[Clang] Disallow VLA type compound literals (llvm#91891)

C99-C23 6.5.2.5 says: The type name shall specify an object type or an
array of unknown size, but not a variable length array type.

Fixes llvm#89835.

Author: J-MR-T

clang/docs/ReleaseNotes.rst

@@ -570,6 +570,9 @@ Bug Fixes in This Version
 - Clang will no longer emit a duplicate -Wunused-value warning for an expression
   `(A, B)` which evaluates to glvalue `B` that can be converted to non ODR-use. (#GH45783)
 
+- Clang now correctly disallows VLA type compound literals, e.g. ``(int[size]){}``,
+  as the C standard mandates. (#GH89835)
+
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -3371,6 +3371,8 @@ def err_field_with_address_space : Error<
   "field may not be qualified with an address space">;
 def err_compound_literal_with_address_space : Error<
   "compound literal in function scope may not be qualified with an address space">;
+def err_compound_literal_with_vla_type : Error<
+  "compound literal cannot be of variable-length array type">;
 def err_address_space_mismatch_templ_inst : Error<
   "conflicting address space qualifiers are provided between types %0 and %1">;
 def err_attr_objc_ownership_redundant : Error<

clang/lib/Sema/SemaExpr.cpp

@@ -7130,12 +7130,19 @@ Sema::BuildCompoundLiteralExpr(SourceLocation LParenLoc, TypeSourceInfo *TInfo,
       // init a VLA in C++ in all cases (such as with non-trivial constructors).
       // FIXME: should we allow this construct in C++ when it makes sense to do
       // so?
-      std::optional<unsigned> NumInits;
-      if (const auto *ILE = dyn_cast<InitListExpr>(LiteralExpr))
-        NumInits = ILE->getNumInits();
-      if ((LangOpts.CPlusPlus || NumInits.value_or(0)) &&
-          !tryToFixVariablyModifiedVarType(TInfo, literalType, LParenLoc,
-                                           diag::err_variable_object_no_init))
+      //
+      // But: C99-C23 6.5.2.5 Compound literals constraint 1: The type name
+      // shall specify an object type or an array of unknown size, but not a
+      // variable length array type. This seems odd, as it allows int a[size] =
+      // {}; but forbids int a[size] = (int[size]){}; As this is what the
+      // standard says, this is what's implemented here for C (except for the
+      // extension that permits constant foldable size arrays)
+
+      auto diagID = LangOpts.CPlusPlus
+                        ? diag::err_variable_object_no_init
+                        : diag::err_compound_literal_with_vla_type;
+      if (!tryToFixVariablyModifiedVarType(TInfo, literalType, LParenLoc,
+                                           diagID))
         return ExprError();
     }
   } else if (!literalType->isDependentType() &&

clang/test/C/C2x/n2900_n3011.c

@@ -27,8 +27,14 @@ void test(void) {
                               compat-warning {{use of an empty initializer is incompatible with C standards before C23}}
   int vla[i] = {}; // compat-warning {{use of an empty initializer is incompatible with C standards before C23}} \
                       pedantic-warning {{use of an empty initializer is a C23 extension}}
+  // C99 6.5.2.5 Compound literals constraint 1: The type name shall specify an
+  // object type or an array of unknown size, but not a variable length array
+  // type.
   int *compound_literal_vla = (int[i]){}; // compat-warning {{use of an empty initializer is incompatible with C standards before C23}} \
-                                             pedantic-warning {{use of an empty initializer is a C23 extension}}
+                                             pedantic-warning {{use of an empty initializer is a C23 extension}}\
+                                             compat-error {{compound literal cannot be of variable-length array type}} \
+                                             pedantic-error {{compound literal cannot be of variable-length array type}}\
+
 
   struct T {
 	int i;

clang/test/C/C2x/n2900_n3011_2.c

@@ -76,22 +76,6 @@ void test_zero_size_vla() {
   // CHECK-NEXT: call void @llvm.memset.p0.i64(ptr {{.*}} %[[VLA]], i8 0, i64 %[[BYTES_TO_COPY]], i1 false)
 }
 
-void test_compound_literal_vla() {
-  int num_elts = 12;
-  int *compound_literal_vla = (int[num_elts]){};
-  // CHECK: define {{.*}} void @test_compound_literal_vla
-  // CHECK-NEXT: entry:
-  // CHECK-NEXT: %[[NUM_ELTS_PTR:.+]] = alloca i32
-  // CHECK-NEXT: %[[COMP_LIT_VLA:.+]] = alloca ptr
-  // CHECK-NEXT: %[[COMP_LIT:.+]] = alloca i32
-  // CHECK-NEXT: store i32 12, ptr %[[NUM_ELTS_PTR]]
-  // CHECK-NEXT: %[[NUM_ELTS:.+]] = load i32, ptr %[[NUM_ELTS_PTR]]
-  // CHECK-NEXT: %[[NUM_ELTS_EXT:.+]] = zext i32 %[[NUM_ELTS]] to i64
-  // CHECK-NEXT: %[[BYTES_TO_COPY:.+]] = mul nuw i64 %[[NUM_ELTS_EXT]], 4
-  // CHECK-NEXT: call void @llvm.memset.p0.i64(ptr {{.*}} %[[COMP_LIT]], i8 0, i64 %[[BYTES_TO_COPY]], i1 false)
-  // CHECK-NEXT: store ptr %[[COMP_LIT]], ptr %[[COMP_LIT_VLA]]
-}
-
 void test_nested_structs() {
   struct T t1 = { 1, {} };
   struct T t2 = { 1, { 2, {} } };

clang/test/Sema/compound-literal.c

@@ -29,7 +29,7 @@ int main(int argc, char **argv) {
 struct Incomplete; // expected-note{{forward declaration of 'struct Incomplete'}}
 struct Incomplete* I1 = &(struct Incomplete){1, 2, 3}; // expected-error {{variable has incomplete type}}
 void IncompleteFunc(unsigned x) {
-  struct Incomplete* I2 = (struct foo[x]){1, 2, 3}; // expected-error {{variable-sized object may not be initialized}}
+  struct Incomplete* I2 = (struct foo[x]){1, 2, 3}; // expected-error {{compound literal cannot be of variable-length array type}}
   (void){1,2,3}; // expected-error {{variable has incomplete type}}
   (void(void)) { 0 }; // expected-error{{illegal initializer type 'void (void)'}}
 }
@@ -42,3 +42,14 @@ int (^block)(int) = ^(int i) {
   int *array = (int[]) {i, i + 2, i + 4};
   return array[i];
 };
+
+// C99 6.5.2.5 Compound literals constraint 1: The type name shall specify an object type or an array of unknown size, but not a variable length array type.
+// So check that VLA type compound literals are rejected (see https://github.com/llvm/llvm-project/issues/89835).
+void vla(int n) {
+  int size = 5;
+  (void)(int[size]){}; // expected-warning {{use of an empty initializer is a C23 extension}}
+                       // expected-error@-1 {{compound literal cannot be of variable-length array type}}
+  (void)(int[size]){1}; // expected-error {{compound literal cannot be of variable-length array type}}
+  (void)(int[size]){1,2,3}; // expected-error {{compound literal cannot be of variable-length array type}}
+  (void)(int[size]){1,2,3,4,5}; // expected-error {{compound literal cannot be of variable-length array type}}
+}