Add a new AST matcher 'optionally'.

This matcher matches any node and at the same time executes all its
inner matchers to produce any possbile result bindings.

This is useful when a user wants certain supplementary information
that's not always present along with the main match result.

clang/docs/LibASTMatchersReference.html

@@ -4689,6 +4689,32 @@ Usable as: Any Matcher
 </pre></td></tr>
 
 
+<tr><td>Matcher&lt;*&gt;</td><td class="name" onclick="toggle('optionally0')"><a name="optionally0Anchor">optionally</a></td><td>Matcher&lt;*&gt;, ..., Matcher&lt;*&gt;</td></tr>
+<tr><td colspan="4" class="doc" id="optionally0"><pre>Matches any node regardless of the submatchers.
+
+However, optionally will generate a result binding for each matching
+submatcher.
+
+Useful when additional information which may or may not present about a
+main matching node is desired.
+
+For example, in:
+  class Foo {
+    int bar;
+  }
+The matcher:
+  cxxRecordDecl(
+    optionally(has(
+      fieldDecl(hasName("bar")).bind("var")
+  ))).bind("record")
+will produce a result binding for both "record" and "var".
+The matcher will produce a "record" binding for even if there is no data
+member named "bar" in that class.
+
+Usable as: Any Matcher
+</pre></td></tr>
+
+
 <tr><td>Matcher&lt;<a href="https://clang.llvm.org/doxygen/classclang_1_1AbstractConditionalOperator.html">AbstractConditionalOperator</a>&gt;</td><td class="name" onclick="toggle('hasCondition5')"><a name="hasCondition5Anchor">hasCondition</a></td><td>Matcher&lt;<a href="https://clang.llvm.org/doxygen/classclang_1_1Expr.html">Expr</a>&gt; InnerMatcher</td></tr>
 <tr><td colspan="4" class="doc" id="hasCondition5"><pre>Matches the condition expression of an if statement, for loop,
 switch statement or conditional operator.

clang/include/clang/ASTMatchers/ASTMatchers.h

@@ -2540,6 +2540,36 @@ extern const internal::VariadicOperatorMatcherFunc<
     2, std::numeric_limits<unsigned>::max()>
     allOf;
 
+/// Matches any node regardless of the submatchers.
+///
+/// However, \c optionally will generate a result binding for each matching
+/// submatcher.
+///
+/// Useful when additional information which may or may not present about a
+/// main matching node is desired.
+///
+/// For example, in:
+/// \code
+///   class Foo {
+///     int bar;
+///   }
+/// \endcode
+/// The matcher:
+/// \code
+///   cxxRecordDecl(
+///     optionally(has(
+///       fieldDecl(hasName("bar")).bind("var")
+///   ))).bind("record")
+/// \endcode
+/// will produce a result binding for both "record" and "var".
+/// The matcher will produce a "record" binding for even if there is no data
+/// member named "bar" in that class.
+///
+/// Usable as: Any Matcher
+extern const internal::VariadicOperatorMatcherFunc<
+    1, std::numeric_limits<unsigned>::max()>
+    optionally;
+
 /// Matches sizeof (C99), alignof (C++11) and vec_step (OpenCL)
 ///
 /// Given

clang/include/clang/ASTMatchers/ASTMatchersInternal.h

@@ -363,6 +363,10 @@ public:
     /// matches, but doesn't stop at the first match.
     VO_EachOf,
 
+    /// Matches any node but executes all inner matchers to find result
+    /// bindings.
+    VO_Optionally,
+
     /// Matches nodes that do not match the provided matcher.
     ///
     /// Uses the variadic matcher interface, but fails if

clang/lib/ASTMatchers/ASTMatchersInternal.cpp

@@ -68,6 +68,11 @@ bool AnyOfVariadicOperator(const ast_type_traits::DynTypedNode &DynNode,
                            BoundNodesTreeBuilder *Builder,
                            ArrayRef<DynTypedMatcher> InnerMatchers);
 
+bool OptionallyVariadicOperator(const ast_type_traits::DynTypedNode &DynNode,
+                                ASTMatchFinder *Finder,
+                                BoundNodesTreeBuilder *Builder,
+                                ArrayRef<DynTypedMatcher> InnerMatchers);
+
 void BoundNodesTreeBuilder::visitMatches(Visitor *ResultVisitor) {
   if (Bindings.empty())
     Bindings.push_back(BoundNodesMap());
@@ -184,6 +189,11 @@ DynTypedMatcher DynTypedMatcher::constructVariadic(
         SupportedKind, RestrictKind,
         new VariadicMatcher<EachOfVariadicOperator>(std::move(InnerMatchers)));
 
+  case VO_Optionally:
+    return DynTypedMatcher(SupportedKind, RestrictKind,
+                           new VariadicMatcher<OptionallyVariadicOperator>(
+                               std::move(InnerMatchers)));
+
   case VO_UnaryNot:
     // FIXME: Implement the Not operator to take a single matcher instead of a
     // vector.
@@ -347,6 +357,20 @@ bool AnyOfVariadicOperator(const ast_type_traits::DynTypedNode &DynNode,
   return false;
 }
 
+bool OptionallyVariadicOperator(const ast_type_traits::DynTypedNode &DynNode,
+                                ASTMatchFinder *Finder,
+                                BoundNodesTreeBuilder *Builder,
+                                ArrayRef<DynTypedMatcher> InnerMatchers) {
+  BoundNodesTreeBuilder Result;
+  for (const DynTypedMatcher &InnerMatcher : InnerMatchers) {
+    BoundNodesTreeBuilder BuilderInner(*Builder);
+    if (InnerMatcher.matches(DynNode, Finder, &BuilderInner))
+      Result.addMatch(BuilderInner);
+  }
+  *Builder = std::move(Result);
+  return true;
+}
+
 inline static
 std::vector<std::string> vectorFromRefs(ArrayRef<const StringRef *> NameRefs) {
   std::vector<std::string> Names;
@@ -797,6 +821,9 @@ const internal::VariadicOperatorMatcherFunc<
 const internal::VariadicOperatorMatcherFunc<
     2, std::numeric_limits<unsigned>::max()>
     allOf = {internal::DynTypedMatcher::VO_AllOf};
+const internal::VariadicOperatorMatcherFunc<
+    1, std::numeric_limits<unsigned>::max()>
+    optionally = {internal::DynTypedMatcher::VO_Optionally};
 const internal::VariadicFunction<internal::Matcher<NamedDecl>, StringRef,
                                  internal::hasAnyNameFunc>
     hasAnyName = {};

clang/lib/ASTMatchers/Dynamic/Registry.cpp

@@ -456,6 +456,7 @@ RegistryMaps::RegistryMaps() {
   REGISTER_MATCHER(on);
   REGISTER_MATCHER(onImplicitObjectArgument);
   REGISTER_MATCHER(opaqueValueExpr);
+  REGISTER_MATCHER(optionally);
   REGISTER_MATCHER(parameterCountIs);
   REGISTER_MATCHER(parenExpr);
   REGISTER_MATCHER(parenListExpr);

clang/unittests/ASTMatchers/ASTMatchersNarrowingTest.cpp

@@ -1866,6 +1866,27 @@ TEST(EachOf, BehavesLikeAnyOfUnlessBothMatch) {
                       has(fieldDecl(hasName("b")).bind("v"))))));
 }
 
+TEST(Optionally, SubmatchersDoNotMatch) {
+  EXPECT_TRUE(matchAndVerifyResultFalse(
+      "class A { int a; int b; };",
+      recordDecl(optionally(has(fieldDecl(hasName("c")).bind("v")),
+                            has(fieldDecl(hasName("d")).bind("v")))),
+      std::make_unique<VerifyIdIsBoundTo<FieldDecl>>("v")));
+}
+
+TEST(Optionally, SubmatchersMatch) {
+  EXPECT_TRUE(matchAndVerifyResultTrue(
+      "class A { int a; int c; };",
+      recordDecl(optionally(has(fieldDecl(hasName("a")).bind("v")),
+                            has(fieldDecl(hasName("b")).bind("v")))),
+      std::make_unique<VerifyIdIsBoundTo<FieldDecl>>("v", 1)));
+  EXPECT_TRUE(matchAndVerifyResultTrue(
+      "class A { int c; int b; };",
+      recordDecl(optionally(has(fieldDecl(hasName("c")).bind("v")),
+                            has(fieldDecl(hasName("b")).bind("v")))),
+      std::make_unique<VerifyIdIsBoundTo<FieldDecl>>("v", 2)));
+}
+
 TEST(IsTemplateInstantiation, MatchesImplicitClassTemplateInstantiation) {
   // Make sure that we can both match the class by name (::X) and by the type
   // the template was instantiated with (via a field).