Correctly diagnose incomplete arrays with static storage in C (#134374)

A file scope declaration without an initializer which is neither extern
nor thread_local is a tentative definition. If the declaration of an
identifier for an object is a tentative definition and has internal
linkage, the declared type shall not be an incomplete type.

Clang was previously failing to diagnose this in -pedantic mode.

Fixes #50661

---------

Co-authored-by: Mariya Podchishchaeva <[email protected]>

Author: AaronBallman

clang/docs/ReleaseNotes.rst

@@ -319,6 +319,9 @@ Improvements to Clang's diagnostics
 - ``-Wc++98-compat`` no longer diagnoses use of ``__auto_type`` or
   ``decltype(auto)`` as though it was the extension for ``auto``. (#GH47900)
 
+- Now correctly diagnose a tentative definition of an array with static
+  storage duration in pedantic mode in C. (#GH50661)
+
 Improvements to Clang's time-trace
 ----------------------------------
 

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -7350,8 +7350,9 @@ def err_typecheck_pointer_arith_void_type : Error<
   "arithmetic on%select{ a|}0 pointer%select{|s}0 to void">;
 def err_typecheck_decl_incomplete_type : Error<
   "variable has incomplete type %0">;
-def ext_typecheck_decl_incomplete_type : ExtWarn<
-  "tentative definition of variable with internal linkage has incomplete non-array type %0">,
+def ext_typecheck_decl_incomplete_type : Extension<
+  "tentative definition of variable with internal linkage has incomplete "
+  "%select{non-array|array}0 type %1">,
   InGroup<DiagGroup<"tentative-definition-incomplete-type">>;
 def err_tentative_def_incomplete_type : Error<
   "tentative definition has type %0 that is never completed">;

clang/lib/Sema/SemaDecl.cpp

@@ -14246,7 +14246,8 @@ void Sema::ActOnUninitializedDecl(Decl *RealDecl) {
                   Var->getLocation(), ArrayT->getElementType(),
                   diag::err_array_incomplete_or_sizeless_type))
             Var->setInvalidDecl();
-        } else if (Var->getStorageClass() == SC_Static) {
+        }
+        if (Var->getStorageClass() == SC_Static) {
           // C99 6.9.2p3: If the declaration of an identifier for an object is
           // a tentative definition and has internal linkage (C99 6.2.2p3), the
           // declared type shall not be an incomplete type.
@@ -14258,7 +14259,8 @@ void Sema::ActOnUninitializedDecl(Decl *RealDecl) {
           // NOTE: to avoid multiple warnings, only check the first declaration.
           if (Var->isFirstDecl())
             RequireCompleteType(Var->getLocation(), Type,
-                                diag::ext_typecheck_decl_incomplete_type);
+                                diag::ext_typecheck_decl_incomplete_type,
+                                Type->isArrayType());
         }
       }
 

clang/test/C/drs/dr0xx.c

@@ -139,7 +139,9 @@ int dr010_c = sizeof(dr010_t); /* expected-error {{invalid application of 'sizeo
  * Note: DR034 has a question resolved by DR011 and another question where the
  * result is UB.
  */
-static int dr011_a[]; /* expected-warning {{tentative array definition assumed to have one element}} */
+static int dr011_a[]; /* expected-warning {{tentative array definition assumed to have one element}}
+                         expected-warning {{tentative definition of variable with internal linkage has incomplete array type 'int[]'}}
+                       */
 void dr011(void) {
   extern int i[];
   {

clang/test/Sema/incomplete-decl.c

@@ -3,16 +3,15 @@
 
 
 
-struct foo; // c-note 5 {{forward declaration of 'struct foo'}} \
+struct foo; // c-note 4 {{forward declaration of 'struct foo'}} \
                cxx-note 3 {{forward declaration of 'foo'}}
 
 void b;  // expected-error {{variable has incomplete type 'void'}}
 struct foo f; // c-error {{tentative definition has type 'struct foo' that is never completed}} \
                  cxx-error {{variable has incomplete type 'struct foo'}}
 
 static void c; // expected-error {{variable has incomplete type 'void'}}
-static struct foo g;  // c-warning {{tentative definition of variable with internal linkage has incomplete non-array type 'struct foo'}} \
-                         c-error {{tentative definition has type 'struct foo' that is never completed}} \
+static struct foo g;  // c-error {{tentative definition has type 'struct foo' that is never completed}} \
                          cxx-error {{variable has incomplete type 'struct foo'}}
 
 extern void d; // cxx-error {{variable has incomplete type 'void'}}

clang/test/Sema/tentative-decls.c

@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 %s -fsyntax-only -Wprivate-extern -verify
+// RUN: %clang_cc1 %s -fsyntax-only -Wprivate-extern -pedantic -verify
 
 // PR3310
 struct a x1; // expected-note 2{{forward declaration of 'struct a'}}
@@ -58,7 +58,7 @@ void func2(void)
   extern double *p;
 }
 
-static int a0[];
+static int a0[]; // expected-warning {{tentative definition of variable with internal linkage has incomplete array type 'int[]'}}
 static int b0;
 
 static int a0[] = { 4 };