[CodeCompletion] Ignore implicit decl when finding equivalent decl

TypeChecker sometimes (e.g. property wrappers) inserts implicit decls
(e.g. 'PatternBindingDecl's and 'VarDecl's) between decls in the AST.
This used to confuse 'getEquivalentDeclContextFromSourceFile()'. It
canceled fast-completion, caused crashes, or completed in a wrong
context.

Ignore implicit decls in the AST so that we can find the correct decl
context.

rdar://problem/58665268

Author: rintaro

lib/IDE/CompletionInstance.cpp

@@ -56,6 +56,8 @@ template <typename Range>
 unsigned findIndexInRange(Decl *D, const Range &Decls) {
   unsigned N = 0;
   for (auto I = Decls.begin(), E = Decls.end(); I != E; ++I) {
+    if ((*I)->isImplicit())
+      continue;
     if (*I == D)
       return N;
     ++N;
@@ -66,6 +68,8 @@ unsigned findIndexInRange(Decl *D, const Range &Decls) {
 /// Return the element at \p N in \p Decls .
 template <typename Range> Decl *getElementAt(const Range &Decls, unsigned N) {
   for (auto I = Decls.begin(), E = Decls.end(); I != E; ++I) {
+    if ((*I)->isImplicit())
+      continue;
     if (N == 0)
       return *I;
     --N;
@@ -140,6 +144,8 @@ static DeclContext *getEquivalentDeclContextFromSourceFile(DeclContext *DC,
       llvm_unreachable("invalid DC kind for finding equivalent DC (query)");
 
     if (auto storage = dyn_cast<AbstractStorageDecl>(D)) {
+      if (IndexStack.empty())
+        return nullptr;
       auto accessorN = IndexStack.pop_back_val();
       D = getElementAt(storage->getAllAccessors(), accessorN);
     }

test/SourceKit/CodeComplete/complete_sequence_property_wrapper.swift

@@ -0,0 +1,36 @@
+@propertyWrapper
+struct TwelveOrLess {
+  private var number = 0
+  var wrappedValue: Int {
+    get { return number }
+    set { number = min(newValue, 12) }
+  }
+}
+
+struct MyStruct {
+  @TwelveOrLess var value = 12
+
+  func foo() {}
+
+  func bar(barParam: Int) {
+
+  }
+}
+
+// RUN: %sourcekitd-test \
+// RUN:   -req=track-compiles == \
+// RUN:   -req=complete -req-opts=reuseastcontext=1 -pos=16:1 -repeat-request=2 %s -- %s > %t.response
+// RUN: %FileCheck --check-prefix=RESULT  %s < %t.response
+// RUN: %FileCheck --check-prefix=TRACE  %s < %t.response
+
+// RESULT-LABEL: key.results: [
+// RESULT-DAG: key.description: "barParam"
+// RESULT: ]
+// RESULT-LABEL: key.results: [
+// RESULT-DAG: key.description: "barParam"
+// RESULT: ]
+
+// TRACE-LABEL: key.notification: source.notification.compile-did-finish,
+// TRACE-NOT: key.description: "completion reusing previous ASTContext (benign diagnostic)"
+// TRACE-LABEL: key.notification: source.notification.compile-did-finish,
+// TRACE: key.description: "completion reusing previous ASTContext (benign diagnostic)"