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, 8 insertions, 7 deletions

diff --git a/clang/test/Parser/cxx1z-decomposition.cpp b/clang/test/Parser/cxx1z-decomposition.cpp
index 9cbe70e3c69..cf4ba77723a 100644
--- a/clang/test/Parser/cxx1z-decomposition.cpp
+++ b/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 {}