diff --git a/clang/lib/Sema/SemaTemplateInstantiate.cpp b/clang/lib/Sema/SemaTemplateInstantiate.cpp
--- a/clang/lib/Sema/SemaTemplateInstantiate.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiate.cpp
@@ -1348,6 +1348,13 @@
         Expr *UninstExpr = PVD->getUninstantiatedDefaultArg();
         // FIXME: Obtain the source location for the '=' token.
         SourceLocation EqualLoc = UninstExpr->getBeginLoc();
+
+        // Push an empty lambda scope. This is needed as SubstDefaultArgument
+        // pushes the context for the lambda's function call operator and we
+        // need to keep the contexts and the function scopes in sync.
+        getSema().PushLambdaScope();
+        Sema::FunctionScopeRAII FuncScopeCleanup(getSema());
+
         if (SemaRef.SubstDefaultArgument(EqualLoc, PVD, TemplateArgs)) {
           // If substitution fails, the default argument is set to a
           // RecoveryExpr that wraps the uninstantiated default argument so
diff --git a/clang/test/SemaCXX/lambda-default-arg.cpp b/clang/test/SemaCXX/lambda-default-arg.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/SemaCXX/lambda-default-arg.cpp
@@ -0,0 +1,19 @@
+// RUN: %clang_cc1 -std=c++17 -verify -fsyntax-only %s
+
+template <class T> bool test0() {
+  constexpr float a = 1e-5f;
+  return [=](float b = a) -> bool {
+    return a < 0;
+  }();
+}
+
+bool b0 = test0<int>();
+
+template <class T> bool test1() {
+  float a = 1e-5f;
+  return [=](float b = a) -> bool { // expected-error {{default argument references local variable 'a'}}
+    return a < 0;
+  }();
+}
+
+bool b1 = test1<int>();