[clang][DebugInfo] Expand detection of structured bindings to account for std::get free function

When we generate the debug-info for a `VarDecl` we try
to determine whether it was introduced as part of a structure
binding (aka a "holding var"). If it was,
we don't mark it as `artificial`.

The heuristic to determine a holding var uses
`IgnoreUnlessSpelledInSource` to unwrap the `VarDecl` initializer
until we hit a `DeclRefExpr` that refers to a `Decomposition`.
For "tuple-like decompositions", Clang will generate a call to
a `template<size_t I> Foo get(Bar)` function that retrieves the
`Ith` element from the tuple-like structure. If that function is a
member function, we get an AST that looks as follows:
```
VarDecl implicit used z1 'std::tuple_element<0, B>::type &&' cinit
`-ExprWithCleanups <col:10> 'int' xvalue
  `-MaterializeTemporaryExpr <col:10> 'int' xvalue extended by Var 0x11d110cf8 'z1' 'std::tuple_element<0, B>::type &&'
    `-CXXMemberCallExpr <col:10> 'int'
      `-MemberExpr <col:10> '<bound member function type>' .get 0x11d104390
        `-ImplicitCastExpr <col:10> 'B' xvalue <NoOp>
          `-DeclRefExpr <col:10> 'B' lvalue Decomposition 0x11d1100a8 '' 'B'
```
`IgnoreUnlessSpelledInSource` happily unwraps this down to the
`DeclRefExpr`. However, when the `get` helper is a free function
(which it is for `std::pair` in libc++ for example), then the AST
is:
```
VarDecl col:16 implicit used k 'std::tuple_element<0, const std::tuple<int, int>>::type &' cinit
`-CallExpr <col:16> 'const typename tuple_element<0UL, tuple<int, int>>::type':'const int' lvalue adl
  |-ImplicitCastExpr <col:16> 'const typename tuple_element<0UL, tuple<int, int>>::type &(*)(const tuple<int, int> &) noexcept' <FunctionToPointerDecay>
  | `-DeclRefExpr <col:16> 'const typename tuple_element<0UL, tuple<int, int>>::type &(const tuple<int, int> &) noexcept' lvalue Function 0x1210262d8 'get' 'const typename tuple_element<0UL, tuple<int, int>>::type &(const tuple<int, int> &) noexcept' (FunctionTemplate 0x11d068088 'get')
  `-DeclRefExpr <col:16> 'const std::tuple<int, int>' lvalue Decomposition 0x121021518 '' 'const std::tuple<int, int> &'
```
`IgnoreUnlessSpelledInSource` doesn't unwrap this `CallExpr`, so we
incorrectly mark the binding as `artificial` in debug-info.

This patch adjusts our heuristic to account for `CallExpr` nodes.

Fixes llvm#122028

Author: Michael137

clang/lib/CodeGen/CGDebugInfo.cpp

@@ -85,12 +85,38 @@ static bool IsDecomposedVarDecl(VarDecl const *VD) {
   if (!Init)
     return false;
 
+  Init = Init->IgnoreUnlessSpelledInSource();
+  if (!Init)
+    return false;
+
+  // For tuple-like decompositions, if the `get` function
+  // is not a member of the decomposed type, but instead a
+  // free function (e.g., how std::get is specialized for
+  // std::pair), then the initializer is a `CallExpr` and
+  // we need to dig into the argument before unwrapping it
+  // with IgnoreUnlessSpelledInSource.
+  if (auto const *CE = llvm::dyn_cast<CallExpr>(Init)) {
+    if (CE->getNumArgs() == 0)
+      return false;
+
+    // The first argument will be the type we're decomposing.
+    // Technically the getter could have more than 1 argument
+    // (e.g., if they're defaulted arguments), but we don't care
+    // about them because we just need to check whether the
+    // first argument is a DecompositionDecl.
+    auto const *Arg = CE->getArg(0);
+    if (!Arg)
+      return false;
+
+    Init = Arg;
+  }
+
   auto const *RefExpr =
       llvm::dyn_cast_or_null<DeclRefExpr>(Init->IgnoreUnlessSpelledInSource());
   if (!RefExpr)
     return false;
 
-  return llvm::dyn_cast_or_null<DecompositionDecl>(RefExpr->getDecl());
+  return llvm::isa_and_nonnull<DecompositionDecl>(RefExpr->getDecl());
 }
 
 /// Returns true if \ref VD is a compiler-generated variable

clang/test/CodeGenCXX/debug-info-structured-binding.cpp

@@ -1,19 +1,26 @@
 // RUN: %clang_cc1 -emit-llvm -debug-info-kind=standalone -triple %itanium_abi_triple %s -o - | FileCheck %s --implicit-check-not="call void @llvm.dbg.declare"
 
+// CHECK: define noundef i32 @_Z1fv
 // CHECK: #dbg_declare(ptr %{{[a-z]+}}, ![[VAR_0:[0-9]+]], !DIExpression(),
 // CHECK: #dbg_declare(ptr %{{[0-9]+}}, ![[VAR_1:[0-9]+]], !DIExpression(),
 // CHECK: #dbg_declare(ptr %{{[0-9]+}}, ![[VAR_2:[0-9]+]], !DIExpression(DW_OP_plus_uconst, 4),
 // CHECK: #dbg_declare(ptr %{{[0-9]+}}, ![[VAR_3:[0-9]+]], !DIExpression(DW_OP_deref),
 // CHECK: #dbg_declare(ptr %{{[0-9]+}}, ![[VAR_4:[0-9]+]], !DIExpression(DW_OP_deref, DW_OP_plus_uconst, 4),
 // CHECK: #dbg_declare(ptr %z1, ![[VAR_5:[0-9]+]], !DIExpression()
 // CHECK: #dbg_declare(ptr %z2, ![[VAR_6:[0-9]+]], !DIExpression()
+// CHECK: #dbg_declare(ptr %k, ![[VAR_7:[0-9]+]], !DIExpression()
+// CHECK: #dbg_declare(ptr %v, ![[VAR_8:[0-9]+]], !DIExpression()
+// CHECK: #dbg_declare(ptr %w, ![[VAR_9:[0-9]+]], !DIExpression()
 // CHECK: ![[VAR_0]] = !DILocalVariable(name: "a"
 // CHECK: ![[VAR_1]] = !DILocalVariable(name: "x1", scope: !{{[0-9]+}}, file: !{{[0-9]+}}, line: {{[0-9]+}}, type: !{{[0-9]+}})
 // CHECK: ![[VAR_2]] = !DILocalVariable(name: "y1", scope: !{{[0-9]+}}, file: !{{[0-9]+}}, line: {{[0-9]+}}, type: !{{[0-9]+}})
 // CHECK: ![[VAR_3]] = !DILocalVariable(name: "x2", scope: !{{[0-9]+}}, file: !{{[0-9]+}}, line: {{[0-9]+}}, type: !{{[0-9]+}})
 // CHECK: ![[VAR_4]] = !DILocalVariable(name: "y2", scope: !{{[0-9]+}}, file: !{{[0-9]+}}, line: {{[0-9]+}}, type: !{{[0-9]+}})
 // CHECK: ![[VAR_5]] = !DILocalVariable(name: "z1", scope: !{{[0-9]+}}, file: !{{[0-9]+}}, line: {{[0-9]+}}, type: !{{[0-9]+}})
 // CHECK: ![[VAR_6]] = !DILocalVariable(name: "z2", scope: !{{[0-9]+}}, file: !{{[0-9]+}}, line: {{[0-9]+}}, type: !{{[0-9]+}})
+// CHECK: ![[VAR_7]] = !DILocalVariable(name: "k", scope: !{{[0-9]+}}, file: !{{[0-9]+}}, line: {{[0-9]+}}, type: !{{[0-9]+}})
+// CHECK: ![[VAR_8]] = !DILocalVariable(name: "v", scope: !{{[0-9]+}}, file: !{{[0-9]+}}, line: {{[0-9]+}}, type: !{{[0-9]+}})
+// CHECK: ![[VAR_9]] = !DILocalVariable(name: "w", scope: !{{[0-9]+}}, file: !{{[0-9]+}}, line: {{[0-9]+}}, type: !{{[0-9]+}})
 
 struct A {
   int x;
@@ -39,12 +46,31 @@ struct tuple_size<B> {
 };
 
 template<unsigned, typename T> struct tuple_element { using type = int; };
+
+// Decomposition of tuple-like bindings but where the `get` methods
+// are declared as free functions.
+struct triple {
+  int k;
+  int v;
+  int w;
+};
+
+template<>
+struct tuple_size<triple> {
+    static constexpr unsigned value = 3;
+};
+
+template <unsigned I> int get(triple);
+template <> int get<0>(triple p) { return p.k; }
+template <> int get<1>(triple p) { return p.v; }
+template <> int get<2>(triple p) { return p.w; }
 } // namespace std
 
 int f() {
   A a{10, 20};
   auto [x1, y1] = a;
   auto &[x2, y2] = a;
   auto [z1, z2] = B{1, 2};
-  return x1 + y1 + x2 + y2 + z1 + z2;
+  auto [k, v, w] = std::triple{3, 4, 5};
+  return x1 + y1 + x2 + y2 + z1 + z2 + k + v + w;
 }

lldb/test/API/lang/cpp/structured-binding/TestStructuredBinding.py

@@ -98,3 +98,14 @@ def test(self):
         self.expect_expr("tx2", result_value="4")
         self.expect_expr("ty2", result_value="'z'")
         self.expect_expr("tz2", result_value="10")
+
+        self.expect("frame variable",
+            substrs=[
+                "tx1 =",
+                "ty1 =",
+                "tz1 =",
+                "tx2 =",
+                "ty2 =",
+                "tz2 =",
+                "mp1 =",
+                "mp2 ="])

lldb/test/API/lang/cpp/structured-binding/main.cpp

@@ -1,13 +1,80 @@
 // Structured binding in C++ can bind identifiers to subobjects of an object.
 //
-// There are three cases we need to test:
+// There are four cases we need to test:
 // 1) arrays
-// 2) tuples like objects
-// 3) non-static data members
+// 2) tuple-like objects with `get` member functions
+// 3) tuple-like objects with `get` free functions
+// 4) non-static data members
 //
 // They can also bind by copy, reference or rvalue reference.
 
-#include <tuple>
+struct MyPair {
+  int m1;
+  int m2;
+
+  // Helpers to enable tuple-like decomposition.
+  template <unsigned> int get();
+  template <> int get<0>() { return m1; }
+  template <> int get<1>() { return m2; }
+};
+
+namespace std {
+template <typename T1, typename T2, typename T3> struct mock_tuple {
+  T1 m1;
+  T2 m2;
+  T3 m3;
+};
+
+template <typename T> struct tuple_size;
+
+template <unsigned, typename T> struct tuple_element;
+
+// Helpers to enable tuple-like decomposition for MyPair
+template <unsigned I> struct tuple_element<I, MyPair> {
+  using type = int;
+};
+
+template <> struct tuple_size<MyPair> {
+  static constexpr unsigned value = 2;
+};
+
+// Helpers to enable tuple-like decomposition for mock_tuple
+template <typename T1, typename T2, typename T3>
+struct tuple_element<0, mock_tuple<T1, T2, T3>> {
+  using type = T1;
+};
+
+template <typename T1, typename T2, typename T3>
+struct tuple_element<1, mock_tuple<T1, T2, T3>> {
+  using type = T2;
+};
+
+template <typename T1, typename T2, typename T3>
+struct tuple_element<2, mock_tuple<T1, T2, T3>> {
+  using type = T3;
+};
+
+template <typename T1, typename T2, typename T3>
+struct tuple_size<mock_tuple<T1, T2, T3>> {
+  static constexpr unsigned value = 3;
+};
+
+template <unsigned I, typename T1, typename T2, typename T3>
+typename tuple_element<I, mock_tuple<T1, T2, T3>>::type
+get(mock_tuple<T1, T2, T3> p) {
+  switch (I) {
+  case 0:
+    return p.m1;
+  case 1:
+    return p.m2;
+  case 2:
+    return p.m3;
+  default:
+    __builtin_trap();
+  }
+}
+
+} // namespace std
 
 struct A {
   int x;
@@ -54,16 +121,18 @@ int main() {
   char y{'z'};
   int z{10};
 
-  std::tuple<float, char, int> tpl(x, y, z);
+  std::mock_tuple<float, char, int> tpl{.m1 = x, .m2 = y, .m3 = z};
   auto [tx1, ty1, tz1] = tpl;
   auto &[tx2, ty2, tz2] = tpl;
 
+  auto [mp1, mp2] = MyPair{.m1 = 1, .m2 = 2};
+
   return a1.x + b1 + c1 + d1 + e1 + f1 + a2.y + b2 + c2 + d2 + e2 + f2 + a3.x +
          b3 + c3 + d3 + e3 + f3 + carr_copy1 + carr_copy2 + carr_copy3 +
          sarr_copy1 + sarr_copy2 + sarr_copy3 + iarr_copy1 + iarr_copy2 +
          iarr_copy3 + carr_ref1 + carr_ref2 + carr_ref3 + sarr_ref1 +
          sarr_ref2 + sarr_ref3 + iarr_ref1 + iarr_ref2 + iarr_ref3 +
          carr_rref1 + carr_rref2 + carr_rref3 + sarr_rref1 + sarr_rref2 +
          sarr_rref3 + iarr_rref1 + iarr_rref2 + iarr_rref3 + tx1 + ty1 + tz1 +
-         tx2 + ty2 + tz2; // break here
+         tx2 + ty2 + tz2 + mp1 + mp2; // break here
 }