[ast] Fix a series of potential nullptr violations in CaptureInfo.

Specifically, CaptureInfo previously as a pattern in CaptureInfo.cpp would
attempt to ascertain if a CapturedValue was a local capture by checking the
CapturedValue's decl. Unfortunately, when we have captured dynamic self
metadata, we do not even have a getDecl() and should return false.

I fixed this by adding a new API to CaptureValue that checks if it has a decl
before checking if the decl is a local capture. This avoids this problem.

Author: gottesmm

include/swift/AST/CaptureInfo.h

@@ -100,6 +100,12 @@ class CapturedValue {
     return Value.getPointer().is<OpaqueValueExpr *>();
   }
 
+  /// Returns true if this captured value has a local capture.
+  ///
+  /// NOTE: This implies that the value is not dynamic self metadata, since
+  /// values with decls are the only values that are able to be local captures.
+  bool isLocalCapture() const;
+
   CapturedValue mergeFlags(CapturedValue cv) {
     assert(Value.getPointer() == cv.Value.getPointer() &&
            "merging flags on two different value decls");

lib/AST/CaptureInfo.cpp

@@ -17,6 +17,10 @@
 
 using namespace swift;
 
+//===----------------------------------------------------------------------===//
+//                             MARK: CaptureInfo
+//===----------------------------------------------------------------------===//
+
 CaptureInfo::CaptureInfo(ASTContext &ctx, ArrayRef<CapturedValue> captures,
                          DynamicSelfType *dynamicSelf,
                          OpaqueValueExpr *opaqueValue,
@@ -56,9 +60,10 @@ CaptureInfo CaptureInfo::empty() {
 }
 
 bool CaptureInfo::hasLocalCaptures() const {
-  for (auto capture : getCaptures())
-    if (capture.getDecl()->isLocalCapture())
+  for (auto capture : getCaptures()) {
+    if (capture.isLocalCapture())
       return true;
+  }
   return false;
 }
 
@@ -71,7 +76,7 @@ getLocalCaptures(SmallVectorImpl<CapturedValue> &Result) const {
 
   // Filter out global variables.
   for (auto capture : getCaptures()) {
-    if (!capture.getDecl()->isLocalCapture())
+    if (!capture.isLocalCapture())
       continue;
 
     Result.push_back(capture);
@@ -83,10 +88,12 @@ VarDecl *CaptureInfo::getIsolatedParamCapture() const {
     return nullptr;
 
   for (const auto &capture : getCaptures()) {
-    if (!capture.getDecl()->isLocalCapture())
+    // Check for dynamic self metadata before checking if we have a local
+    // capture since dynamic self metadata doesn't have a decl.
+    if (capture.isDynamicSelfMetadata())
       continue;
 
-    if (capture.isDynamicSelfMetadata())
+    if (!capture.isLocalCapture())
       continue;
 
     // If we captured an isolated parameter, return it.
@@ -134,3 +141,11 @@ void CaptureInfo::print(raw_ostream &OS) const {
   OS << ')';
 }
 
+//===----------------------------------------------------------------------===//
+//                            MARK: CapturedValue
+//===----------------------------------------------------------------------===//
+
+bool CapturedValue::isLocalCapture() const {
+  auto *decl = Value.getPointer().dyn_cast<ValueDecl *>();
+  return decl && decl->isLocalCapture();
+}

test/SILGen/capture_info_invariants.swift

@@ -0,0 +1,21 @@
+// RUN: %target-swift-frontend -o - -emit-silgen %s
+
+// REQUIRES: concurrency
+
+// READ THIS! This test is meant to test invariants around CaptureInfo usage in
+// SILGen. Only add examples to this if we hit a crasher in capture info and the
+// example makes sure we do not crash again.
+
+class GetIsolatedParamTest {
+  public static var rootType: Any.Type? { nil }
+}
+
+extension GetIsolatedParamTest : CustomDebugStringConvertible {
+    public var debugDescription: String {
+      let description = "\\\(String(describing: Self.rootType!))"
+      let x: Any.Type? = nil
+      // The error is triggered by the ?? autoclosure.
+      var valueType: Any.Type? = x ?? Self.rootType
+      return description
+    }
+}