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
diff --git a/clang/test/Parser/cxx1z-decomposition.cpp b/clang/test/Parser/cxx1z-decomposition.cpp
index 9cbe70e..cf4ba77 100644
--- a/clang/test/Parser/cxx1z-decomposition.cpp
+++ b/clang/test/Parser/cxx1z-decomposition.cpp
@@ -32,13 +32,14 @@
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 {}
diff --git a/clang/test/Parser/decomposed-condition.cpp b/clang/test/Parser/decomposed-condition.cpp
new file mode 100644
index 0000000..c41c822
--- /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
diff --git a/clang/test/SemaCXX/decomposed-condition.cpp b/clang/test/SemaCXX/decomposed-condition.cpp
new file mode 100644
index 0000000..ab011f6
--- /dev/null
+++ b/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