P0859R0: List-initialization is potentially-constant-evaluated and

triggers instantiation of constexpr functions.

We mostly implemented this since Clang 6, but missed the template
instantiation case.

We do not implement the '&cast-expression' special case. It appears to
be a mistake / oversight. I've mailed CWG to see if we can remove it.

llvm-svn: 343064

2 files changed, 51 insertions, 0 deletions

diff --git a/clang/test/CXX/expr/expr.const/p6.cpp b/clang/test/CXX/expr/expr.const/p6.cpp
new file mode 100644
index 00000000000..a8fc7754e7d
--- /dev/null
+++ b/clang/test/CXX/expr/expr.const/p6.cpp
@@ -0,0 +1,39 @@
+// RUN: %clang_cc1 -std=c++17 -verify %s
+
+template<typename T> int not_constexpr() { return T::error; }
+template<typename T> constexpr int is_constexpr() { return T::error; } // expected-error {{'::'}}
+
+template<typename T> int not_constexpr_var = T::error;
+template<typename T> constexpr int is_constexpr_var = T::error; // expected-error {{'::'}}
+template<typename T> const int is_const_var = T::error; // expected-error {{'::'}}
+template<typename T> const volatile int is_const_volatile_var = T::error;
+template<typename T> T is_dependent_var = T::error; // expected-error {{'::'}}
+template<typename T> int &is_reference_var = T::error; // expected-error {{'::'}}
+template<typename T> float is_float_var = T::error;
+
+void test() {
+  // Do not instantiate functions referenced in unevaluated operands...
+  (void)sizeof(not_constexpr<long>());
+  (void)sizeof(is_constexpr<long>());
+  (void)sizeof(not_constexpr_var<long>);
+  (void)sizeof(is_constexpr_var<long>);
+  (void)sizeof(is_const_var<long>);
+  (void)sizeof(is_const_volatile_var<long>);
+  (void)sizeof(is_dependent_var<long>);
+  (void)sizeof(is_dependent_var<const long>);
+  (void)sizeof(is_reference_var<long>);
+  (void)sizeof(is_float_var<long>);
+
+  // ... but do if they are potentially constant evaluated, and refer to
+  // constexpr functions or to variables usable in constant expressions.
+  (void)sizeof(int{not_constexpr<int>()});
+  (void)sizeof(int{is_constexpr<int>()}); // expected-note {{instantiation of}}
+  (void)sizeof(int{not_constexpr_var<int>});
+  (void)sizeof(int{is_constexpr_var<int>}); // expected-note {{instantiation of}}
+  (void)sizeof(int{is_const_var<int>}); // expected-note {{instantiation of}}
+  (void)sizeof(int{is_const_volatile_var<int>});
+  (void)sizeof(int{is_dependent_var<int>});
+  (void)sizeof(int{is_dependent_var<const int>}); // expected-note {{instantiation of}}
+  (void)sizeof(int{is_reference_var<int>}); // expected-note {{instantiation of}}
+  (void)sizeof(int{is_float_var<int>}); // expected-error {{cannot be narrowed}} expected-note {{cast}}
+}
diff --git a/clang/test/CXX/temp/temp.spec/temp.inst/p7.cpp b/clang/test/CXX/temp/temp.spec/temp.inst/p7.cpp
new file mode 100644
index 00000000000..5145dafdc5b
--- /dev/null
+++ b/clang/test/CXX/temp/temp.spec/temp.inst/p7.cpp
@@ -0,0 +1,12 @@
+// RUN: %clang_cc1 -verify -std=c++17 %s
+
+template<typename T> constexpr int f() { return T::value; } // expected-error {{'::'}}
+template<bool B, typename T> void g(decltype(B ? f<T>() : 0));
+template<bool B, typename T> void g(...);
+template<bool B, typename T> void h(decltype(int{B ? f<T>() : 0})); // expected-note {{instantiation of}}
+template<bool B, typename T> void h(...);
+void x() {
+  g<false, int>(0); // ok
+  g<true, int>(0); // ok
+  h<false, int>(0); // expected-note {{while substituting}}
+}