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

1 files changed, 61 insertions, 0 deletions

diff --git a/clang/test/Parser/decomposed-condition.cpp b/clang/test/Parser/decomposed-condition.cpp
new file mode 100644
index 00000000000..c41c82292e8
--- /dev/null
+++ b/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