lichray created this revision.
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 query 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.
https://reviews.llvm.org/D39284
Files:
include/clang/Sema/DeclSpec.h
lib/Parse/ParseExprCXX.cpp
lib/Sema/SemaDeclCXX.cpp
test/Parser/cxx2a-decomposition.cpp
test/SemaCXX/cxx2a-decomposition.cpp
Index: test/SemaCXX/cxx2a-decomposition.cpp
===================================================================
--- /dev/null
+++ test/SemaCXX/cxx2a-decomposition.cpp
@@ -0,0 +1,96 @@
+// RUN: %clang_cc1 -std=c++2a -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;
+}
+
+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
Index: test/Parser/cxx2a-decomposition.cpp
===================================================================
--- /dev/null
+++ test/Parser/cxx2a-decomposition.cpp
@@ -0,0 +1,61 @@
+// RUN: %clang_cc1 -std=c++2a %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())
+ ;
+ if (auto [ok, d] = g()) // expected-error {{value of type 'Na' is not contextually convertible to 'bool'}}
+ ;
+}
+} // namespace CondInIf
+
+namespace CondInWhile {
+void h() {
+ while (auto [ok, d] = f())
+ ;
+ while (auto [ok, d] = g()) // expected-error {{value of type 'Na' is not contextually convertible to 'bool'}}
+ ;
+}
+} // namespace CondInWhile
+
+namespace CondInFor {
+void h() {
+ for (; auto [ok, d] = f();)
+ ;
+ for (; auto [ok, d] = g();) // 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)
+ ;
+ switch (auto [ok, d] = g()) // expected-error {{statement requires expression of integer type ('Na' invalid)}}
+ ;
+}
+} // namespace CondInSwitch
Index: lib/Sema/SemaDeclCXX.cpp
===================================================================
--- lib/Sema/SemaDeclCXX.cpp
+++ lib/Sema/SemaDeclCXX.cpp
@@ -692,9 +692,10 @@
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.
- if (!D.mayHaveDecompositionDeclarator()) {
+ // The syntax only allows a decomposition declarator as a simple-declaration,
+ // a for-range-declaration, or a condition in C++2a, but we parse it in more
+ // cases than that.
+ if (!D.mayHaveDecompositionDeclarator(getLangOpts())) {
Diag(Decomp.getLSquareLoc(), diag::err_decomp_decl_context)
<< Decomp.getSourceRange();
return nullptr;
Index: lib/Parse/ParseExprCXX.cpp
===================================================================
--- lib/Parse/ParseExprCXX.cpp
+++ lib/Parse/ParseExprCXX.cpp
@@ -1708,6 +1708,8 @@
/// type-specifier-seq declarator '=' assignment-expression
/// [C++11] type-specifier-seq declarator '=' initializer-clause
/// [C++11] type-specifier-seq declarator braced-init-list
+/// [C++2a] 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
///
Index: include/clang/Sema/DeclSpec.h
===================================================================
--- include/clang/Sema/DeclSpec.h
+++ include/clang/Sema/DeclSpec.h
@@ -2001,7 +2001,7 @@
}
/// Return true if the context permits a C++17 decomposition declarator.
- bool mayHaveDecompositionDeclarator() const {
+ bool mayHaveDecompositionDeclarator(const LangOptions &Lang) const {
switch (Context) {
case FileContext:
// FIXME: It's not clear that the proposal meant to allow file-scope
@@ -2012,6 +2012,7 @@
return true;
case ConditionContext:
+ return Lang.CPlusPlus2a;
case MemberContext:
case PrototypeContext:
case TemplateParamContext:
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