[c++20] P1331R2: Allow transient use of uninitialized objects in

constant evaluation.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@372237 91177308-0d34-0410-b5e6-96231b3b80d8

3 files changed, 48 insertions, 29 deletions

diff --git a/include/clang/AST/DeclCXX.h b/include/clang/AST/DeclCXX.h
index f9668f1dce..49a0e6e982 100644
--- a/include/clang/AST/DeclCXX.h
+++ b/include/clang/AST/DeclCXX.h
@@ -1352,7 +1352,8 @@ public:
   /// would be constexpr.
   bool defaultedDefaultConstructorIsConstexpr() const {
     return data().DefaultedDefaultConstructorIsConstexpr &&
-           (!isUnion() || hasInClassInitializer() || !hasVariantMembers());
+           (!isUnion() || hasInClassInitializer() || !hasVariantMembers() ||
+            getASTContext().getLangOpts().CPlusPlus2a);
   }
 
   /// Determine whether this class has a constexpr default constructor.
diff --git a/include/clang/Basic/DiagnosticASTKinds.td b/include/clang/Basic/DiagnosticASTKinds.td
index 7732be5659..98c1269e03 100644
--- a/include/clang/Basic/DiagnosticASTKinds.td
+++ b/include/clang/Basic/DiagnosticASTKinds.td
@@ -37,7 +37,7 @@ def note_constexpr_virtual_call : Note<
 def note_constexpr_pure_virtual_call : Note<
   "pure virtual function %q0 called">;
 def note_constexpr_polymorphic_unknown_dynamic_type : Note<
-  "%select{||||virtual function called on|dynamic_cast applied to|"
+  "%select{|||||virtual function called on|dynamic_cast applied to|"
   "typeid applied to}0 object '%1' whose dynamic type is not constant">;
 def note_constexpr_dynamic_cast_to_reference_failed : Note<
   "reference dynamic_cast failed: %select{"
@@ -113,12 +113,12 @@ def note_constexpr_this : Note<
   "%select{|implicit }0use of 'this' pointer is only allowed within the "
   "evaluation of a call to a 'constexpr' member function">;
 def note_constexpr_lifetime_ended : Note<
-  "%select{read of|assignment to|increment of|decrement of|member call on|"
-  "dynamic_cast of|typeid applied to}0 "
+  "%select{read of|read of|assignment to|increment of|decrement of|"
+  "member call on|dynamic_cast of|typeid applied to}0 "
   "%select{temporary|variable}1 whose lifetime has ended">;
 def note_constexpr_access_uninit : Note<
-  "%select{read of|assignment to|increment of|decrement of|member call on|"
-  "dynamic_cast of|typeid applied to}0 "
+  "%select{read of|read of|assignment to|increment of|decrement of|"
+  "member call on|dynamic_cast of|typeid applied to}0 "
   "%select{object outside its lifetime|uninitialized object}1 "
   "is not allowed in a constant expression">;
 def note_constexpr_use_uninit_reference : Note<
@@ -128,10 +128,12 @@ def note_constexpr_modify_const_type : Note<
   "modification of object of const-qualified type %0 is not allowed "
   "in a constant expression">;
 def note_constexpr_access_volatile_type : Note<
-  "%select{read of|assignment to|increment of|decrement of|<ERROR>|<ERROR>}0 "
+  "%select{read of|read of|assignment to|increment of|decrement of|"
+  "<ERROR>|<ERROR>}0 "
   "volatile-qualified type %1 is not allowed in a constant expression">;
 def note_constexpr_access_volatile_obj : Note<
-  "%select{read of|assignment to|increment of|decrement of|<ERROR>|<ERROR>}0 "
+  "%select{read of|read of|assignment to|increment of|decrement of|"
+  "<ERROR>|<ERROR>}0 "
   "volatile %select{temporary|object %2|member %2}1 is not allowed in "
   "a constant expression">;
 def note_constexpr_volatile_here : Note<
@@ -145,31 +147,33 @@ def note_constexpr_ltor_non_constexpr : Note<
 def note_constexpr_ltor_incomplete_type : Note<
   "read of incomplete type %0 is not allowed in a constant expression">;
 def note_constexpr_access_null : Note<
-  "%select{read of|assignment to|increment of|decrement of|member call on|"
-  "dynamic_cast of|typeid applied to}0 "
+  "%select{read of|read of|assignment to|increment of|decrement of|"
+  "member call on|dynamic_cast of|typeid applied to}0 "
   "dereferenced null pointer is not allowed in a constant expression">;
 def note_constexpr_access_past_end : Note<
-  "%select{read of|assignment to|increment of|decrement of|member call on|"
-  "dynamic_cast of|typeid applied to}0 "
-  "dereferenced one-past-the-end pointer is not allowed in a constant expression">;
+  "%select{read of|read of|assignment to|increment of|decrement of|"
+  "member call on|dynamic_cast of|typeid applied to}0 "
+  "dereferenced one-past-the-end pointer is not allowed "
+  "in a constant expression">;
 def note_constexpr_access_unsized_array : Note<
-  "%select{read of|assignment to|increment of|decrement of|member call on|"
-  "dynamic_cast of|typeid applied to}0 "
+  "%select{read of|read of|assignment to|increment of|decrement of|"
+  "member call on|dynamic_cast of|typeid applied to}0 "
   "element of array without known bound "
   "is not allowed in a constant expression">;
 def note_constexpr_access_inactive_union_member : Note<
-  "%select{read of|assignment to|increment of|decrement of|member call on|"
-  "dynamic_cast of|typeid applied to}0 "
+  "%select{read of|read of|assignment to|increment of|decrement of|"
+  "member call on|dynamic_cast of|typeid applied to}0 "
   "member %1 of union with %select{active member %3|no active member}2 "
   "is not allowed in a constant expression">;
 def note_constexpr_access_static_temporary : Note<
-  "%select{read of|assignment to|increment of|decrement of|member call on|"
-  "dynamic_cast of|typeid applied to}0 temporary "
+  "%select{read of|read of|assignment to|increment of|decrement of|"
+  "member call on|dynamic_cast of|typeid applied to}0 temporary "
   "is not allowed in a constant expression outside the expression that "
   "created the temporary">;
 def note_constexpr_access_unreadable_object : Note<
-  "%select{read of|assignment to|increment of|decrement of|member call on|"
-  "dynamic_cast of|typeid applied to}0 object '%1' whose value is not known">;
+  "%select{read of|read of|assignment to|increment of|decrement of|"
+  "member call on|dynamic_cast of|typeid applied to}0 object '%1' "
+  "whose value is not known">;
 def note_constexpr_modify_global : Note<
   "a constant expression cannot modify an object that is visible outside "
   "that expression">;
diff --git a/include/clang/Basic/DiagnosticSemaKinds.td b/include/clang/Basic/DiagnosticSemaKinds.td
index 35cca1303b..b89bd36757 100644
--- a/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/include/clang/Basic/DiagnosticSemaKinds.td
@@ -2438,9 +2438,13 @@ def err_constexpr_local_var_static : Error<
 def err_constexpr_local_var_non_literal_type : Error<
   "variable of non-literal type %1 cannot be defined in a constexpr "
   "%select{function|constructor}0">;
-def err_constexpr_local_var_no_init : Error<
-  "variables defined in a constexpr %select{function|constructor}0 must be "
-  "initialized">;
+def ext_constexpr_local_var_no_init : ExtWarn<
+  "uninitialized variable in a constexpr %select{function|constructor}0 "
+  "is a C++20 extension">, InGroup<CXX2a>;
+def warn_cxx17_compat_constexpr_local_var_no_init : Warning<
+  "uninitialized variable in a constexpr %select{function|constructor}0 "
+  "is incompatible with C++ standards before C++20">,
+  InGroup<CXXPre2aCompat>, DefaultIgnore;
 def ext_constexpr_function_never_constant_expr : ExtWarn<
   "constexpr %select{function|constructor}0 never produces a "
   "constant expression">, InGroup<DiagGroup<"invalid-constexpr">>, DefaultError;
@@ -2468,7 +2472,7 @@ def note_constexpr_body_previous_return : Note<
 def err_constexpr_function_try_block : Error<
   "function try block not allowed in constexpr %select{function|constructor}0">;
 
-// c++2a function try blocks in constexpr
+// C++2a function try blocks in constexpr
 def ext_constexpr_function_try_block_cxx2a : ExtWarn<
   "function try block in constexpr %select{function|constructor}0 is "
   "a C++2a extension">, InGroup<CXX2a>;
@@ -2477,10 +2481,20 @@ def warn_cxx17_compat_constexpr_function_try_block : Warning<
   "incompatible with C++ standards before C++2a">,
   InGroup<CXXPre2aCompat>, DefaultIgnore;
 
-def err_constexpr_union_ctor_no_init : Error<
-  "constexpr union constructor does not initialize any member">;
-def err_constexpr_ctor_missing_init : Error<
-  "constexpr constructor must initialize all members">;
+def ext_constexpr_union_ctor_no_init : ExtWarn<
+  "constexpr union constructor that does not initialize any member "
+  "is a C++20 extension">, InGroup<CXX2a>;
+def warn_cxx17_compat_constexpr_union_ctor_no_init : Warning<
+  "constexpr union constructor that does not initialize any member "
+  "is incompatible with C++ standards before C++20">,
+  InGroup<CXXPre2aCompat>, DefaultIgnore;
+def ext_constexpr_ctor_missing_init : ExtWarn<
+  "constexpr constructor that does not initialize all members "
+  "is a C++20 extension">, InGroup<CXX2a>;
+def warn_cxx17_compat_constexpr_ctor_missing_init : Warning<
+  "constexpr constructor that does not initialize all members "
+  "is incompatible with C++ standards before C++20">,
+  InGroup<CXXPre2aCompat>, DefaultIgnore;
 def note_constexpr_ctor_missing_init : Note<
   "member not initialized by constructor">;
 def note_non_literal_no_constexpr_ctors : Note<