[CodeGen] Fix the type of the constant that is used to zero-initialize a

flexible array member

A zero-element array type was incorrectly being used when an incomplete
array was being initialized with a non-empty initializer.

This fixes an assertion failure in AddInitializerToStaticVarDecl. See
the discussion here: https://reviews.llvm.org/D123649#4362210

Differential Revision: https://reviews.llvm.org/D151172

clang/lib/CodeGen/CGExprConstant.cpp

@@ -2189,6 +2189,11 @@ llvm::Constant *ConstantEmitter::tryEmitPrivate(const APValue &Value,
 
     llvm::ArrayType *Desired =
         cast<llvm::ArrayType>(CGM.getTypes().ConvertType(DestType));
+
+    // Fix the type of incomplete arrays if the initializer isn't empty.
+    if (DestType->isIncompleteArrayType() && !Elts.empty())
+      Desired = llvm::ArrayType::get(Desired->getElementType(), Elts.size());
+
     return EmitArrayConstant(CGM, Desired, CommonElementType, NumElements, Elts,
                              Filler);
   }

clang/test/CodeGenCXX/flexible-array-init.cpp

@@ -23,3 +23,14 @@ void g() {
 struct B { int x; char y; char z[]; };
 B e = {f(), f(), f(), f()}; // expected-error {{cannot compile this flexible array initializer yet}}
 #endif
+
+namespace zero_initializer {
+A a0{0, 0}, a1{0, {0, 0}};
+// CHECK: @_ZN16zero_initializer2a0E = global { i32, [1 x i32] } zeroinitializer,
+// CHECK: @_ZN16zero_initializer2a1E = global { i32, [2 x i32] } zeroinitializer,
+
+struct S { int x[]; };
+
+S s{0};
+// CHECK: @_ZN16zero_initializer1sE = global { [1 x i32] } zeroinitializer,
+}