Go-to-def does not see through call to function template with non-dependent return type

[HighCommander4]

In the following code:

struct Cat {
  void meow();
};

struct Unrelated {
  void meow();
};

template <typename T> Cat foo(T);

template <typename T>
void bar(T t) {
  auto cat = foo(t);
  cat.meow();
}

go-to-def on the meow token in cat.meow offers both Cat::meow and Unrelated::meow as candidates, suggesting that it's falling back to the textual heuristic.

This is a variation of the test case fixed in #897, but unlike the variation in llvm/llvm-project#124445, I don't think this case should require Sema to handle. Also, code completion handles this case fine (completion at cat.^ offers meow), suggesting that this is somehow a deficiency in the go-to-def codepath compared to code completion.

[HighCommander4]

Findings so far:

• HeuristicResolver (which is now used by both go-to-def and code completion) fails to resolve the type of cat to Cat. The reason code completion works must be that it does some additional tricks beyond HeuristicResolver (which should probably be identified and moved into HeuristicResolver).
• The issue is actually unrelated to auto type deduction -- the type of cat's initializer, foo(t), already cannot be resolved (and so e.g. go-to-def does not work properly on meow in foo(t).meow either.

[HighCommander4]

The issue is actually unrelated to auto type deduction -- the type of cat's initializer, foo(t), already cannot be resolved

When trying to resolve the type of foo, we get into resolveDeclsToType, which handles FunctionDecl (via the ValueDecl branch), but not FunctionTemplateDecl.

(Adjusting issue title to reflect my improved understanding of the problem.)

[HighCommander4]

The reason code completion works must be that it does some additional tricks beyond HeuristicResolver (which should probably be identified and moved into HeuristicResolver).

Code completion handles calls to a function template here.

I think it would make sense to unify the entire getApproximateType() function in SemaCodeComplete.cpp with HeuristicResolver::resolveExprToType().

[HighCommander4]

Proposed fix: llvm/llvm-project#151643

[HighCommander4]

Proposed fix: llvm/llvm-project#151643

(Now at llvm/llvm-project#156282)