Index: include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- include/clang/Basic/DiagnosticSemaKinds.td
+++ include/clang/Basic/DiagnosticSemaKinds.td
@@ -319,6 +319,11 @@
 def warn_unused_private_field: Warning<"private field %0 is not used">,
   InGroup<UnusedPrivateField>, DefaultIgnore;
 
+def err_return_value_linkage: Error<
+  "%0 has C-linkage specified, but return type %1 has internal linkage">;
+def err_parameter_value_linkage: Error<
+  "%0 has C-linkage specified, but parameter type %1 has internal linkage">;
+
 def warn_parameter_size: Warning<
   "%0 is a large (%1 bytes) pass-by-value argument; "
   "pass it by reference instead ?">, InGroup<LargeByValueCopy>;
Index: lib/Sema/SemaDecl.cpp
===================================================================
--- lib/Sema/SemaDecl.cpp
+++ lib/Sema/SemaDecl.cpp
@@ -8206,6 +8206,25 @@
       Diag(NewFD->getLocation(), diag::ext_out_of_line_declaration)
         << D.getCXXScopeSpec().getRange();
     }
+
+    if (NewFD->isExternC()) {
+      // Check return type linkage
+      QualType R = NewFD->getReturnType();
+      if (R.getTypePtr()->getLinkage() != Linkage::ExternalLinkage) {
+        Diag(NewFD->getLocation(), diag::err_return_value_linkage)
+          << NewFD << R;
+        NewFD->setInvalidDecl();
+      }
+      // Check parameter type linkage
+      for (auto param : NewFD->parameters()) {
+        QualType P = param->getOriginalType();
+        if (P.getTypePtr()->getLinkage() != Linkage::ExternalLinkage) {
+          Diag(NewFD->getLocation(), diag::err_parameter_value_linkage)
+            << NewFD << P;
+          NewFD->setInvalidDecl();
+        }
+      }
+    }
   }
 
   ProcessPragmaWeak(S, NewFD);
Index: test/Sema/pr23090-crash-on-invalid.cpp
===================================================================
--- /dev/null
+++ test/Sema/pr23090-crash-on-invalid.cpp
@@ -0,0 +1,16 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s
+// Don't crash (PR23090).
+
+namespace {
+
+// check return type
+struct A;
+extern "C" A *foo(); // expected-error {{'foo' has C-linkage specified, but return type '(anonymous namespace)::A *' has internal linkage}}
+A *foo();
+
+// check parameter
+struct B;
+extern "C" void bar(B*); // expected-error {{'bar' has C-linkage specified, but parameter type '(anonymous namespace)::B *' has internal linkage}}
+void bar(B*);
+
+}