Allow conditions to be decomposed with structured bindings

Summary:
This feature was discussed but not yet proposed.  It allows a structured binding to appear as a //condition//

    if (auto [ok, val] = f(...))

So the user can save an extra //condition// if the statement can test the value to-be-decomposed instead.  Formally, it makes the value of the underlying object of the structured binding declaration also the value of a //condition// that is an initialized declaration.

Considering its logicality which is entirely evident from its trivial implementation, I think it might be acceptable to land it as an extension for now before I write the paper.

Reviewers: rsmith, faisalv, aaron.ballman

Reviewed By: rsmith

Subscribers: aaron.ballman, cfe-commits

Differential Revision: https://reviews.llvm.org/D39284

llvm-svn: 320011

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -414,6 +414,9 @@ def warn_cxx14_compat_decomp_decl : Warning<
   "C++ standards before C++17">, DefaultIgnore, InGroup<CXXPre17Compat>;
 def ext_decomp_decl : ExtWarn<
   "decomposition declarations are a C++17 extension">, InGroup<CXX17>;
+def ext_decomp_decl_cond : ExtWarn<
+  "ISO C++17 does not permit structured binding declaration in a condition">,
+  InGroup<DiagGroup<"binding-in-condition">>;
 def err_decomp_decl_spec : Error<
   "decomposition declaration cannot be declared "
   "%plural{1:'%1'|:with '%1' specifiers}0">;

clang/include/clang/Sema/DeclSpec.h

@@ -1995,9 +1995,9 @@ public:
     case BlockContext:
     case ForContext:
     case InitStmtContext:
+    case ConditionContext:
       return true;
 
-    case ConditionContext:
     case MemberContext:
     case PrototypeContext:
     case TemplateParamContext:

clang/lib/Parse/ParseExprCXX.cpp

@@ -1715,6 +1715,8 @@ Parser::ParseCXXTypeConstructExpression(const DeclSpec &DS) {
 ///         type-specifier-seq declarator '=' assignment-expression
 /// [C++11] type-specifier-seq declarator '=' initializer-clause
 /// [C++11] type-specifier-seq declarator braced-init-list
+/// [Clang] type-specifier-seq ref-qualifier[opt] '[' identifier-list ']'
+///             brace-or-equal-initializer
 /// [GNU]   type-specifier-seq declarator simple-asm-expr[opt] attributes[opt]
 ///             '=' assignment-expression
 ///

clang/lib/Sema/SemaDeclCXX.cpp

@@ -692,8 +692,9 @@ Sema::ActOnDecompositionDeclarator(Scope *S, Declarator &D,
   assert(D.isDecompositionDeclarator());
   const DecompositionDeclarator &Decomp = D.getDecompositionDeclarator();
 
-  // The syntax only allows a decomposition declarator as a simple-declaration
-  // or a for-range-declaration, but we parse it in more cases than that.
+  // The syntax only allows a decomposition declarator as a simple-declaration,
+  // a for-range-declaration, or a condition in Clang, but we parse it in more
+  // cases than that.
   if (!D.mayHaveDecompositionDeclarator()) {
     Diag(Decomp.getLSquareLoc(), diag::err_decomp_decl_context)
       << Decomp.getSourceRange();
@@ -708,9 +709,12 @@ Sema::ActOnDecompositionDeclarator(Scope *S, Declarator &D,
     return nullptr;
   }
 
-  Diag(Decomp.getLSquareLoc(), getLangOpts().CPlusPlus17
-                                   ? diag::warn_cxx14_compat_decomp_decl
-                                   : diag::ext_decomp_decl)
+  Diag(Decomp.getLSquareLoc(),
+       !getLangOpts().CPlusPlus17
+           ? diag::ext_decomp_decl
+           : D.getContext() == Declarator::ConditionContext
+                 ? diag::ext_decomp_decl_cond
+                 : diag::warn_cxx14_compat_decomp_decl)
       << Decomp.getSourceRange();
 
   // The semantic context is always just the current context.

clang/test/Parser/cxx1z-decomposition.cpp

@@ -32,13 +32,14 @@ namespace OtherDecl {
   void f(auto [a, b, c]); // expected-error {{'auto' not allowed in function prototype}} expected-error {{'a'}}
 
   void g() {
-    // A condition is not a simple-declaration.
-    for (; auto [a, b, c] = S(); ) {} // expected-error {{not permitted in this context}}
-    if (auto [a, b, c] = S()) {} // expected-error {{not permitted in this context}}
-    if (int n; auto [a, b, c] = S()) {} // expected-error {{not permitted in this context}}
-    switch (auto [a, b, c] = S()) {} // expected-error {{not permitted in this context}}
-    switch (int n; auto [a, b, c] = S()) {} // expected-error {{not permitted in this context}}
-    while (auto [a, b, c] = S()) {} // expected-error {{not permitted in this context}}
+    // A condition is allowed as a Clang extension.
+    // See commentary in test/Parser/decomposed-condition.cpp
+    for (; auto [a, b, c] = S(); ) {} // expected-warning {{ISO C++17 does not permit structured binding declaration in a condition}} expected-error {{value of type 'S' is not contextually convertible to 'bool'}}
+    if (auto [a, b, c] = S()) {} // expected-warning {{ISO C++17 does not permit structured binding declaration in a condition}} expected-error {{value of type 'S' is not contextually convertible to 'bool'}}
+    if (int n; auto [a, b, c] = S()) {} // expected-warning {{ISO C++17 does not permit structured binding declaration in a condition}} expected-error {{value of type 'S' is not contextually convertible to 'bool'}}
+    switch (auto [a, b, c] = S()) {} // expected-warning {{ISO C++17 does not permit structured binding declaration in a condition}} expected-error {{statement requires expression of integer type ('S' invalid)}}
+    switch (int n; auto [a, b, c] = S()) {} // expected-warning {{ISO C++17 does not permit structured binding declaration in a condition}} expected-error {{statement requires expression of integer type ('S' invalid)}}
+    while (auto [a, b, c] = S()) {} // expected-warning {{ISO C++17 does not permit structured binding declaration in a condition}} expected-error {{value of type 'S' is not contextually convertible to 'bool'}}
 
     // An exception-declaration is not a simple-declaration.
     try {}

clang/test/Parser/decomposed-condition.cpp

@@ -0,0 +1,61 @@
+// RUN: %clang_cc1 -std=c++1z %s -verify
+
+struct Na {
+  bool flag;
+  float data;
+};
+
+struct Rst {
+  bool flag;
+  float data;
+  explicit operator bool() const {
+    return flag;
+  }
+};
+
+Rst f();
+Na g();
+
+namespace CondInIf {
+void h() {
+  if (auto [ok, d] = f()) // expected-warning {{ISO C++17 does not permit structured binding declaration in a condition}}
+    ;
+  if (auto [ok, d] = g()) // expected-warning {{ISO C++17 does not permit structured binding declaration in a condition}} expected-error {{value of type 'Na' is not contextually convertible to 'bool'}}
+    ;
+}
+} // namespace CondInIf
+
+namespace CondInWhile {
+void h() {
+  while (auto [ok, d] = f()) // expected-warning {{ISO C++17 does not permit structured binding declaration in a condition}}
+    ;
+  while (auto [ok, d] = g()) // expected-warning {{ISO C++17 does not permit structured binding declaration in a condition}} expected-error {{value of type 'Na' is not contextually convertible to 'bool'}}
+    ;
+}
+} // namespace CondInWhile
+
+namespace CondInFor {
+void h() {
+  for (; auto [ok, d] = f();) // expected-warning {{ISO C++17 does not permit structured binding declaration in a condition}}
+    ;
+  for (; auto [ok, d] = g();) // expected-warning {{ISO C++17 does not permit structured binding declaration in a condition}} expected-error {{value of type 'Na' is not contextually convertible to 'bool'}}
+    ;
+}
+} // namespace CondInFor
+
+struct IntegerLike {
+  bool flag;
+  float data;
+  operator int() const {
+    return int(data);
+  }
+};
+
+namespace CondInSwitch {
+void h(IntegerLike x) {
+  switch (auto [ok, d] = x) // expected-warning {{ISO C++17 does not permit structured binding declaration in a condition}}
+    ;
+  switch (auto [ok, d] = g()) // expected-warning {{ISO C++17 does not permit structured binding declaration in a condition}} expected-error {{statement requires expression of integer type ('Na' invalid)}}
+    ;
+}
+} // namespace CondInSwitch

clang/test/SemaCXX/decomposed-condition.cpp

@@ -0,0 +1,99 @@
+// RUN: %clang_cc1 -std=c++1z -Wno-binding-in-condition -verify %s
+
+struct X {
+  bool flag;
+  int data;
+  constexpr explicit operator bool() const {
+    return flag;
+  }
+  constexpr operator int() const {
+    return data;
+  }
+};
+
+namespace CondInIf {
+constexpr int f(X x) {
+  if (auto [ok, d] = x)
+    return d + int(ok);
+  else
+    return d * int(ok);
+  ok = {}; // expected-error {{use of undeclared identifier 'ok'}}
+  d = {};  // expected-error {{use of undeclared identifier 'd'}}
+}
+
+static_assert(f({true, 2}) == 3);
+static_assert(f({false, 2}) == 0);
+
+constexpr char g(char const (&x)[2]) {
+  if (auto &[a, b] = x)
+    return a;
+  else
+    return b;
+
+  if (auto [a, b] = x) // expected-error {{an array type is not allowed here}}
+    ;
+}
+
+static_assert(g("x") == 'x');
+} // namespace CondInIf
+
+namespace CondInSwitch {
+constexpr int f(int n) {
+  switch (X s = {true, n}; auto [ok, d] = s) {
+    s = {};
+  case 0:
+    return int(ok);
+  case 1:
+    return d * 10;
+  case 2:
+    return d * 40;
+  default:
+    return 0;
+  }
+  ok = {}; // expected-error {{use of undeclared identifier 'ok'}}
+  d = {};  // expected-error {{use of undeclared identifier 'd'}}
+  s = {};  // expected-error {{use of undeclared identifier 's'}}
+}
+
+static_assert(f(0) == 1);
+static_assert(f(1) == 10);
+static_assert(f(2) == 80);
+} // namespace CondInSwitch
+
+namespace CondInWhile {
+constexpr int f(int n) {
+  int m = 1;
+  while (auto [ok, d] = X{n > 1, n}) {
+    m *= d;
+    --n;
+  }
+  return m;
+  return ok; // expected-error {{use of undeclared identifier 'ok'}}
+}
+
+static_assert(f(0) == 1);
+static_assert(f(1) == 1);
+static_assert(f(4) == 24);
+} // namespace CondInWhile
+
+namespace CondInFor {
+constexpr int f(int n) {
+  int a = 1, b = 1;
+  for (X x = {true, n}; auto &[ok, d] = x; --d) {
+    if (d < 2)
+      ok = false;
+    else {
+      int x = b;
+      b += a;
+      a = x;
+    }
+  }
+  return b;
+  return d; // expected-error {{use of undeclared identifier 'd'}}
+}
+
+static_assert(f(0) == 1);
+static_assert(f(1) == 1);
+static_assert(f(2) == 2);
+static_assert(f(5) == 8);
+} // namespace CondInFor