Index: lib/Transforms/InstCombine/InstCombineCasts.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -1915,6 +1915,11 @@
       if (Instruction *V = PromoteCastOfAllocation(CI, *AI))
         return V;
 
+    Type *PointeeType =
+        cast<PointerType>(Src->getType()->getScalarType())->getElementType();
+    if (!PointeeType->isSized())
+      return nullptr;
+
     // If the source and destination are pointers, and this cast is equivalent
     // to a getelementptr X, 0, 0, 0...  turn it into the appropriate gep.
     // This can enhance SROA and other transforms that want type-safe pointers.
Index: test/Transforms/InstCombine/opaque.ll
===================================================================
--- /dev/null
+++ test/Transforms/InstCombine/opaque.ll
@@ -0,0 +1,32 @@
+; RUN: opt < %s -instcombine -disable-output
+; Checks that bitcasts are not converted into GEP when
+; when the size of an aggregate cannot be determined.
+%swift.opaque = type opaque
+%SQ = type <{ [8 x i8] }>
+%Si = type <{ i64 }>
+
+%V = type <{ <{ %Vs4Int8, %Vs4Int8, %Vs4Int8, %Vs4Int8, %Vs4Int8, %Vs4Int8, %Vs4Int8, %Vs4Int8 }>, %Si, %SQ, %SQ, %Si, %swift.opaque }>
+%Vs4Int8 = type <{ i8 }>
+%swift.type = type { i64 }
+
+declare void @llvm.memcpy.p0i8.p0i8.i64(i8* nocapture, i8* nocapture readonly,
+i64, i32, i1) #8
+
+@_swift_slowAlloc = external global i8* (i64, i64)*
+
+declare i8* @rt_swift_slowAlloc(i64, i64)
+
+define  %swift.opaque* @_TwTkV([24 x i8]* %dest, %swift.opaque* %src,
+%swift.type* %bios_boot_params) #0 {
+entry:
+  %0 = bitcast %swift.opaque* %src to %V*
+  %1 = call noalias i8* @rt_swift_slowAlloc(i64 40, i64 0) #11
+  %2 = bitcast [24 x i8]* %dest to i8**
+  store i8* %1, i8** %2, align 8
+  %3 = bitcast i8* %1 to %V*
+  %4 = bitcast %V* %3 to i8*
+  %5 = bitcast %V* %0 to i8*
+  call void @llvm.memcpy.p0i8.p0i8.i64(i8* %4, i8* %5, i64 40, i32 1, i1 false)
+  %6 = bitcast %V* %3 to %swift.opaque*
+  ret %swift.opaque* %6
+}