[clang-format] Allow decltype in requires clause (#78847)

If clang-format is not sure whether a `requires` keyword starts a
requires clause or a requires expression, it looks ahead to see if any
token disqualifies it from being a requires clause. Among these tokens
was `decltype`, since it fell through the switch.

This patch allows decltype to exist in a require clause.

I'm not 100% sure this change won't have repercussions, but that just
means we need more test coverage!

Fixes #78645

Author: rymiel

clang/lib/Format/UnwrappedLineParser.cpp

@@ -3446,18 +3446,14 @@ bool clang::format::UnwrappedLineParser::parseRequires() {
         return false;
       }
       break;
-    case tok::r_paren:
-    case tok::pipepipe:
-      FormatTok = Tokens->setPosition(StoredPosition);
-      parseRequiresClause(RequiresToken);
-      return true;
     case tok::eof:
       // Break out of the loop.
       Lookahead = 50;
       break;
     case tok::coloncolon:
       LastWasColonColon = true;
       break;
+    case tok::kw_decltype:
     case tok::identifier:
       if (FoundType && !LastWasColonColon && OpenAngles == 0) {
         FormatTok = Tokens->setPosition(StoredPosition);

clang/unittests/Format/TokenAnnotatorTest.cpp

@@ -1076,6 +1076,28 @@ TEST_F(TokenAnnotatorTest, UnderstandsRequiresClausesAndConcepts) {
                     "concept C = (!Foo<T>) && Bar;");
   ASSERT_EQ(Tokens.size(), 19u) << Tokens;
   EXPECT_TOKEN(Tokens[15], tok::ampamp, TT_BinaryOperator);
+
+  Tokens = annotate("void f() & requires(C<decltype(x)>) {}");
+  ASSERT_EQ(Tokens.size(), 18u) << Tokens;
+  EXPECT_TOKEN(Tokens[4], tok::amp, TT_PointerOrReference);
+  EXPECT_TOKEN(Tokens[5], tok::kw_requires, TT_RequiresClause);
+
+  Tokens = annotate("auto f() -> int& requires(C<decltype(x)>) {}");
+  ASSERT_EQ(Tokens.size(), 20u) << Tokens;
+  EXPECT_TOKEN(Tokens[6], tok::amp, TT_PointerOrReference);
+  EXPECT_TOKEN(Tokens[7], tok::kw_requires, TT_RequiresClause);
+
+  Tokens = annotate("bool x = t && requires(decltype(t) x) { x.foo(); };");
+  ASSERT_EQ(Tokens.size(), 23u) << Tokens;
+  EXPECT_TOKEN(Tokens[5], tok::kw_requires, TT_RequiresExpression);
+
+  Tokens = annotate("bool x = t && requires(Foo<decltype(t)> x) { x.foo(); };");
+  ASSERT_EQ(Tokens.size(), 26u) << Tokens;
+  EXPECT_TOKEN(Tokens[5], tok::kw_requires, TT_RequiresExpression);
+
+  Tokens = annotate("bool x = t && requires(Foo<C1 || C2> x) { x.foo(); };");
+  ASSERT_EQ(Tokens.size(), 25u) << Tokens;
+  EXPECT_TOKEN(Tokens[5], tok::kw_requires, TT_RequiresExpression);
 }
 
 TEST_F(TokenAnnotatorTest, UnderstandsRequiresExpressions) {