[CodeCompletion] Fix a crash in collectPossibleReturnTypesFromContext

Avoid re-typechecking for nested closures. For example:

  Something {
    Other {
      #^COMPLETE^#
    }
  }

If 'Something' is successfully type checked but 'Other' is failed, the
outer closure has type, but the inner closure doesn't. In such state,
when the type of 'Other' is requested, the outer closure used to be
re-typechecked. That may cause crash because it may contain expressions
CSGen doesn't expect.

rdar://problem/69246891

Author: rintaro

lib/Sema/TypeCheckStmt.cpp

@@ -1933,11 +1933,13 @@ bool TypeCheckASTNodeAtLocRequest::evaluate(Evaluator &evaluator,
   // The enclosing closure might be a single expression closure or a function
   // builder closure. In such cases, the body elements are type checked with
   // the closure itself. So we need to try type checking the enclosing closure
-  // signature first.
+  // signature first unless it has already been type checked.
   if (auto CE = dyn_cast<ClosureExpr>(DC)) {
-    swift::typeCheckASTNodeAtLoc(CE->getParent(), CE->getLoc());
-    if (CE->getBodyState() != ClosureExpr::BodyState::ReadyForTypeChecking)
-      return false;
+    if (CE->getBodyState() == ClosureExpr::BodyState::Parsed) {
+      swift::typeCheckASTNodeAtLoc(CE->getParent(), CE->getLoc());
+      if (CE->getBodyState() != ClosureExpr::BodyState::ReadyForTypeChecking)
+        return false;
+    }
   }
 
   TypeChecker::typeCheckASTNode(finder.getRef(), DC,

validation-test/IDE/crashers_2_fixed/rdar69246891.swift

@@ -0,0 +1,12 @@
+// RUN: %target-swift-ide-test -code-completion -code-completion-token=COMPLETE -source-filename=%s
+
+class MyCls {
+  public init(body: (Int) throws -> Void) {}
+}
+
+func foo() {
+  MyCls { arg in
+    MyCls { #^COMPLETE^#
+    }
+  }
+}