[AST][RecoveryAST] Preseve invalid return stmt, and suppress the diagnostics for missing return stmt in constexpr func.

[sammccall]

This is a nice coverage improvement but diagnostic changes don't seem to particularly be improvements... not opposed to landing as-is but want to consider if we can mitigate these...

clang/test/SemaCXX/constant-expression-cxx11.cpp

1812

an idea (not sure if feasible) - do we want to suppress "not valid in constant expression" errors when the expressions contains errors?
If so, may want a (different) FIXME here

clang/test/SemaCXX/lambda-expressions.cpp

522 ↗

(On Diff #272340)

this seems like a regression - what exactly is going on here?

in a perfect world I guess we'd e.g. leave the type undeduced from the first return, rather than deduce it as undeduced-auto (at least for the purposes of diagnostics).

clang/test/SemaCXX/typo-correction-crash.cpp

19–20

this is also a regression, though this code is pretty broken and is really just asserting no crash.

[sammccall]

Any interest in chasing these diagnostic regressions (they seem like they might be tractable) or should we abandon this?

[hokein]

rebase and update

[hokein]

sorry for the long delay, picking it up.

clang/test/SemaCXX/constant-expression-cxx11.cpp

1812

the extra diagnostics don't exit any more in the trunk (I think it is because we support the error-dependent expression in const evaluator).

clang/test/SemaCXX/lambda-expressions.cpp

522 ↗

(On Diff #272340)

this is an exiting issue in clang, and our tests don't cover that. https://reviews.llvm.org/D92211 will fix that.

clang/test/SemaCXX/typo-correction-crash.cpp

19–20

this seems non-trivial to fix (we have a too strong invalidation for decltype on an error expression, the declarator is marked invalid), I'd defer the fix (since this is not a critical case, and the code is pretty broken).

[sammccall]

LG once D92211 lands.

clang/lib/Sema/SemaStmt.cpp

bool Sema::DeduceFunctionTypeFromReturnExpr(FunctionDecl *FD,
   return false;
 }
 
 StmtResult
 Sema::ActOnReturnStmt(SourceLocation ReturnLoc, Expr *RetValExp,
                       Scope *CurScope) {
   // Correct typos, in case the containing function returns 'auto' and
   // RetValExp should determine the deduced type.
-  ExprResult RetVal = CorrectDelayedTyposInExpr(RetValExp);
+  ExprResult RetVal = CorrectDelayedTyposInExpr(
+      RetValExp, nullptr, /*RecoverUncorrectedTypos=*/true);
   if (RetVal.isInvalid())
     return StmtError();
   StmtResult R = BuildReturnStmt(ReturnLoc, RetVal.get());
   if (R.isInvalid() || ExprEvalContexts.back().Context ==
                            ExpressionEvaluationContext::DiscardedStatement)
     return R;
 
   if (VarDecl *VD =

clang/test/SemaCXX/constant-expression-cxx11.cpp

namespace PR15884 {
   constexpr S *p = &f();
   // expected-error@-1 {{taking the address of a temporary}}
   // expected-error@-2 {{constexpr variable 'p' must be initialized by a constant expression}}
   // expected-note@-3 {{pointer to temporary is not a constant expression}}
   // expected-note@-4 {{temporary created here}}
 }
 
 namespace AfterError {
-  // FIXME: Suppress the 'no return statements' diagnostic if the body is invalid.
-  constexpr int error() { // expected-error {{no return statement}}
+  constexpr int error() {
     return foobar; // expected-error {{undeclared identifier}}
   }
-  constexpr int k = error();
+  constexpr int k = error(); // expected-error {{constexpr variable 'k' must be initialized by a constant expression}}
 }
 
 namespace std {
   typedef decltype(sizeof(int)) size_t;
 
   template <class _E>
   class initializer_list
   {

clang/test/SemaCXX/typo-correction-crash.cpp

return tes; // expected-error {{use of undeclared identifier 'tes'; did you mean 'test'?}}
               // expected-error@-1 {{deduced as 'int' here but deduced as 'const char *' in earlier}}
 }
 
 template <class A, class B> struct is_same { static constexpr bool value = false; };
 template <class A> struct is_same<A,A> { static constexpr bool value = true; };
 
 auto L1 = [] { return s; }; // expected-error {{use of undeclared identifier 's'}}
 using T1 = decltype(L1());
-static_assert(is_same<T1, void>::value, "Return statement should be discarded");
+// FIXME: Suppress the 'undeclared identifier T1' diagnostic, the UsingDecl T1 is discarded because of an invalid L1().
+static_assert(is_same<T1, void>::value, "Return statement should be discarded"); // expected-error {{use of undeclared identifier 'T1'}}
 auto L2 = [] { return tes; }; // expected-error {{use of undeclared identifier 'tes'; did you mean 'test'?}}
 using T2 = decltype(L2());
 static_assert(is_same<T2, int>::value, "Return statement was corrected");
 
 namespace BarNamespace {
 namespace NestedNamespace { // expected-note {{'BarNamespace::NestedNamespace' declared here}}
 typedef int type;
 }