[clang] Fix a nullptr dereference bug on invalid code

When working with invalid code, we would try to dereference a nullptr
while deducing template arguments in some dependend code operating on a
lambda with invalid return type.

Differential Revision: https://reviews.llvm.org/D95145

Author: gislan

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

@@ -4189,6 +4189,9 @@ TemplateDeclInstantiator::SubstFunctionType(FunctionDecl *D,
       for (unsigned OldIdx = 0, NumOldParams = OldProtoLoc.getNumParams();
            OldIdx != NumOldParams; ++OldIdx) {
         ParmVarDecl *OldParam = OldProtoLoc.getParam(OldIdx);
+        if (!OldParam)
+          return nullptr;
+
         LocalInstantiationScope *Scope = SemaRef.CurrentInstantiationScope;
 
         Optional<unsigned> NumArgumentsInExpansion;

clang/test/SemaCXX/subst-func-type-invalid-ret-type.cpp

@@ -0,0 +1,16 @@
+// RUN: %clang -fsyntax-only -std=c++17 %s -Xclang -verify
+
+// The important part is that we do not crash.
+
+template<typename T> T declval();
+
+template <typename T>
+auto Call(T x) -> decltype(declval<T>()(0)) {} // expected-note{{candidate template ignored}}
+
+class Status {};
+
+void fun() {
+  // The Status() (instead of Status) here used to cause a crash.
+  Call([](auto x) -> Status() {}); // expected-error{{function cannot return function type 'Status ()}}
+  // expected-error@-1{{no matching function for call to 'Call'}}
+}