert(compoundOperatorType.positionalParameters.length > 0);
-          _checkAssignment(expressionChecksOrigin,
+          _checkAssignment(edgeOrigin,
               source: sourceType,
               destination: compoundOperatorType.positionalParameters[0],
               hard: _postDominatedLocals.isReferenceInScope(expression));
@@ -1655,7 +1661,7 @@ class EdgeBuilder extends GeneralizingAstVisitor<DecoratedType>
           sourceType = _dynamicType;
         }
       } else {
-        _checkAssignment(expressionChecksOrigin,
+        _checkAssignment(edgeOrigin,
             source: sourceType,
             destination: destinationType,
             hard: questionAssignNode == null &&

pkg/nnbd_migration/lib/src/edge_origin.dart

@@ -28,6 +28,12 @@ class CompoundAssignmentOrigin extends EdgeOrigin {
   AssignmentExpression get node => super.node as AssignmentExpression;
 }
 
+/// An edge origin used for edges that originated because of a default value on
+/// a parameter.
+class DefaultValueOrigin extends EdgeOrigin {
+  DefaultValueOrigin(Source source, Expression node) : super(source, node);
+}
+
 /// Common interface for classes providing information about how an edge came
 /// to be; that is, what was found in the source code that led the migration
 /// tool to create the edge.