std: non-byte-multiple sized integers have no definite representation

lib/std/hash/auto_hash.zig

@@ -99,7 +99,16 @@ pub fn hash(hasher: anytype, key: anytype, comptime strat: HashStrategy) void {
 
         // Help the optimizer see that hashing an int is easy by inlining!
         // TODO Check if the situation is better after #561 is resolved.
-        .Int => @call(.{ .modifier = .always_inline }, hasher.update, .{std.mem.asBytes(&key)}),
+        .Int => {
+            if (comptime meta.trait.hasUniqueRepresentation(Key)) {
+                @call(.{ .modifier = .always_inline }, hasher.update, .{std.mem.asBytes(&key)});
+            } else {
+                // Take only the part containing the key value, the remaining
+                // bytes are undefined and must not be hashed!
+                const byte_size = comptime std.math.divCeil(comptime_int, @bitSizeOf(Key), 8) catch unreachable;
+                @call(.{ .modifier = .always_inline }, hasher.update, .{std.mem.asBytes(&key)[0..byte_size]});
+            }
+        },
 
         .Bool => hash(hasher, @boolToInt(key), strat),
         .Enum => hash(hasher, @enumToInt(key), strat),

lib/std/meta/trait.zig

@@ -476,15 +476,19 @@ pub fn hasUniqueRepresentation(comptime T: type) bool {
         else => return false, // TODO can we know if it's true for some of these types ?
 
         .AnyFrame,
-        .Bool,
         .BoundFn,
         .Enum,
         .ErrorSet,
         .Fn,
-        .Int, // TODO check that it is still true
         .Pointer,
         => return true,
 
+        .Bool => return false,
+
+        // The padding bits are undefined.
+        .Int => |info| return (info.bits % 8) == 0 and
+            (info.bits == 0 or std.math.isPowerOfTwo(info.bits)),
+
         .Array => |info| return comptime hasUniqueRepresentation(info.child),
 
         .Struct => |info| {
@@ -525,14 +529,10 @@ test "std.meta.trait.hasUniqueRepresentation" {
 
     testing.expect(hasUniqueRepresentation(TestStruct3));
 
-    testing.expect(hasUniqueRepresentation(i1));
-    testing.expect(hasUniqueRepresentation(u2));
-    testing.expect(hasUniqueRepresentation(i3));
-    testing.expect(hasUniqueRepresentation(u4));
-    testing.expect(hasUniqueRepresentation(i5));
-    testing.expect(hasUniqueRepresentation(u6));
-    testing.expect(hasUniqueRepresentation(i7));
-    testing.expect(hasUniqueRepresentation(u8));
-    testing.expect(hasUniqueRepresentation(i9));
-    testing.expect(hasUniqueRepresentation(u10));
+    inline for ([_]type{ i0, u8, i16, u32, i64 }) |T| {
+        testing.expect(hasUniqueRepresentation(T));
+    }
+    inline for ([_]type{ i1, u9, i17, u33, i24 }) |T| {
+        testing.expect(!hasUniqueRepresentation(T));
+    }
 }