[TypeChecker] SE-0326: Allow ExprAvailabilityWalker to walk into multi-statement closures

When `experimental-multi-statement-closures` flag is enabled,
multi-statement closures are type-checked together with enclosing
context, which means that availability checking is allowed to walk
into them.

Author: xedin

lib/Sema/TypeCheckAvailability.cpp

@@ -2849,16 +2849,7 @@ class ExprAvailabilityWalker : public ASTWalker {
     }
 
     if (AbstractClosureExpr *closure = dyn_cast<AbstractClosureExpr>(E)) {
-      // Multi-statement closures are collected by ExprWalker::rewriteFunction
-      // and checked by ExprWalker::processDelayed in CSApply.cpp.
-      // Single-statement closures only have the attributes checked
-      // by TypeChecker::checkClosureAttributes in that rewriteFunction.
-      // Multi-statement closures will be checked explicitly later (as the decl
-      // context in the Where). Single-expression closures will not be
-      // revisited, and are not automatically set as the context of the 'where'.
-      // Don't double-check multi-statement closures, but do check
-      // single-statement closures, setting the closure as the decl context.
-      if (closure->hasSingleExpressionBody()) {
+      if (shouldWalkIntoClosure(closure)) {
         walkAbstractClosure(closure);
         return skipChildren();
       }
@@ -3006,6 +2997,24 @@ class ExprAvailabilityWalker : public ASTWalker {
     walkInContext(E, E->getSubExpr(), MemberAccessContext::InOut);
   }
 
+  bool shouldWalkIntoClosure(AbstractClosureExpr *closure) const {
+    // Multi-statement closures are collected by ExprWalker::rewriteFunction
+    // and checked by ExprWalker::processDelayed in CSApply.cpp.
+    // Single-statement closures only have the attributes checked
+    // by TypeChecker::checkClosureAttributes in that rewriteFunction.
+    // Multi-statement closures will be checked explicitly later (as the decl
+    // context in the Where). Single-expression closures will not be
+    // revisited, and are not automatically set as the context of the 'where'.
+    // Don't double-check multi-statement closures, but do check
+    // single-statement closures, setting the closure as the decl context.
+    //
+    // Note about SE-0326: When a flag is enabled multi-statement closures
+    // are type-checked together with enclosing context, so walker behavior
+    // should match that of single-expression closures.
+    return closure->hasSingleExpressionBody() ||
+           Context.TypeCheckerOpts.EnableMultiStatementClosureInference;
+  }
+
   /// Walk an abstract closure expression, checking for availability
   void walkAbstractClosure(AbstractClosureExpr *closure) {
     // Do the walk with the closure set as the decl context of the 'where'