[cfe] Move assignability checking for for-element conditions earlier.

Previously, the logic to validate that the condition part of a
for-element (a "for" loop inside a collection) was done in
`InferenceVisitorImpl.checkElement` and
`InferenceVisitorImpl.checkMapEntry` (which execute after the type of
the collection has been fully inferred). This CL moves the logic to
`InferenceVisitorImpl._inferForElementBase` and
`InferenceVisitorImpl._inferForMapEntryBase` (so that the
assignability check happens right after the condition has been visited
by the type inferrer).

Moving the assignability checking earlier ensures that if there is an
assignability error, the flow analysis "why not promoted" information
will still be valid, increasing the chances that the user can be given
a helpful "why not promoted" context message.

Change-Id: Ie239a88c94da1bf476fa9e1361053df2d1a56ba2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/388223
Reviewed-by: Johnni Winther <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

Author: stereotype441

pkg/front_end/lib/src/type_inference/inference_visitor.dart

@@ -2658,7 +2658,11 @@ class InferenceVisitorImpl extends InferenceVisitorBase
       ExpressionInferenceResult conditionResult = inferExpression(
           element.condition!, coreTypes.boolRawType(Nullability.nonNullable),
           isVoidAllowed: false);
-      element.condition = conditionResult.expression..parent = element;
+      Expression assignableCondition = ensureAssignable(
+          coreTypes.boolRawType(Nullability.nonNullable),
+          conditionResult.inferredType,
+          conditionResult.expression);
+      element.condition = assignableCondition..parent = element;
       inferredConditionTypes[element.condition!] = conditionResult.inferredType;
     }
     flowAnalysis.for_bodyBegin(null, element.condition);
@@ -2816,28 +2820,12 @@ class InferenceVisitorImpl extends InferenceVisitorBase
           checkElement(otherwise, item, typeArgument, inferredSpreadTypes,
               inferredConditionTypes);
         }
-      case ForElement(:Expression? condition, :Expression body):
-        if (condition != null) {
-          DartType conditionType = inferredConditionTypes[condition]!;
-          Expression assignableCondition = ensureAssignable(
-              coreTypes.boolRawType(Nullability.nonNullable),
-              conditionType,
-              condition);
-          item.condition = assignableCondition..parent = item;
-        }
+      case ForElement(:Expression body):
         if (body is ControlFlowElement) {
           checkElement(body, item, typeArgument, inferredSpreadTypes,
               inferredConditionTypes);
         }
-      case PatternForElement(:Expression? condition, :Expression body):
-        if (condition != null) {
-          DartType conditionType = inferredConditionTypes[condition]!;
-          Expression assignableCondition = ensureAssignable(
-              coreTypes.boolRawType(Nullability.nonNullable),
-              conditionType,
-              condition);
-          item.condition = assignableCondition..parent = item;
-        }
+      case PatternForElement(:Expression body):
         if (body is ControlFlowElement) {
           checkElement(body, item, typeArgument, inferredSpreadTypes,
               inferredConditionTypes);
@@ -4954,8 +4942,11 @@ class InferenceVisitorImpl extends InferenceVisitorBase
       ExpressionInferenceResult conditionResult = inferExpression(
           entry.condition!, coreTypes.boolRawType(Nullability.nonNullable),
           isVoidAllowed: false);
-      entry.condition = conditionResult.expression..parent = entry;
-      // TODO(johnniwinther): Ensure assignability of condition?
+      Expression condition = ensureAssignable(
+          coreTypes.boolRawType(Nullability.nonNullable),
+          conditionResult.inferredType,
+          conditionResult.expression);
+      entry.condition = condition..parent = entry;
       inferredConditionTypes[entry.condition!] = conditionResult.inferredType;
     }
     flowAnalysis.for_bodyBegin(null, entry.condition);
@@ -5210,26 +5201,10 @@ class InferenceVisitorImpl extends InferenceVisitorBase
             entry.otherwise = otherwise..parent = entry;
           }
         case ForMapEntry():
-          if (entry.condition != null) {
-            DartType conditionType = inferredConditionTypes[entry.condition]!;
-            Expression condition = ensureAssignable(
-                coreTypes.boolRawType(Nullability.nonNullable),
-                conditionType,
-                entry.condition!);
-            entry.condition = condition..parent = entry;
-          }
           MapLiteralEntry body = checkMapEntry(entry.body, keyType, valueType,
               inferredSpreadTypes, inferredConditionTypes, offsets);
           entry.body = body..parent = entry;
         case PatternForMapEntry():
-          if (entry.condition != null) {
-            DartType conditionType = inferredConditionTypes[entry.condition]!;
-            Expression condition = ensureAssignable(
-                coreTypes.boolRawType(Nullability.nonNullable),
-                conditionType,
-                entry.condition!);
-            entry.condition = condition..parent = entry;
-          }
           MapLiteralEntry body = checkMapEntry(entry.body, keyType, valueType,
               inferredSpreadTypes, inferredConditionTypes, offsets);
           entry.body = body..parent = entry;