diff --git a/clang/include/clang/Basic/DiagnosticParseKinds.td b/clang/include/clang/Basic/DiagnosticParseKinds.td
--- a/clang/include/clang/Basic/DiagnosticParseKinds.td
+++ b/clang/include/clang/Basic/DiagnosticParseKinds.td
@@ -934,6 +934,8 @@
   "the name of the capture">;
 def err_lambda_capture_multiple_ellipses : Error<
   "multiple ellipses in pack capture">;
+def err_capture_default_first : Error<
+  "capture default must be first">;
 // C++17 lambda expressions
 def err_expected_star_this_capture : Error<
   "expected 'this' following '*' in lambda capture list">;
diff --git a/clang/lib/Parse/ParseExprCXX.cpp b/clang/lib/Parse/ParseExprCXX.cpp
--- a/clang/lib/Parse/ParseExprCXX.cpp
+++ b/clang/lib/Parse/ParseExprCXX.cpp
@@ -926,6 +926,16 @@
     } else if (Tok.is(tok::kw_this)) {
       Kind = LCK_This;
       Loc = ConsumeToken();
+    } else if (Tok.isOneOf(tok::amp, tok::equal) &&
+               NextToken().isOneOf(tok::comma, tok::r_square) &&
+               Intro.Default == LCD_None) {
+      // We have a lone "&" or "=" which is either a misplaced capture-default
+      // or the start of a capture (in the "&" case) with the rest of the
+      // capture missing. Both are an error but a misplaced capture-default
+      // is more likely if we don't already have a capture default.
+      return Invalid([&] {
+        Diag(Tok.getLocation(), diag::err_capture_default_first);
+      });
     } else {
       TryConsumeToken(tok::ellipsis, EllipsisLocs[0]);
 
diff --git a/clang/test/Parser/lambda-misplaced-capture-default.cpp b/clang/test/Parser/lambda-misplaced-capture-default.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/Parser/lambda-misplaced-capture-default.cpp
@@ -0,0 +1,18 @@
+// RUN: %clang_cc1 -std=c++17 -Wno-unused-value -fsyntax-only -verify %s
+
+namespace misplaced_capture_default {
+void Test() {
+  int i = 0;
+  [&, i, &] {};   // expected-error {{expected variable name or 'this' in lambda capture list}}
+  [&, i, = ] {};  // expected-error {{expected variable name or 'this' in lambda capture list}}
+  [=, &i, &] {};  // expected-error {{expected variable name or 'this' in lambda capture list}}
+  [=, &i, = ] {}; // expected-error {{expected variable name or 'this' in lambda capture list}}
+
+  [i, &] {}; // expected-error {{capture default must be first}}
+  [i, = ] {}; // expected-error {{capture default must be first}}
+  [i, = x] {}; // expected-error {{expected variable name or 'this' in lambda capture list}}
+
+  [i, &, x = 2] {}; // expected-error {{capture default must be first}}
+  [i, =, x = 2] {}; // expected-error {{capture default must be first}}
+}
+} // namespace misplaced_capture_default