[CodeCompletion] Workaround fast-completion issue in UnqualifiedLookup

In fast-completion, a function body can be replaced with another function
body parsed from a new buffer. In such cases, during typechecking the
expressions in the *new* function body, a source location range check in
UnqualifiedLookup didn't work well because they are not from the same
buffer.

This patch workaround it by skipping the source range checks and returns
'success' in such cases.

rdar://problem/58881999

Author: rintaro

include/swift/Basic/SourceManager.h

@@ -76,6 +76,10 @@ class SourceManager {
     CodeCompletionOffset = Offset;
   }
 
+  bool hasCodeCompletionBuffer() const {
+    return CodeCompletionBufferID != 0U;
+  }
+
   unsigned getCodeCompletionBufferID() const {
     return CodeCompletionBufferID;
   }

lib/AST/UnqualifiedLookup.cpp

@@ -58,19 +58,34 @@ namespace {
     // lookup.
     if (loc.isInvalid() || decl->getBraces().isInvalid())
       return true;
-    
+
+    SourceManager &SM = decl->getASTContext().SourceMgr;
+
+    // If a code completion happens inside a function body, some lookups may
+    // happen from the 'loc' that is in a different buffer from the 'decl'.
+    // In such cases, look for members of the 'decl' because we know 'loc' is
+    // inside a function body in the 'decl'.
+    if (SM.hasCodeCompletionBuffer()) {
+      auto completionBufferID = SM.getCodeCompletionBufferID();
+      if (SM.getRangeForBuffer(completionBufferID).contains(loc)) {
+        auto declBufferID =
+            decl->getDeclContext()->getParentSourceFile()->getBufferID();
+        if (completionBufferID != declBufferID)
+          return true;
+      }
+    }
+
     // Within the braces, always look for members.
-    auto &ctx = decl->getASTContext();
     auto braces = decl->getBraces();
     if (braces.Start != braces.End &&
-        ctx.SourceMgr.rangeContainsTokenLoc(braces, loc))
+        SM.rangeContainsTokenLoc(braces, loc))
       return true;
 
     // Within 'where' clause, we can also look for members.
     if (auto *whereClause = decl->getTrailingWhereClause()) {
       SourceRange whereClauseRange = whereClause->getSourceRange();
       if (whereClauseRange.isValid() &&
-          ctx.SourceMgr.rangeContainsTokenLoc(whereClauseRange, loc)) {
+          SM.rangeContainsTokenLoc(whereClauseRange, loc)) {
         return true;
       }
     }

test/SourceKit/CodeComplete/complete_sequence_nestedtype.swift

@@ -0,0 +1,50 @@
+struct Outer {
+  enum Inner {
+    case east, west
+    static func staticMethod() {}
+    func instanceMethod() {}
+
+    func test() {
+      Inner.
+    }
+  }
+
+  func test() {
+    Inner.
+  }
+}
+
+// RUN: %sourcekitd-test \
+// RUN:   -req=track-compiles == \
+// RUN:   -req=complete -req-opts=reuseastcontext=1 -pos=8:13 %s -- %s == \
+// RUN:   -req=complete -req-opts=reuseastcontext=1 -pos=8:13 %s -- %s == \
+// RUN:   -req=complete -req-opts=reuseastcontext=1 -pos=13:11 %s -- %s \
+// RUN:   > %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: "east"
+// RESULT-DAG: key.description: "west"
+// RESULT-DAG: key.description: "staticMethod()"
+// RESULT-DAG: key.description: "instanceMethod(self: Outer.Inner)"
+// RESULT: ]
+// RESULT-LABEL: key.results: [
+// RESULT-DAG: key.description: "east"
+// RESULT-DAG: key.description: "west"
+// RESULT-DAG: key.description: "staticMethod()"
+// RESULT-DAG: key.description: "instanceMethod(self: Outer.Inner)"
+// RESULT: ]
+// RESULT-LABEL: key.results: [
+// RESULT-DAG: key.description: "east"
+// RESULT-DAG: key.description: "west"
+// RESULT-DAG: key.description: "staticMethod()"
+// RESULT-DAG: key.description: "instanceMethod(self: Inner)"
+// 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)"
+// TRACE-LABEL: key.notification: source.notification.compile-did-finish,
+// TRACE: key.description: "completion reusing previous ASTContext (benign diagnostic)"