[clang] Improve diagnostics for uninitialized constexpr variables

Instead of complaining about default initialization, tell users that
constexpr variables need to be initialized by a constant expression.

Differential Revision: https://reviews.llvm.org/D131662

clang/docs/ReleaseNotes.rst

@@ -101,6 +101,8 @@ Improvements to Clang's diagnostics
 - Clang will now print more information about failed static assertions. In
   particular, simple static assertion expressions are evaluated to their
   compile-time value and printed out if the assertion fails.
+- Diagnostics about uninitialized ``constexpr`` varaibles have been improved
+  to mention the missing constant initializer.
 
 Non-comprehensive list of changes in this release
 -------------------------------------------------

clang/lib/Sema/SemaDecl.cpp

@@ -13372,8 +13372,12 @@ void Sema::ActOnUninitializedDecl(Decl *RealDecl) {
     // Provide a specific diagnostic for uninitialized variable
     // definitions with incomplete array type.
     if (Type->isIncompleteArrayType()) {
-      Diag(Var->getLocation(),
-           diag::err_typecheck_incomplete_array_needs_initializer);
+      if (Var->isConstexpr())
+        Diag(Var->getLocation(), diag::err_constexpr_var_requires_const_init)
+            << Var;
+      else
+        Diag(Var->getLocation(),
+             diag::err_typecheck_incomplete_array_needs_initializer);
       Var->setInvalidDecl();
       return;
     }

clang/lib/Sema/SemaInit.cpp

@@ -8053,19 +8053,29 @@ ExprResult InitializationSequence::Perform(Sema &S,
     return ExprError();
   }
   if (!ZeroInitializationFixit.empty()) {
-    unsigned DiagID = diag::err_default_init_const;
-    if (Decl *D = Entity.getDecl())
-      if (S.getLangOpts().MSVCCompat && D->hasAttr<SelectAnyAttr>())
-        DiagID = diag::ext_default_init_const;
+    const Decl *D = Entity.getDecl();
+    const auto *VD = dyn_cast_or_null<VarDecl>(D);
+    QualType DestType = Entity.getType();
 
     // The initialization would have succeeded with this fixit. Since the fixit
     // is on the error, we need to build a valid AST in this case, so this isn't
     // handled in the Failed() branch above.
-    QualType DestType = Entity.getType();
-    S.Diag(Kind.getLocation(), DiagID)
-        << DestType << (bool)DestType->getAs<RecordType>()
-        << FixItHint::CreateInsertion(ZeroInitializationFixitLoc,
-                                      ZeroInitializationFixit);
+    if (!DestType->isRecordType() && VD && VD->isConstexpr()) {
+      // Use a more useful diagnostic for constexpr variables.
+      S.Diag(Kind.getLocation(), diag::err_constexpr_var_requires_const_init)
+          << VD
+          << FixItHint::CreateInsertion(ZeroInitializationFixitLoc,
+                                        ZeroInitializationFixit);
+    } else {
+      unsigned DiagID = diag::err_default_init_const;
+      if (S.getLangOpts().MSVCCompat && D && D->hasAttr<SelectAnyAttr>())
+        DiagID = diag::ext_default_init_const;
+
+      S.Diag(Kind.getLocation(), DiagID)
+          << DestType << (bool)DestType->getAs<RecordType>()
+          << FixItHint::CreateInsertion(ZeroInitializationFixitLoc,
+                                        ZeroInitializationFixit);
+    }
   }
 
   if (getKind() == DependentSequence) {
@@ -9464,6 +9474,10 @@ bool InitializationSequence::Diagnose(Sema &S,
         << Entity.getName();
       S.Diag(Entity.getDecl()->getLocation(), diag::note_previous_decl)
         << Entity.getName();
+    } else if (const auto *VD = dyn_cast_if_present<VarDecl>(Entity.getDecl());
+               VD && VD->isConstexpr()) {
+      S.Diag(Kind.getLocation(), diag::err_constexpr_var_requires_const_init)
+          << VD;
     } else {
       S.Diag(Kind.getLocation(), diag::err_default_init_const)
           << DestType << (bool)DestType->getAs<RecordType>();

clang/test/CXX/dcl.dcl/dcl.spec/dcl.constexpr/p1.cpp

@@ -37,7 +37,7 @@ struct s2 {
 #if __cplusplus <= 201402L && !defined(MS_ABI)
   // expected-error@-2 {{requires an initializer}}
 #else
-  // expected-error@-4 {{default initialization of an object of const}}
+  // expected-error@-4 {{constexpr variable 'mi2' must be initialized by a constant expression}}
 #endif
   mutable constexpr int mi3 = 3; // expected-error-re {{non-static data member cannot be constexpr{{$}}}} expected-error {{'mutable' and 'const' cannot be mixed}}
 };

clang/test/CXX/dcl.dcl/dcl.spec/dcl.constexpr/p9.cpp

@@ -18,7 +18,7 @@ extern int (*const d)(int);
 
 // A variable declaration which uses the constexpr specifier shall have an
 // initializer and shall be initialized by a constant expression.
-constexpr int ni1; // expected-error {{default initialization of an object of const type 'const int'}}
+constexpr int ni1; // expected-error {{constexpr variable 'ni1' must be initialized by a constant expression}}
 constexpr struct C { C(); } ni2; // expected-error {{cannot have non-literal type 'const struct C'}} expected-note 3{{has no constexpr constructors}}
 constexpr double &ni3; // expected-error {{declaration of reference variable 'ni3' requires an initializer}}
 

clang/test/SemaCXX/constant-expression-cxx11.cpp

@@ -27,6 +27,10 @@ struct MemberZero {
   constexpr int zero() const { return 0; }
 };
 
+constexpr int arr[]; // expected-error {{constexpr variable 'arr' must be initialized by a constant expression}}
+constexpr int arr2[2]; // expected-error {{constexpr variable 'arr2' must be initialized by a constant expression}}
+constexpr int arr3[2] = {};
+
 namespace DerivedToVBaseCast {
 
   struct U { int n; };
@@ -1298,7 +1302,7 @@ namespace ExternConstexpr {
   void f() {
     extern constexpr int i; // expected-error {{constexpr variable declaration must be a definition}}
     constexpr int j = 0;
-    constexpr int k; // expected-error {{default initialization of an object of const type}}
+    constexpr int k; // expected-error {{constexpr variable 'k' must be initialized by a constant expression}}
   }
 
   extern const int q;