[PATCH] D20130: Fix enable_if evaluation in value-dependent contexts.

[George Burgess IV via cfe-commits]

george.burgess.iv created this revision.
george.burgess.iv added a reviewer: rsmith.
george.burgess.iv added a subscriber: cfe-commits.

This patch is meant to be applied instead of D18425, but is a new review because we're taking an entirely different approach.

This patch makes us fail `enable_if` conditions in a value dependent context, if not all values are present when we're trying to evaluate the `enable_if`. e.g.

```
void foo(int i) __attribute__((enable_if(i, "")));
template <int N> void callFoo() { foo(N); } // Error: No viable overload (emitted before any instantiations of callFoo are made), because the call to foo is resolved prior to instantiating callFoo, so we don't have a single value for N.
```

Our current behavior is that we'll happily resolve `foo` to the only existing `foo` overload, and be totally fine with `callFoo<0>();` as a result, which is incorrect. Were there two `foo` overloads with different `enable_if` attributes, we would always emit an error about an ambiguous overload set.

A complete, consistent fix for this would require that we're able to arbitrarily defer overload resolution. Additionally, we would need to handle type dependence appearing out of nowhere, as in:

```
double bar(int N) __attribute__((enable_if(N, "")));
void *bar(int N) __attribute__((enable_if(!N, "")));
template <int N>
auto callBar() { return bar(N); }
```

Which seems like a very large, bug-prone change.

Given that it seems no one has tried to use `enable_if` in a value-dependent context yet (if they were, I'd assume we would have received a bug report about `bar(N)` always being ambiguous by now), I think this is the least bad of all of our options.

http://reviews.llvm.org/D20130

Files:
  include/clang/Basic/AttrDocs.td
  lib/Sema/SemaOverload.cpp
  test/SemaCXX/enable_if.cpp

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