diff --git a/src/mtype.c b/src/mtype.c
index 86de838a3c1c..1b59d457b875 100644
--- a/src/mtype.c
+++ b/src/mtype.c
@@ -1055,7 +1055,7 @@ StorageClass ModToStc(unsigned mod)
  * Apply MODxxxx bits to existing type.
  */
 
-Type *Type::castMod(unsigned mod)
+Type *Type::castMod(MOD mod)
 {   Type *t;
 
     switch (mod)
@@ -1108,7 +1108,7 @@ Type *Type::castMod(unsigned mod)
  * a shared type => "shared const"
  */
 
-Type *Type::addMod(unsigned mod)
+Type *Type::addMod(MOD mod)
 {
     /* Add anything to immutable, and it remains immutable
      */
@@ -1387,10 +1387,10 @@ Type *Type::toBasetype()
 /***************************
  * Return !=0 if modfrom can be implicitly converted to modto
  */
-int MODimplicitConv(unsigned char modfrom, unsigned char modto)
+bool MODimplicitConv(MOD modfrom, MOD modto)
 {
     if (modfrom == modto)
-        return 1;
+        return true;
 
     //printf("MODimplicitConv(from = %x, to = %x)\n", modfrom, modto);
     #define X(m, n) (((m) << 4) | (n))
@@ -1404,10 +1404,10 @@ int MODimplicitConv(unsigned char modfrom, unsigned char modto)
 
         case X(MODimmutable, MODconst):
         case X(MODimmutable, MODwildconst):
-            return 1;
+            return true;
 
         default:
-            return 0;
+            return false;
     }
     #undef X
 }
@@ -1415,10 +1415,10 @@ int MODimplicitConv(unsigned char modfrom, unsigned char modto)
 /***************************
  * Return !=0 if a method of type '() modfrom' can call a method of type '() modto'.
  */
-int MODmethodConv(unsigned char modfrom, unsigned char modto)
+bool MODmethodConv(MOD modfrom, MOD modto)
 {
     if (MODimplicitConv(modfrom, modto))
-        return 1;
+        return true;
 
     #define X(m, n) (((m) << 4) | (n))
     switch (X(modfrom, modto))
@@ -1429,10 +1429,10 @@ int MODmethodConv(unsigned char modfrom, unsigned char modto)
         case X(MODshared, MODshared|MODwild):
         case X(MODshared|MODimmutable, MODshared|MODwild):
         case X(MODshared|MODconst, MODshared|MODwild):
-            return 1;
+            return true;
 
         default:
-            return 0;
+            return false;
     }
     #undef X
 }
@@ -1440,13 +1440,13 @@ int MODmethodConv(unsigned char modfrom, unsigned char modto)
 /***************************
  * Merge mod bits to form common mod.
  */
-unsigned char MODmerge(unsigned char mod1, unsigned char mod2)
+MOD MODmerge(MOD mod1, MOD mod2)
 {
     if (mod1 == mod2)
         return mod1;
 
     //printf("MODmerge(1 = %x, 2 = %x)\n", mod1, mod2);
-    unsigned char result = 0;
+    MOD result = 0;
     if ((mod1 | mod2) & MODshared)
     {
         // If either type is shared, the result will be shared
@@ -1474,7 +1474,7 @@ unsigned char MODmerge(unsigned char mod1, unsigned char mod2)
 /*********************************
  * Mangling for mod.
  */
-void MODtoDecoBuffer(OutBuffer *buf, unsigned char mod)
+void MODtoDecoBuffer(OutBuffer *buf, MOD mod)
 {
     switch (mod)
     {   case 0:
@@ -1511,7 +1511,7 @@ void MODtoDecoBuffer(OutBuffer *buf, unsigned char mod)
 /*********************************
  * Store modifier name into buf.
  */
-void MODtoBuffer(OutBuffer *buf, unsigned char mod)
+void MODtoBuffer(OutBuffer *buf, MOD mod)
 {
     switch (mod)
     {
@@ -1561,7 +1561,7 @@ void MODtoBuffer(OutBuffer *buf, unsigned char mod)
 /*********************************
  * Return modifier name.
  */
-char *MODtoChars(unsigned char mod)
+char *MODtoChars(MOD mod)
 {
     OutBuffer buf;
     buf.reserve(16);
diff --git a/src/mtype.h b/src/mtype.h
index 32b39375492b..163bfdc79c77 100644
--- a/src/mtype.h
+++ b/src/mtype.h
@@ -115,20 +115,26 @@ extern int Tsize_t;
 extern int Tptrdiff_t;
 
 
-/* pick this order of numbers so switch statements work better
+/**
+ * type modifiers
+ * pick this order of numbers so switch statements work better
  */
-#define MODconst     1  // type is const
-#define MODimmutable 4  // type is immutable
-#define MODshared    2  // type is shared
-#define MODwild      8  // type is wild
-#define MODwildconst (MODwild | MODconst)  // type is wild const
-#define MODmutable   0x10       // type is mutable (only used in wildcard matching)
+enum MODFlags
+{
+    MODconst     = 1, // type is const
+    MODimmutable = 4, // type is immutable
+    MODshared    = 2, // type is shared
+    MODwild      = 8, // type is wild
+    MODwildconst = (MODwild | MODconst), // type is wild const
+    MODmutable   = 0x10       // type is mutable (only used in wildcard matching)
+};
+typedef unsigned char MOD;
 
 class Type : public RootObject
 {
 public:
     TY ty;
-    unsigned char mod;  // modifiers MODxxxx
+    MOD mod;  // modifiers MODxxxx
     char *deco;
 
     /* These are cached values that are lazily evaluated by constOf(), immutableOf(), etc.
@@ -296,8 +302,8 @@ class Type : public RootObject
     void fixTo(Type *t);
     void check();
     Type *addSTC(StorageClass stc);
-    Type *castMod(unsigned mod);
-    Type *addMod(unsigned mod);
+    Type *castMod(MOD mod);
+    Type *addMod(MOD mod);
     virtual Type *addStorageClass(StorageClass stc);
     Type *pointerTo();
     Type *referenceTo();
@@ -1014,11 +1020,11 @@ class Parameter : public RootObject
 
 int arrayTypeCompatible(Loc loc, Type *t1, Type *t2);
 int arrayTypeCompatibleWithoutCasting(Loc loc, Type *t1, Type *t2);
-void MODtoBuffer(OutBuffer *buf, unsigned char mod);
-char *MODtoChars(unsigned char mod);
-int MODimplicitConv(unsigned char modfrom, unsigned char modto);
-int MODmethodConv(unsigned char modfrom, unsigned char modto);
-unsigned char MODmerge(unsigned char mod1, unsigned char mod2);
+void MODtoBuffer(OutBuffer *buf, MOD mod);
+char *MODtoChars(MOD mod);
+bool MODimplicitConv(MOD modfrom, MOD modto);
+bool MODmethodConv(MOD modfrom, MOD modto);
+MOD MODmerge(MOD mod1, MOD mod2);
 void identifierToDocBuffer(Identifier* ident, OutBuffer *buf, HdrGenState *hgs);
 
 #endif /* DMD_MTYPE_H */
diff --git a/src/opover.c b/src/opover.c
index a3a53d1f6cf9..2e2c55a926cd 100644
--- a/src/opover.c
+++ b/src/opover.c
@@ -1704,7 +1704,7 @@ static Dsymbol *inferApplyArgTypesX(Expression *ethis, FuncDeclaration *fstart,
   struct ParamOpOver
   {
     Parameters *arguments;
-    unsigned char mod;
+    MOD mod;
     MATCH match;
     FuncDeclaration *fd_best;
     FuncDeclaration *fd_ambig;