Index: lib/Sema/SemaTemplateInstantiate.cpp
===================================================================
--- lib/Sema/SemaTemplateInstantiate.cpp
+++ lib/Sema/SemaTemplateInstantiate.cpp
@@ -2892,7 +2892,7 @@
       unsigned i = PV->getFunctionScopeIndex();
       // This parameter might be from a freestanding function type within the
       // function and isn't necessarily referring to one of FD's parameters.
-      if (FD->getParamDecl(i) == PV)
+      if (i < FD->getNumParams() && FD->getParamDecl(i) == PV)
         return FD->getCanonicalDecl()->getParamDecl(i);
     }
   }
Index: test/SemaCXX/PR41139.cpp
===================================================================
--- test/SemaCXX/PR41139.cpp
+++ test/SemaCXX/PR41139.cpp
@@ -0,0 +1,15 @@
+// RUN: %clang_cc1 -fsyntax-only -std=c++17 -verify %s
+
+// expected-no-diagnostics
+
+// This test should not crash.
+int f1( unsigned ) { return 0; }
+
+template <class R, class... Args>
+struct S1 {
+    S1( R(*f)(Args...) ) {}
+};
+
+int main() {
+    S1 s1( f1 );
+}
Index: test/SemaCXX/cxx1y-generic-lambdas.cpp
===================================================================
--- test/SemaCXX/cxx1y-generic-lambdas.cpp
+++ test/SemaCXX/cxx1y-generic-lambdas.cpp
@@ -944,6 +944,15 @@
   }(0)(0);
 }
 
+namespace PR41139 {
+  int y = [](auto outer) {
+    return [](auto inner) {
+      using T = int(decltype(outer), decltype(inner));
+      return 0;
+    };
+  }(0)(0);
+}
+
 namespace PR23716 {
 template<typename T>
 auto f(T x) {