diff --git a/clang/lib/Serialization/ASTReaderDecl.cpp b/clang/lib/Serialization/ASTReaderDecl.cpp
--- a/clang/lib/Serialization/ASTReaderDecl.cpp
+++ b/clang/lib/Serialization/ASTReaderDecl.cpp
@@ -868,7 +868,10 @@
   FD->setInlineSpecified(Record.readInt());
   FD->setImplicitlyInline(Record.readInt());
   FD->setVirtualAsWritten(Record.readInt());
-  FD->setPure(Record.readInt());
+  // We defer calling `FunctionDecl::setPure()` here as for methods of
+  // `CXXTemplateSpecializationDecl`s, we may not have connected up the
+  // definition (which is required for `setPure`).
+  const bool Pure = Record.readInt();
   FD->setHasInheritedPrototype(Record.readInt());
   FD->setHasWrittenPrototype(Record.readInt());
   FD->setDeletedAsWritten(Record.readInt());
@@ -1015,6 +1018,10 @@
   }
   }
 
+  // Defer calling `setPure` until merging above has guaranteed we've set
+  // `DefinitionData` (as this will need to access it).
+  FD->setPure(Pure);
+
   // Read in the parameters.
   unsigned NumParams = Record.readInt();
   SmallVector<ParmVarDecl *, 16> Params;
diff --git a/clang/test/Modules/Inputs/set-pure-crash/a.h b/clang/test/Modules/Inputs/set-pure-crash/a.h
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/Inputs/set-pure-crash/a.h
@@ -0,0 +1,11 @@
+#pragma once
+
+struct Tag {};
+
+template <typename T>
+class Base {
+public:
+  virtual void func() = 0;
+};
+
+Base<Tag> bar();
diff --git a/clang/test/Modules/Inputs/set-pure-crash/b.h b/clang/test/Modules/Inputs/set-pure-crash/b.h
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/Inputs/set-pure-crash/b.h
@@ -0,0 +1,14 @@
+#pragma once
+
+#include "a.h"
+#include "c.h"
+
+template <typename Fun, typename = simple<Fun>>
+void foo(Fun) {}
+
+class Child : public Base<Tag> {
+public:
+  void func() {
+    foo([]() {});
+  }
+};
diff --git a/clang/test/Modules/Inputs/set-pure-crash/c.h b/clang/test/Modules/Inputs/set-pure-crash/c.h
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/Inputs/set-pure-crash/c.h
@@ -0,0 +1,5 @@
+#pragma once
+
+template <typename>
+struct simple {
+};
diff --git a/clang/test/Modules/Inputs/set-pure-crash/module.modulemap b/clang/test/Modules/Inputs/set-pure-crash/module.modulemap
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/Inputs/set-pure-crash/module.modulemap
@@ -0,0 +1,11 @@
+module a {
+  header "a.h"
+}
+
+module b {
+  header "b.h"
+}
+
+module c {
+  header "c.h"
+}
diff --git a/clang/test/Modules/set-pure-crash.cpp b/clang/test/Modules/set-pure-crash.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/set-pure-crash.cpp
@@ -0,0 +1,9 @@
+// RUN: rm -rf %t
+// RUN: %clang_cc1 -fsyntax-only -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -x c++ -I %S/Inputs/set-pure-crash -verify %s -o %t
+
+// expected-no-diagnostics
+
+#include "b.h"
+#include "c.h"
+
+auto t = simple<const char *>();