[Sema] Fold VLAs to constant arrays in a few more contexts

552c6c2 removed support for promoting VLAs to constant arrays when the bounds
isn't an ICE, since this can result in miscompiling a conforming program that
assumes that the array is a VLA. Promoting VLAs for fields is still supported,
since clang doesn't support VLAs in fields, so no conforming program could have
a field VLA.

This change is really disruptive, so this commit carves out two more cases
where we promote VLAs which can't miscompile a conforming program:

 - When the VLA appears in an ivar -- this seems like a corollary to the field thing
 - When the VLA has an initializer -- VLAs can't have an initializer

Differential revision: https://reviews.llvm.org/D90871

clang/include/clang/Sema/DeclSpec.h

@@ -1844,6 +1844,9 @@ private:
   /// Indicates whether the InlineParams / InlineBindings storage has been used.
   unsigned InlineStorageUsed : 1;
 
+  /// Indicates whether this declarator has an initializer.
+  unsigned HasInitializer : 1;
+
   /// Attrs - Attributes.
   ParsedAttributes Attrs;
 
@@ -1892,8 +1895,8 @@ public:
                                    FunctionDefinitionKind::Declaration)),
         Redeclaration(false), Extension(false), ObjCIvar(false),
         ObjCWeakProperty(false), InlineStorageUsed(false),
-        Attrs(ds.getAttributePool().getFactory()), AsmLabel(nullptr),
-        TrailingRequiresClause(nullptr),
+        HasInitializer(false), Attrs(ds.getAttributePool().getFactory()),
+        AsmLabel(nullptr), TrailingRequiresClause(nullptr),
         InventedTemplateParameterList(nullptr) {}
 
   ~Declarator() {
@@ -1976,6 +1979,7 @@ public:
     Attrs.clear();
     AsmLabel = nullptr;
     InlineStorageUsed = false;
+    HasInitializer = false;
     ObjCIvar = false;
     ObjCWeakProperty = false;
     CommaLoc = SourceLocation();
@@ -2574,6 +2578,9 @@ public:
     return (FunctionDefinitionKind)FunctionDefinition;
   }
 
+  void setHasInitializer(bool Val = true) { HasInitializer = Val; }
+  bool hasInitializer() const { return HasInitializer; }
+
   /// Returns true if this declares a real member and not a friend.
   bool isFirstDeclarationOfMember() {
     return getContext() == DeclaratorContext::Member &&

clang/lib/Parse/ParseDecl.cpp

@@ -2192,7 +2192,22 @@ Decl *Parser::ParseDeclarationAfterDeclaratorAndAttributes(
     }
   };
 
-  // Inform the current actions module that we just parsed this declarator.
+  enum class InitKind { Uninitialized, Equal, CXXDirect, CXXBraced };
+  InitKind TheInitKind;
+  // If a '==' or '+=' is found, suggest a fixit to '='.
+  if (isTokenEqualOrEqualTypo())
+    TheInitKind = InitKind::Equal;
+  else if (Tok.is(tok::l_paren))
+    TheInitKind = InitKind::CXXDirect;
+  else if (getLangOpts().CPlusPlus11 && Tok.is(tok::l_brace) &&
+           (!CurParsedObjCImpl || !D.isFunctionDeclarator()))
+    TheInitKind = InitKind::CXXBraced;
+  else
+    TheInitKind = InitKind::Uninitialized;
+  if (TheInitKind != InitKind::Uninitialized)
+    D.setHasInitializer();
+
+  // Inform Sema that we just parsed this declarator.
   Decl *ThisDecl = nullptr;
   Decl *OuterDecl = nullptr;
   switch (TemplateInfo.Kind) {
@@ -2254,9 +2269,9 @@ Decl *Parser::ParseDeclarationAfterDeclaratorAndAttributes(
     }
   }
 
+  switch (TheInitKind) {
   // Parse declarator '=' initializer.
-  // If a '==' or '+=' is found, suggest a fixit to '='.
-  if (isTokenEqualOrEqualTypo()) {
+  case InitKind::Equal: {
     SourceLocation EqualLoc = ConsumeToken();
 
     if (Tok.is(tok::kw_delete)) {
@@ -2311,7 +2326,9 @@ Decl *Parser::ParseDeclarationAfterDeclaratorAndAttributes(
         Actions.AddInitializerToDecl(ThisDecl, Init.get(),
                                      /*DirectInit=*/false);
     }
-  } else if (Tok.is(tok::l_paren)) {
+    break;
+  }
+  case InitKind::CXXDirect: {
     // Parse C++ direct initializer: '(' expression-list ')'
     BalancedDelimiterTracker T(*this, tok::l_paren);
     T.consumeOpen();
@@ -2365,8 +2382,9 @@ Decl *Parser::ParseDeclarationAfterDeclaratorAndAttributes(
       Actions.AddInitializerToDecl(ThisDecl, Initializer.get(),
                                    /*DirectInit=*/true);
     }
-  } else if (getLangOpts().CPlusPlus11 && Tok.is(tok::l_brace) &&
-             (!CurParsedObjCImpl || !D.isFunctionDeclarator())) {
+    break;
+  }
+  case InitKind::CXXBraced: {
     // Parse C++0x braced-init-list.
     Diag(Tok, diag::warn_cxx98_compat_generalized_initializer_lists);
 
@@ -2381,9 +2399,12 @@ Decl *Parser::ParseDeclarationAfterDeclaratorAndAttributes(
       Actions.ActOnInitializerError(ThisDecl);
     } else
       Actions.AddInitializerToDecl(ThisDecl, Init.get(), /*DirectInit=*/true);
-
-  } else {
+    break;
+  }
+  case InitKind::Uninitialized: {
     Actions.ActOnUninitializedDecl(ThisDecl);
+    break;
+  }
   }
 
   Actions.FinalizeDeclaration(ThisDecl);

clang/lib/Sema/SemaDecl.cpp

@@ -6023,6 +6023,31 @@ TryToFixInvalidVariablyModifiedTypeSourceInfo(TypeSourceInfo *TInfo,
   return FixedTInfo;
 }
 
+/// Attempt to fold a variable-sized type to a constant-sized type, returning
+/// true if we were successful.
+static bool tryToFixVariablyModifiedVarType(Sema &S, TypeSourceInfo *&TInfo,
+                                            QualType &T, SourceLocation Loc,
+                                            unsigned FailedFoldDiagID) {
+  bool SizeIsNegative;
+  llvm::APSInt Oversized;
+  TypeSourceInfo *FixedTInfo = TryToFixInvalidVariablyModifiedTypeSourceInfo(
+      TInfo, S.Context, SizeIsNegative, Oversized);
+  if (FixedTInfo) {
+    S.Diag(Loc, diag::ext_vla_folded_to_constant);
+    TInfo = FixedTInfo;
+    T = FixedTInfo->getType();
+    return true;
+  }
+
+  if (SizeIsNegative)
+    S.Diag(Loc, diag::err_typecheck_negative_array_size);
+  else if (Oversized.getBoolValue())
+    S.Diag(Loc, diag::err_array_too_large) << Oversized.toString(10);
+  else if (FailedFoldDiagID)
+    S.Diag(Loc, FailedFoldDiagID);
+  return false;
+}
+
 /// Register the given locally-scoped extern "C" declaration so
 /// that it can be found later for redeclarations. We include any extern "C"
 /// declaration that is not visible in the translation unit here, not just
@@ -6861,6 +6886,12 @@ NamedDecl *Sema::ActOnVariableDeclarator(
     }
   }
 
+  // If this variable has a variable-modified type and an initializer, try to
+  // fold to a constant-sized type. This is otherwise invalid.
+  if (D.hasInitializer() && R->isVariablyModifiedType())
+    tryToFixVariablyModifiedVarType(*this, TInfo, R, D.getIdentifierLoc(),
+                                    /*DiagID=*/0);
+
   bool IsMemberSpecialization = false;
   bool IsVariableTemplateSpecialization = false;
   bool IsPartialSpecialization = false;
@@ -16658,27 +16689,9 @@ FieldDecl *Sema::CheckFieldDecl(DeclarationName Name, QualType T,
   // C99 6.7.2.1p8: A member of a structure or union may have any type other
   // than a variably modified type.
   if (!InvalidDecl && T->isVariablyModifiedType()) {
-    bool SizeIsNegative;
-    llvm::APSInt Oversized;
-
-    TypeSourceInfo *FixedTInfo =
-      TryToFixInvalidVariablyModifiedTypeSourceInfo(TInfo, Context,
-                                                    SizeIsNegative,
-                                                    Oversized);
-    if (FixedTInfo) {
-      Diag(Loc, diag::ext_vla_folded_to_constant);
-      TInfo = FixedTInfo;
-      T = FixedTInfo->getType();
-    } else {
-      if (SizeIsNegative)
-        Diag(Loc, diag::err_typecheck_negative_array_size);
-      else if (Oversized.getBoolValue())
-        Diag(Loc, diag::err_array_too_large)
-          << Oversized.toString(10);
-      else
-        Diag(Loc, diag::err_typecheck_field_variable_size);
+    if (!tryToFixVariablyModifiedVarType(
+            *this, TInfo, T, Loc, diag::err_typecheck_field_variable_size))
       InvalidDecl = true;
-    }
   }
 
   // Fields can not have abstract class types
@@ -16904,8 +16917,9 @@ Decl *Sema::ActOnIvar(Scope *S,
   // C99 6.7.2.1p8: A member of a structure or union may have any type other
   // than a variably modified type.
   else if (T->isVariablyModifiedType()) {
-    Diag(Loc, diag::err_typecheck_ivar_variable_size);
-    D.setInvalidType();
+    if (!tryToFixVariablyModifiedVarType(
+            *this, TInfo, T, Loc, diag::err_typecheck_ivar_variable_size))
+      D.setInvalidType();
   }
 
   // Get the visibility (access control) for this ivar.

clang/test/CXX/basic/basic.types/p10.cpp

@@ -140,8 +140,7 @@ constexpr int arb(int n) {
   int a[n]; // expected-error {{variable of non-literal type 'int [n]' cannot be defined in a constexpr function}}
 }
 // expected-warning@+1 {{variable length array folded to constant array as an extension}}
-constexpr long Overflow[ // expected-error {{constexpr variable cannot have non-literal type 'long const[(1 << 30) << 2]'}}
-    (1 << 30) << 2]{};   // expected-warning {{requires 34 bits to represent}}
+constexpr long Overflow[(1 << 30) << 2]{}; // expected-warning {{requires 34 bits to represent}}
 
 namespace inherited_ctor {
   struct A { constexpr A(int); };

clang/test/Sema/decl-in-prototype.c

@@ -49,7 +49,7 @@ void f(struct q *, struct __attribute__((aligned(4))) q *); // expected-warning
 // function.
 enum { BB = 0 };
 void enum_in_fun_in_fun(void (*fp)(enum { AA, BB } e)) { // expected-warning {{will not be visible}}
-  SA(1, AA == 5); // expected-error {{variable-sized object may not be initialized}}
+  SA(1, AA == 5); // expected-warning{{variable length array folded to constant array as an extension}}
   SA(2, BB == 0);
 }
 

clang/test/Sema/vla.c

@@ -100,3 +100,33 @@ const int pr44406_a = 32;
 typedef struct {
   char c[pr44406_a]; // expected-warning {{folded to constant array as an extension}}
 } pr44406_s;
+
+void test_fold_to_constant_array() {
+  const int ksize = 4;
+
+  goto jump_over_a1; // expected-error{{cannot jump from this goto statement to its label}}
+  char a1[ksize]; // expected-note{{variable length array}}
+ jump_over_a1:;
+
+  goto jump_over_a2;
+  char a2[ksize] = "foo"; // expected-warning{{variable length array folded to constant array as an extension}}
+ jump_over_a2:;
+
+  goto jump_over_a3;
+  char a3[ksize] = {}; // expected-warning {{variable length array folded to constant array as an extension}} expected-warning{{use of GNU empty initializer}}
+ jump_over_a3:;
+
+  goto jump_over_a4; // expected-error{{cannot jump from this goto statement to its label}}
+  char a4[ksize][2]; // expected-note{{variable length array}}
+ jump_over_a4:;
+
+  char a5[ksize][2] = {}; // expected-warning {{variable length array folded to constant array as an extension}} expected-warning{{use of GNU empty initializer}}
+
+  int a6[ksize] = {1,2,3,4}; // expected-warning{{variable length array folded to constant array as an extension}}
+
+  // expected-warning@+1{{variable length array folded to constant array as an extension}}
+  int a7[ksize] __attribute__((annotate("foo"))) = {1,2,3,4};
+
+  // expected-warning@+1{{variable length array folded to constant array as an extension}}
+  char a8[2][ksize] = {{1,2,3,4},{4,3,2,1}};
+}

clang/test/SemaCXX/vla.cpp

@@ -20,3 +20,9 @@ namespace PR18581 {
 
 void pr23151(int (&)[*]) { // expected-error {{variable length array must be bound in function definition}}
 }
+
+void test_fold() {
+  char a1[(unsigned long)(int *)0+1]{}; // expected-warning{{variable length array folded to constant array as an extension}}
+  char a2[(unsigned long)(int *)0+1] = {}; // expected-warning{{variable length array folded to constant array as an extension}}
+  char a3[(unsigned long)(int *)0+1];
+}

clang/test/SemaObjC/variable-size-ivar.m

@@ -0,0 +1,12 @@
+// RUN: %clang_cc1 -fsyntax-only %s -verify
+
+const int ksize = 42;
+int size = 42;
+
+@interface X
+{
+  int arr1[ksize]; // expected-warning{{variable length array folded to constant array}}
+  int arr2[size]; // expected-error{{instance variables must have a constant size}}
+  int arr3[ksize-43]; // expected-error{{array size is negative}}
+}
+@end