diff --git a/clang/lib/Sema/SemaTemplateDeduction.cpp b/clang/lib/Sema/SemaTemplateDeduction.cpp
--- a/clang/lib/Sema/SemaTemplateDeduction.cpp
+++ b/clang/lib/Sema/SemaTemplateDeduction.cpp
@@ -4346,7 +4346,7 @@
     HasDeducedReturnType = true;
   }
 
-  if (!ArgFunctionType.isNull()) {
+  if (!ArgFunctionType.isNull() && !FunctionType.isNull()) {
     unsigned TDF =
         TDF_TopLevelParameterTypeList | TDF_AllowCompatibleFunctionType;
     // Deduce template arguments from the function type.
diff --git a/clang/test/SemaCXX/pr51171-crash.cpp b/clang/test/SemaCXX/pr51171-crash.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/SemaCXX/pr51171-crash.cpp
@@ -0,0 +1,36 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++17 %s
+
+// Ensure that we don't crash if errors are suppressed by an error limit.
+// RUN: not %clang_cc1 -fsyntax-only -std=c++17 -ferror-limit 1 %s
+
+template <bool is_const, typename tag_t = void>
+struct tv_val {
+};
+
+template <bool is_const>
+auto &val(const tv_val<is_const> &val) { return val.val(); } // expected-note {{possible target for call}}
+
+struct Class {
+  template <bool is_const>
+  struct Entry {
+    tv_val<is_const> val;
+  };
+};
+
+enum Types : int {
+  Class = 1, // expected-note {{struct 'Class' is hidden}} \
+               // expected-note {{struct 'Class' is hidden}}
+};
+
+struct Record {
+  Class *val_;            // expected-error {{must use 'struct' tag}}
+  void setClass(Class *); // expected-error {{must use 'struct' tag}}
+};
+
+void Record::setClass(Class *val) { // expected-error {{variable has incomplete type 'void'}} \
+                                   // expected-error {{reference to overloaded function}} \
+                                   // expected-error {{expected ';' after top level declarator}}
+  val_ = val;
+}
+
+/// expected-fatal {{too many errors emitted, stopping now [-ferror-limit=]}}