Detect an incompatible VLA pointer assignment

[jmorse]

Summary

For pointer assignments of VLA types, Clang currently detects when array
dimensions _lower_ than a variable dimension differ, and reports a warning.
However it does not do the same when the _higher_ dimensions differ, a
case that GCC does catch.

These two pointer types

int (*foo)[1][bar][3];
int (*baz)[1][2][3];

are compatible with each another, and the program is well formed if
bar == 2, a matter that is the programmers problem. However the following:

int (*qux)[2][2][3];

would not be compatible with either, because the upper dimension differs
in size. Clang reports baz is incompatible with qux, but not that foo is
incompatible with qux because it doesn't check those higher dimensions.

Fix this by comparing array sizes on higher dimensions: if both are
constants but unequal then report incompatibility; if either dimension is
variable then we can't know either way.

[efriedma]

lib/AST/ASTContext.cpp

8588

E && E->isIntegerConstantExpr?

8603

The != will hit an assertion failure if LSize and RSize don't have the same bitwidth.

[jmorse]

Avoid un-necessary call, use APInt::isSameValue rather than operator!=

[jmorse]

lib/AST/ASTContext.cpp

8588

Done

8603

Erk -- replaced with APInt::isSameValue.

[efriedma]

LGTM

lib/AST/ASTContext.cpp

case Type::ConstantArray:
     if (Unqualified) {
       LHSElem = LHSElem.getUnqualifiedType();
       RHSElem = RHSElem.getUnqualifiedType();
     }
     
     QualType ResultType = mergeTypes(LHSElem, RHSElem, false, Unqualified);
     if (ResultType.isNull())
       return {};
+
+    const VariableArrayType* LVAT = getAsVariableArrayType(LHS);
+    const VariableArrayType* RVAT = getAsVariableArrayType(RHS);
+
+    // If either side is a variable array, and both are complete, check whether
+    // the current dimension is definite.
+    if (LVAT || RVAT) {
+      auto SizeFetch = [this](const VariableArrayType* VAT,
+          const ConstantArrayType* CAT)
+          -> std::pair<bool,llvm::APInt> {
+        if (VAT) {
+          llvm::APSInt TheInt;
+          Expr *E = VAT->getSizeExpr();
+          if (E && E->isIntegerConstantExpr(TheInt, *this))
+            return std::make_pair(true, TheInt);
+          else
+            return std::make_pair(false, TheInt);
+        } else if (CAT) {
+            return std::make_pair(true, CAT->getSize());
+        } else {
+            return std::make_pair(false, llvm::APInt());
+        }
+      };
+
+      bool HaveLSize, HaveRSize;
+      llvm::APInt LSize, RSize;
+      std::tie(HaveLSize, LSize) = SizeFetch(LVAT, LCAT);
+      std::tie(HaveRSize, RSize) = SizeFetch(RVAT, RCAT);
+      if (HaveLSize && HaveRSize && !llvm::APInt::isSameValue(LSize, RSize))
+        return {}; // Definite, but unequal, array dimension
+    }
+
     if (LCAT && getCanonicalType(LHSElem) == getCanonicalType(ResultType))
       return LHS;
     if (RCAT && getCanonicalType(RHSElem) == getCanonicalType(ResultType))
       return RHS;
     if (LCAT) return getConstantArrayType(ResultType, LCAT->getSize(),
                                           ArrayType::ArraySizeModifier(), 0);
     if (RCAT) return getConstantArrayType(ResultType, RCAT->getSize(),
                                           ArrayType::ArraySizeModifier(), 0);
-    const VariableArrayType* LVAT = getAsVariableArrayType(LHS);
-    const VariableArrayType* RVAT = getAsVariableArrayType(RHS);
     if (LVAT && getCanonicalType(LHSElem) == getCanonicalType(ResultType))
       return LHS;
     if (RVAT && getCanonicalType(RHSElem) == getCanonicalType(ResultType))
       return RHS;
     if (LVAT) {
       // FIXME: This isn't correct! But tricky to implement because
       // the array's size has to be the size of LHS, but the type
       // has to be different.

test/Sema/vla.c

 
 // PR36157
 struct {
   int a[ // expected-error {{variable length array in struct}}
     implicitly_declared() // expected-warning {{implicit declaration}}
   ];
 };
 int (*use_implicitly_declared)() = implicitly_declared; // ok, was implicitly declared at file scope
+
+void VLAPtrAssign(int size) {
+  int array[1][2][3][size][4][5];
+  // This is well formed
+  int (*p)[2][3][size][4][5] = array;
+  // Last array dimension too large
+  int (*p2)[2][3][size][4][6] = array; // expected-warning {{incompatible pointer types}}
+  // Second array dimension too large
+  int (*p3)[20][3][size][4][5] = array; // expected-warning {{incompatible pointer types}}
+
+  // Not illegal in C, program _might_ be well formed if size == 3.
+  int (*p4)[2][size][3][4][5] = array;
+}