[CS] Improve SolutionApplicationTarget walking logic

Fill in some nodes that previously weren't being
walked, and bail with `None` if the walk is
terminated, rather than filling the resulting
SolutionApplicationTarget with null pointers.

This method currently isn't called anywhere, but
I'm planning on using it shortly, so this is an
NFC change for now.

Author: hamishknight

include/swift/Sema/SolutionApplicationTarget.h

@@ -869,7 +869,7 @@ class SolutionApplicationTarget {
   }
 
   /// Walk the contents of the application target.
-  SolutionApplicationTarget walk(ASTWalker &walker);
+  Optional<SolutionApplicationTarget> walk(ASTWalker &walker) const;
 };
 
 }

lib/Sema/SolutionApplicationTarget.cpp

@@ -281,50 +281,84 @@ bool SolutionApplicationTarget::contextualTypeIsOnlyAHint() const {
   llvm_unreachable("invalid contextual type");
 }
 
-SolutionApplicationTarget SolutionApplicationTarget::walk(ASTWalker &walker) {
+Optional<SolutionApplicationTarget>
+SolutionApplicationTarget::walk(ASTWalker &walker) const {
+  SolutionApplicationTarget result = *this;
   switch (kind) {
   case Kind::expression: {
-    SolutionApplicationTarget result = *this;
-    result.setExpr(getAsExpr()->walk(walker));
-    return result;
+    if (isForInitialization()) {
+      if (auto *newPattern = getInitializationPattern()->walk(walker)) {
+        result.setPattern(newPattern);
+      } else {
+        return None;
+      }
+    }
+    if (auto *newExpr = getAsExpr()->walk(walker)) {
+      result.setExpr(newExpr);
+    } else {
+      return None;
+    }
+    break;
   }
-
-  case Kind::closure:
-    return *this;
-
-  case Kind::function:
-    return SolutionApplicationTarget(
-        *getAsFunction(),
-        cast_or_null<BraceStmt>(getFunctionBody()->walk(walker)));
-
-  case Kind::stmtCondition:
-    for (auto &condElement : stmtCondition.stmtCondition) {
-      condElement = *condElement.walk(walker);
+  case Kind::closure: {
+    if (auto *newClosure = closure.closure->walk(walker)) {
+      result.closure.closure = cast<ClosureExpr>(newClosure);
+    } else {
+      return None;
+    }
+    break;
+  }
+  case Kind::function: {
+    if (auto *newBody = getFunctionBody()->walk(walker)) {
+      result.function.body = cast<BraceStmt>(newBody);
+    } else {
+      return None;
     }
-    return *this;
+    break;
+  }
+  case Kind::stmtCondition: {
+    for (auto &condElement : stmtCondition.stmtCondition)
+      condElement = *condElement.walk(walker);
 
-  case Kind::caseLabelItem:
-    if (auto newPattern =
-            caseLabelItem.caseLabelItem->getPattern()->walk(walker)) {
-      caseLabelItem.caseLabelItem->setPattern(
-          newPattern, caseLabelItem.caseLabelItem->isPatternResolved());
+    break;
+  }
+  case Kind::caseLabelItem: {
+    auto *item = caseLabelItem.caseLabelItem;
+    if (auto *newPattern = item->getPattern()->walk(walker)) {
+      item->setPattern(newPattern, item->isPatternResolved());
+    } else {
+      return None;
     }
-    if (auto guardExpr = caseLabelItem.caseLabelItem->getGuardExpr()) {
-      if (auto newGuardExpr = guardExpr->walk(walker))
-        caseLabelItem.caseLabelItem->setGuardExpr(newGuardExpr);
+    if (auto guardExpr = item->getGuardExpr()) {
+      if (auto newGuardExpr = guardExpr->walk(walker)) {
+        item->setGuardExpr(newGuardExpr);
+      } else {
+        return None;
+      }
     }
-
-    return *this;
-
-  case Kind::patternBinding:
-    return *this;
-
-  case Kind::uninitializedVar:
-    return *this;
-
-  case Kind::forEachStmt:
-    return *this;
+    break;
   }
-
-  llvm_unreachable("invalid target kind");
+  case Kind::patternBinding: {
+    if (getAsPatternBinding()->walk(walker))
+      return None;
+    break;
+  }
+  case Kind::uninitializedVar: {
+    if (auto *P = getAsUninitializedVar()->walk(walker)) {
+      result.setPattern(P);
+    } else {
+      return None;
+    }
+    break;
+  }
+  case Kind::forEachStmt: {
+    if (auto *newStmt = getAsForEachStmt()->walk(walker)) {
+      result.forEachStmt.stmt = cast<ForEachStmt>(newStmt);
+    } else {
+      return None;
+    }
+    break;
+  }
+  }
+  return result;
 }