[Constraint system] Properly deal with "as" patterns.

Teach pattern matching involving "as" patterns to work properly in
function builders. The code almost handled this, but prematurely
prechecking expressions in patterns broke it.

Author: DougGregor

lib/Sema/BuilderTransform.cpp

@@ -331,7 +331,7 @@ class BuilderClosureVisitor
       addChild(captureExpr(expr, /*oneWay=*/true, node.get<Expr *>()));
     }
 
-    if (!cs)
+    if (!cs || hadError)
       return nullptr;
 
     // Call Builder.buildBlock(... args ...)
@@ -1354,6 +1354,11 @@ class PreCheckFunctionBuilderApplication : public ASTWalker {
     // Otherwise, recurse into the statement normally.
     return std::make_pair(true, S);
   }
+
+  /// Ignore patterns.
+  std::pair<bool, Pattern*> walkToPatternPre(Pattern *pat) override {
+    return { false, pat };
+  }
 };
 
 }

test/Constraints/function_builder.swift

@@ -536,3 +536,30 @@ tuplify(true) { c in
 // CHECK-SAME: Optional("matched without payload")
 // CHECK-SAME: "matched with payload", "hello!", 34
 // CHECK-SAME: "intentional mismatch"
+
+class Super { }
+
+class Sub : Super {
+  func subMethod() -> String {
+    return "subMethod"
+  }
+}
+
+func getSuper(wantSubclass: Bool) -> Super {
+  return wantSubclass ? Sub() : Super()
+}
+
+tuplify(true) { c in
+  "testIfLetAsMatching"
+  if case let sub as Sub = getSuper(wantSubclass: true) {
+    sub.subMethod()
+  }
+  if case let sub as Sub = getSuper(wantSubclass: false) {
+    fatalError("cannot match this")
+  } else {
+    "Superclass instance"
+  }
+}
+// CHECK: testIfLetAsMatching
+// CHECK-SAME: "subMethod"
+// CHECK-SAME: "Superclass instance"