allow HSlice bounded by constants of distinct types

When creating heterogenous slices of distinct types, the compiler does
not initialize the internal type's `size` before accessing it.
This then leads to this crash message:
```
compiler/int128.nim(594, 11) `false` masking only implemented for 1, 2, 4 and 8 bytes [AssertionError]
```
This patch initializes the `size` properly, fixing the problem.

Author: etan-status

compiler/semfold.nim

@@ -133,6 +133,7 @@ proc evalOp(m: TMagic, n, a, b, c: PNode; idgen: IdGenerator; g: ModuleGraph): P
   of mCard: result = newIntNodeT(toInt128(nimsets.cardSet(g.config, a)), n, idgen, g)
   of mBitnotI:
     if n.typ.isUnsigned:
+      computeSizeAlign(g.config, n.typ)
       result = newIntNodeT(bitnot(getInt(a)).maskBytes(int(n.typ.size)), n, idgen, g)
     else:
       result = newIntNodeT(bitnot(getInt(a)), n, idgen, g)
@@ -248,21 +249,26 @@ proc evalOp(m: TMagic, n, a, b, c: PNode; idgen: IdGenerator; g: ModuleGraph): P
   of mBitorI, mOr: result = newIntNodeT(bitor(getInt(a), getInt(b)), n, idgen, g)
   of mBitxorI, mXor: result = newIntNodeT(bitxor(getInt(a), getInt(b)), n, idgen, g)
   of mAddU:
+    computeSizeAlign(g.config, n.typ)
     let val = maskBytes(getInt(a) + getInt(b), int(n.typ.size))
     result = newIntNodeT(val, n, idgen, g)
   of mSubU:
+    computeSizeAlign(g.config, n.typ)
     let val = maskBytes(getInt(a) - getInt(b), int(n.typ.size))
     result = newIntNodeT(val, n, idgen, g)
     # echo "subU: ", val, " n: ", n, " result: ", val
   of mMulU:
+    computeSizeAlign(g.config, n.typ)
     let val = maskBytes(getInt(a) * getInt(b), int(n.typ.size))
     result = newIntNodeT(val, n, idgen, g)
   of mModU:
+    computeSizeAlign(g.config, a.typ)
     let argA = maskBytes(getInt(a), int(a.typ.size))
     let argB = maskBytes(getInt(b), int(a.typ.size))
     if argB != Zero:
       result = newIntNodeT(argA mod argB, n, idgen, g)
   of mDivU:
+    computeSizeAlign(g.config, a.typ)
     let argA = maskBytes(getInt(a), int(a.typ.size))
     let argB = maskBytes(getInt(b), int(a.typ.size))
     if argB != Zero:

compiler/sizealignoffsetimpl.nim

@@ -73,7 +73,7 @@ proc finish(arg: var OffsetAccum): int =
     result = align(arg.offset, arg.maxAlign) - arg.offset
     arg.offset += result
 
-proc computeSizeAlign(conf: ConfigRef; typ: PType)
+proc computeSizeAlign*(conf: ConfigRef; typ: PType)
 
 proc computeSubObjectAlign(conf: ConfigRef; n: PNode): BiggestInt =
   ## returns object alignment
@@ -195,7 +195,7 @@ proc computeUnionObjectOffsetsFoldFunction(conf: ConfigRef; n: PNode; packed: bo
     accum.maxAlign = szUnknownSize
     accum.offset = szUnknownSize
 
-proc computeSizeAlign(conf: ConfigRef; typ: PType) =
+proc computeSizeAlign*(conf: ConfigRef; typ: PType) =
   template setSize(typ, s) =
     typ.size = s
     typ.align = s

tests/slice/tdistinct.nim

@@ -0,0 +1,2 @@
+type Foo = distinct uint64
+const slice = 0 ..< 42.Foo