[Sema][Crash] Correctly handle an non-dependent noexcept expr in function template

[erichkeane]

Summary

It seems that all of the other templated cases are handled correctly,
however the function template case was not correctly handled. This
patch recovers from this condition by setting the function to noexcept
after diagnosing. Previously it simply set NoexceptExpr to null,
which caused an Assert when this was evaluated during substitution.

[erichkeane]

Bump! I realize the two of you are super busy, but if anyone else on cfe-commits wants to take a look, I'd apprecate it!
-Erich

[rnk]

Looks good, thanks!

lib/Sema/SemaDeclCXX.cpp

if (NoexceptExpr) {
               Context.BoolTy) &&
              "Parser should have made sure that the expression is boolean");
       if (IsTopLevel && NoexceptExpr &&
           DiagnoseUnexpandedParameterPack(NoexceptExpr)) {
         ESI.Type = EST_BasicNoexcept;
         return;
       }
 
-      if (!NoexceptExpr->isValueDependent())
-        NoexceptExpr = VerifyIntegerConstantExpression(NoexceptExpr, nullptr,
-                         diag::err_noexcept_needs_constant_expression,
-                         /*AllowFold*/ false).get();
+      if (!NoexceptExpr->isValueDependent()) {
+        ExprResult Result = VerifyIntegerConstantExpression(
+            NoexceptExpr, nullptr, diag::err_noexcept_needs_constant_expression,
+            /*AllowFold*/ false);
+        if (Result.isInvalid()) {
+          ESI.Type = EST_BasicNoexcept;
+          return;
+        }
+        NoexceptExpr = Result.get();
+      }
       ESI.NoexceptExpr = NoexceptExpr;
     }
     return;
   }
 }
 
 void Sema::actOnDelayedExceptionSpecification(Decl *MethodD,
              ExceptionSpecificationType EST,

test/CXX/except/except.spec/p1.cpp

template<int X> void f() {
     int (*p)() noexcept(1/X); // expected-error{{argument to noexcept specifier must be a constant expression}} expected-note{{division by zero}}
   };
 
   void g() {
     A<0>::f(); // expected-note{{in instantiation of exception specification for 'f'}}
     f<0>(); // expected-note{{in instantiation of function template specialization}}
   }
 }
+
+namespace FuncTmplNoexceptError {
+  int a = 0;
+  // expected-error@+1{{argument to noexcept specifier must be a constant expression}}
+  template <class T> T f() noexcept(a++){ return {};}
+  void g(){
+    f<int>();
+  }
+};