[clang-format] Allow trailing return types in macros

[rymiel]

Summary

The trailing return type arrow checker verifies that a declaration is
being parsed, however, this isn't true when inside of macros.

It turns out the existence of the auto keyword is enough to make
sure that we're dealing with a trailing return type, and whether we're
in a declaration doesn't matter.

Fixes https://github.com/llvm/llvm-project/issues/47664

[HazardyKnusperkeks]

What about decltype(auto)?

[rymiel]

In D141811#4055485, @HazardyKnusperkeks wrote:

What about decltype(auto)?

Turns out this is a problem even without this patch:

decltype(auto) a = (b) -> c;

I'm sure this could somehow be valid c++ syntax, but it does have to be at the top scope. I suppose I could fix it in this patch.

[rymiel]

I suppose this means auto a = (b) -> c; is also technically broken, actually, and there's no easy fix for that.

I did have an idea for adding an additional pass taking place somewhere in TokenAnnotator::calculateFormattingInformation, which would detect trailing return types using a bit more context, as a potential fix for https://github.com/llvm/llvm-project/issues/41495. I suppose I could make that theoretically replace all of this AutoFound logic. I should actually try to see if my idea would work at all though.

edit: nevermind, not as easy as i thought, as operator precedence logic wants to know sooner if an arrow is an operator or a trailing return arrow

[HazardyKnusperkeks]

In D141811#4064428, @rymiel wrote:

In D141811#4055485, @HazardyKnusperkeks wrote:

What about decltype(auto)?

Turns out this is a problem even without this patch:

decltype(auto) a = (b) -> c;

I'm sure this could somehow be valid c++ syntax, but it does have to be at the top scope. I suppose I could fix it in this patch.

I thought about auto NAME() -> int { return 42; }.

decltype(auto) a = (b) -> c; is something else...

[rymiel]

In D141811#4068180, @HazardyKnusperkeks wrote:

I thought about auto NAME() -> int { return 42; }.

decltype(auto) a = (b) -> c; is something else...

I'm not sure how that affects this? Using decltype(auto) as in decltype(auto) NAME() -> int { return 42; } will recognize the arrow as a trailing return type, but it doesn't really matter as its invalid syntax. I tried to find valid syntax where the arrow would be seen as a trailing return type, and I did with that second example

[HazardyKnusperkeks]

In D141811#4070629, @rymiel wrote:

In D141811#4068180, @HazardyKnusperkeks wrote:

I thought about auto NAME() -> int { return 42; }.

decltype(auto) a = (b) -> c; is something else...

I'm not sure how that affects this? Using decltype(auto) as in decltype(auto) NAME() -> int { return 42; } will recognize the arrow as a trailing return type, but it doesn't really matter as its invalid syntax. I tried to find valid syntax where the arrow would be seen as a trailing return type, and I did with that second example

I'd just add a test to confirm that it works as expected.

[rymiel]

Okay, I planned changes because I had more ambitious plans for fixing this, but those didn't work out, so instead I made a separate issue (https://github.com/llvm/llvm-project/issues/61469).
This patch in its current state is a simple fix, so it's probably fine as is

[owenpan]

clang/unittests/Format/FormatTest.cpp

8013–8014

I'd reduce ColumnLimit to 60 for example to get rid of the string concatenation.

[rymiel]

Reduce column limit for macro test

clang/lib/Format/TokenAnnotator.cpp

if (Current.is(Keywords.kw_instanceof)) {
       // with multiple increment statements is not confused with a for loop
       // having multiple variable declarations.
       Contexts.back().FirstStartOfName = nullptr;
     } else if (Current.isOneOf(tok::kw_auto, tok::kw___auto_type)) {
       AutoFound = true;
     } else if (Current.is(tok::arrow) &&
                Style.Language == FormatStyle::LK_Java) {
       Current.setType(TT_LambdaArrow);
-    } else if (Current.is(tok::arrow) && AutoFound && Line.MustBeDeclaration &&
+    } else if (Current.is(tok::arrow) && AutoFound &&
+               (Line.MustBeDeclaration || Line.InPPDirective) &&
                Current.NestingLevel == 0 &&
                !Current.Previous->isOneOf(tok::kw_operator, tok::identifier)) {
       // not auto operator->() -> xxx;
       Current.setType(TT_TrailingReturnArrow);
     } else if (Current.is(tok::arrow) && Current.Previous &&
                Current.Previous->is(tok::r_brace)) {
       // Concept implicit conversion constraint needs to be treated like
       // a trailing return type  ... } -> <type>.

clang/unittests/Format/FormatTest.cpp

verifyFormat("template <size_t Order, typename T>\n"
                "    -> alias::tensor<Order, T, mem::tag::cpu> {}");
   verifyFormat("auto SomeFunction(A aaaaaaaaaaaaaaaaaaaaa) const\n"
                "    -> decltype(f(aaaaaaaaaaaaaaaaaaaaa)) {}");
   verifyFormat("auto doSomething(Aaaaaa *aaaaaa) -> decltype(aaaaaa->f()) {}");
   verifyFormat("template <typename T>\n"
                "auto aaaaaaaaaaaaaaaaaaaaaa(T t)\n"
                "    -> decltype(eaaaaaaaaaaaaaaa<T>(t.a).aaaaaaaa());");
 
+  FormatStyle Style = getLLVMStyleWithColumns(60);
+  verifyFormat("#define MAKE_DEF(NAME)                                     \\\n"
+               "  auto NAME() -> int { return 42; }",
+               Style);
+
   // Not trailing return types.
   verifyFormat("void f() { auto a = b->c(); }");
   verifyFormat("auto a = p->foo();");
   verifyFormat("int a = p->foo();");
   verifyFormat("auto lmbd = [] NOEXCEPT -> int { return 0; };");
 }
 
 TEST_F(FormatTest, DeductionGuides) {