fix bugzilla 21995 Struct with size uint.max or greater causes ICE

compiler/src/dmd/aggregate.d

@@ -793,6 +793,7 @@ public uint alignmember(structalign_t alignment, uint memalignsize, uint offset)
 /****************************************
  * Place a field (mem) into an aggregate (agg), which can be a struct, union or class
  * Params:
+ *    loc           = source location for error messages
  *    nextoffset    = location just past the end of the previous field in the aggregate.
  *                    Updated to be just past the end of this field to be placed, i.e. the future nextoffset
  *    memsize       = size of field
@@ -805,8 +806,8 @@ public uint alignmember(structalign_t alignment, uint memalignsize, uint offset)
  *    aligned offset to place field at
  *
  */
-public uint placeField(ref uint nextoffset, uint memsize, uint memalignsize,
-    structalign_t alignment, ref uint aggsize, ref uint aggalignsize, bool isunion) @safe pure nothrow
+public uint placeField(Loc loc, ref uint nextoffset, uint memsize, uint memalignsize,
+    structalign_t alignment, ref uint aggsize, ref uint aggalignsize, bool isunion) @trusted nothrow
 {
     static if (0)
     {
@@ -829,7 +830,12 @@ public uint placeField(ref uint nextoffset, uint memsize, uint memalignsize,
     bool overflow;
     const sz = addu(memsize, actualAlignment, overflow);
     addu(ofs, sz, overflow);
-    if (overflow) assert(0);
+    if (overflow)
+    {
+        error(loc, "max object size %u exceeded from adding field size %u + alignment adjustment %u + field offset %u when placing field in aggregate",
+                uint.max, memsize, actualAlignment, ofs);
+        return 0;
+    }
 
     // Skip no-op for noreturn without custom aligment
     if (memalignsize != 0 || !alignment.isDefault())

compiler/src/dmd/dsymbolsem.d

@@ -7126,7 +7126,7 @@ private extern(C++) class SetFieldOffsetVisitor : Visitor
         assert(sz != SIZE_INVALID && sz < uint.max);
         uint memsize = cast(uint)sz;                // size of member
         uint memalignsize = target.fieldalign(t);   // size of member for alignment purposes
-        vd.offset = placeField(
+        vd.offset = placeField(vd.loc,
             fieldState.offset,
             memsize, memalignsize, vd.alignment,
             ad.structsize, ad.alignsize,
@@ -7193,7 +7193,7 @@ private extern(C++) class SetFieldOffsetVisitor : Visitor
                 alignsize = memsize; // not memalignsize
 
             uint dummy;
-            bfd.offset = placeField(
+            bfd.offset = placeField(bfd.loc,
                 fieldState.offset,
                 memsize, alignsize, bfd.alignment,
                 ad.structsize,
@@ -7395,7 +7395,7 @@ private extern(C++) class SetFieldOffsetVisitor : Visitor
             /* Given the anon 'member's size and alignment,
              * go ahead and place it.
              */
-            anond.anonoffset = placeField(
+            anond.anonoffset = placeField(anond.loc,
                 fieldState.offset,
                 anond.anonstructsize, anond.anonalignsize, alignment,
                 ad.structsize, ad.alignsize,

compiler/test/fail_compilation/test21995.d

@@ -0,0 +1,11 @@
+/* TEST_OUTPUT:
+---
+fail_compilation/test21995.d(10): Error: max object size 4294967295 exceeded from adding field size 3 + alignment adjustment 1 + field offset 4294967292 when placing field in aggregate
+---
+*/
+struct S
+{
+    ubyte[0x7ffffffe] a;
+    ubyte[0x7ffffffe] b;
+    ubyte[3] c;
+}