Index: include/clang/Lex/Preprocessor.h
===================================================================
--- include/clang/Lex/Preprocessor.h
+++ include/clang/Lex/Preprocessor.h
@@ -630,6 +630,13 @@
   /// invoked (at which point the last position is popped).
   std::vector<CachedTokensTy::size_type> BacktrackPositions;
 
+  struct CachedTokensRange {
+    CachedTokensTy::size_type Begin, End;
+  };
+  /// \brief A range of cached tokens that should be erased after lexing
+  /// when backtracking requires the erasure of such cached tokens.
+  Optional<CachedTokensRange> ErasedBacktrackRange;
+
   struct MacroInfoChain {
     MacroInfo MI;
     MacroInfoChain *Next;
Index: lib/Lex/PPCaching.cpp
===================================================================
--- lib/Lex/PPCaching.cpp
+++ lib/Lex/PPCaching.cpp
@@ -32,7 +32,20 @@
 void Preprocessor::CommitBacktrackedTokens() {
   assert(!BacktrackPositions.empty()
          && "EnableBacktrackAtThisPos was not called!");
+  auto PrevCachedLexPos = BacktrackPositions.back();
   BacktrackPositions.pop_back();
+  // When committing in a macro argument pre-expansion when tokens are still
+  // being cached, we want to ensure that the tokens which were just committed
+  // won't remain in the cache and that the caching stops.
+  // Otherwise the caching will interfere with the way macro expansion works,
+  // because we will continue to cache tokens after consuming the backtracked
+  // tokens, which shouldn't happen when we're dealing with macro argument
+  // pre-expansion.
+  if (InMacroArgPreExpansion && !BacktrackPositions.empty()) {
+    CachedTokens.erase(CachedTokens.begin() + PrevCachedLexPos,
+                       CachedTokens.begin() + CachedLexPos);
+    ExitCachingLexMode();
+  }
 }
 
 // Make Preprocessor re-lex the tokens that were lexed since
@@ -40,9 +53,20 @@
 void Preprocessor::Backtrack() {
   assert(!BacktrackPositions.empty()
          && "EnableBacktrackAtThisPos was not called!");
+  auto CurrentCachedLexPos = CachedLexPos;
   CachedLexPos = BacktrackPositions.back();
   BacktrackPositions.pop_back();
   recomputeCurLexerKind();
+  // When backtracking in a macro argument pre-expansion when tokens are still
+  // being cached, we want to ensure that the tokens over which we just
+  // backtracked won't remain in the cache after they're consumed and and that
+  // the caching will stop after consuming them.
+  // Otherwise they will interfere with the way macro expansion works, because
+  // we will continue to cache tokens after consuming the backtracked tokens,
+  // which shouldn't happen when we're dealing with macro argument
+  // pre-expansion.
+  if (InMacroArgPreExpansion && !BacktrackPositions.empty())
+    ErasedBacktrackRange = CachedTokensRange{CachedLexPos, CurrentCachedLexPos};
 }
 
 void Preprocessor::CachingLex(Token &Result) {
@@ -51,6 +75,15 @@
 
   if (CachedLexPos < CachedTokens.size()) {
     Result = CachedTokens[CachedLexPos++];
+    // Erase the cached token range after the whole range is consumed if
+    // necessary.
+    if (ErasedBacktrackRange && ErasedBacktrackRange->End == CachedLexPos) {
+      CachedTokens.erase(CachedTokens.begin() + ErasedBacktrackRange->Begin,
+                         CachedTokens.begin() + CachedLexPos);
+      CachedLexPos = ErasedBacktrackRange->Begin;
+      ErasedBacktrackRange = None;
+      ExitCachingLexMode();
+    }
     return;
   }
 
Index: test/CodeCompletion/pragma-macro-token-caching.c
===================================================================
--- /dev/null
+++ test/CodeCompletion/pragma-macro-token-caching.c
@@ -0,0 +1,18 @@
+
+#define Outer(action) action
+
+void completeParam(int param) {
+    ;
+    Outer(__extension__({ _Pragma("clang diagnostic push") }));
+    param;
+}
+
+// RUN: %clang_cc1 -fsyntax-only -code-completion-at=%s:7:1 %s | FileCheck %s
+// CHECK: param : [#int#]param
+
+void completeParamPragmaError(int param) {
+    Outer(__extension__({ _Pragma(2) })); // expected-error {{_Pragma takes a parenthesized string literal}}
+    param;
+}
+
+// RUN: %clang_cc1 -fsyntax-only -verify -code-completion-at=%s:16:1 %s | FileCheck %s