diff --git a/lldb/source/Plugins/ExpressionParser/Clang/ASTUtils.h b/lldb/source/Plugins/ExpressionParser/Clang/ASTUtils.h
index 769b18d54ced..b70ec223df4d 100644
--- a/lldb/source/Plugins/ExpressionParser/Clang/ASTUtils.h
+++ b/lldb/source/Plugins/ExpressionParser/Clang/ASTUtils.h
@@ -359,15 +359,12 @@ public:
   }
 
   void CompleteType(clang::TagDecl *Tag) override {
-    while (!Tag->isCompleteDefinition())
-      for (size_t i = 0; i < Sources.size(); ++i) {
-        // FIXME: We are technically supposed to loop here too until
-        // Tag->isCompleteDefinition() is true, but if our low quality source
-        // is failing to complete the tag this code will deadlock.
-        Sources[i]->CompleteType(Tag);
-        if (Tag->isCompleteDefinition())
-          break;
-      }
+    for (clang::ExternalSemaSource *S : Sources) {
+      S->CompleteType(Tag);
+      // Stop after the first source completed the type.
+      if (Tag->isCompleteDefinition())
+        break;
+    }
   }
 
   void CompleteType(clang::ObjCInterfaceDecl *Class) override {
diff --git a/lldb/test/API/commands/expression/import-std-module/forward_decl_from_module/Makefile b/lldb/test/API/commands/expression/import-std-module/forward_decl_from_module/Makefile
new file mode 100644
index 000000000000..4915cdae8764
--- /dev/null
+++ b/lldb/test/API/commands/expression/import-std-module/forward_decl_from_module/Makefile
@@ -0,0 +1,9 @@
+# We don't have any standard include directories, so we can't
+# parse the test_common.h header we usually inject as it includes
+# system headers.
+NO_TEST_COMMON_H := 1
+
+CXXFLAGS_EXTRAS = -I $(SRCDIR)/root/usr/include/c++/v1/ -I $(SRCDIR)/root/usr/include/ -nostdinc -nostdinc++
+CXX_SOURCES := main.cpp
+
+include Makefile.rules
diff --git a/lldb/test/API/commands/expression/import-std-module/forward_decl_from_module/TestForwardDeclFromStdModule.py b/lldb/test/API/commands/expression/import-std-module/forward_decl_from_module/TestForwardDeclFromStdModule.py
new file mode 100644
index 000000000000..48459abb9266
--- /dev/null
+++ b/lldb/test/API/commands/expression/import-std-module/forward_decl_from_module/TestForwardDeclFromStdModule.py
@@ -0,0 +1,39 @@
+"""
+Tests forward declarations coming from the `std` module.
+"""
+
+from lldbsuite.test.decorators import *
+from lldbsuite.test.lldbtest import *
+from lldbsuite.test import lldbutil
+import os
+
+class TestCase(TestBase):
+
+    mydir = TestBase.compute_mydir(__file__)
+
+    # We only emulate a fake libc++ in this test and don't use the real libc++,
+    # but we still add the libc++ category so that this test is only run in
+    # test configurations where libc++ is actually supposed to be tested.
+    @add_test_categories(["libc++"])
+    @skipIfRemote
+    @skipIf(compiler=no_match("clang"))
+    def test(self):
+        self.build()
+
+        sysroot = os.path.join(os.getcwd(), "root")
+
+        # Set the sysroot where our dummy libc++ exists.
+        self.runCmd("platform select --sysroot '" + sysroot + "' host", CURRENT_EXECUTABLE_SET)
+
+        lldbutil.run_to_source_breakpoint(self,
+            "// Set break point at this line.", lldb.SBFileSpec("main.cpp"))
+
+        self.runCmd("settings set target.import-std-module true")
+
+        # Print the dummy `std::vector`. It only has the dummy member in it
+        # so the standard `std::vector` formatter can't format it. Instead use
+        # the raw output so LLDB has to show the member variable.
+        # Both `std::vector` and the type of the member have forward
+        # declarations before their definitions.
+        self.expect("expr --raw -- v",
+                    substrs=['(std::__1::vector<int>) $0 = {', 'f = 0x', '}'])
diff --git a/lldb/test/API/commands/expression/import-std-module/forward_decl_from_module/main.cpp b/lldb/test/API/commands/expression/import-std-module/forward_decl_from_module/main.cpp
new file mode 100644
index 000000000000..a0b02d5c6814
--- /dev/null
+++ b/lldb/test/API/commands/expression/import-std-module/forward_decl_from_module/main.cpp
@@ -0,0 +1,8 @@
+#include <vector>
+
+int main(int argc, char **argv) {
+  // Makes sure we have the mock libc headers in the debug information.
+  libc_struct s;
+  std::vector<int> v;
+  return 0; // Set break point at this line.
+}
diff --git a/lldb/test/API/commands/expression/import-std-module/forward_decl_from_module/root/usr/include/c++/v1/module.modulemap b/lldb/test/API/commands/expression/import-std-module/forward_decl_from_module/root/usr/include/c++/v1/module.modulemap
new file mode 100644
index 000000000000..f149be7b7d21
--- /dev/null
+++ b/lldb/test/API/commands/expression/import-std-module/forward_decl_from_module/root/usr/include/c++/v1/module.modulemap
@@ -0,0 +1,3 @@
+module std {
+  module "vector" { header "vector" export * }
+}
diff --git a/lldb/test/API/commands/expression/import-std-module/forward_decl_from_module/root/usr/include/c++/v1/vector b/lldb/test/API/commands/expression/import-std-module/forward_decl_from_module/root/usr/include/c++/v1/vector
new file mode 100644
index 000000000000..c2d77aab0711
--- /dev/null
+++ b/lldb/test/API/commands/expression/import-std-module/forward_decl_from_module/root/usr/include/c++/v1/vector
@@ -0,0 +1,14 @@
+#include "libc_header.h"
+
+namespace std {
+  inline namespace __1 {
+    // A forward decl of `vector`.
+    template<typename T> class vector;
+    // Pretend to be a std::vector template we need to instantiate in LLDB
+    // when import-std-module is enabled.
+    template<typename T>
+    struct vector { class F; F *f; };
+    // The definition of our forward declared nested class.
+    template<typename T> class vector<T>::F { int x; };
+  }
+}
diff --git a/lldb/test/API/commands/expression/import-std-module/forward_decl_from_module/root/usr/include/libc_header.h b/lldb/test/API/commands/expression/import-std-module/forward_decl_from_module/root/usr/include/libc_header.h
new file mode 100644
index 000000000000..47525c9db346
--- /dev/null
+++ b/lldb/test/API/commands/expression/import-std-module/forward_decl_from_module/root/usr/include/libc_header.h
@@ -0,0 +1 @@
+struct libc_struct {};