[spec/class] Tweak alias this docs

Make examples runnable.
Split up long example.

Author: ntrel

spec/class.dd

@@ -1035,6 +1035,7 @@ $(GNAME AliasThis):
         member.
         )
 
+$(SPEC_RUNNABLE_EXAMPLE_RUN
 ---
 struct S
 {
@@ -1044,22 +1045,29 @@ struct S
 
 int foo(int i) { return i * 2; }
 
-void test()
+void main()
 {
     S s;
     s.x = 7;
-    int i = -s;  // i == -7
-    i = s + 8;   // i == 15
-    i = s + s;   // i == 14
-    i = 9 + s;   // i == 16
+    int i = -s;
+    assert(i == -7);
+    i = s + 8;
+    assert(i == 15);
+    i = s + s;
+    assert(i == 14);
+    i = 9 + s;
+    assert(i == 16);
     i = foo(s);  // implicit conversion to int
+    assert(i == 14);
 }
 ---
+)
 
         $(P If the member is a class or struct, undefined lookups will
         be forwarded to the $(I AliasThis) member.
         )
 
+$(SPEC_RUNNABLE_EXAMPLE_RUN
 ---
 struct Foo
 {
@@ -1073,19 +1081,23 @@ class Bar
     alias foo this;
 }
 
-void test()
+void main()
 {
     auto bar = new Bar;
-    int i = bar.baz; // i == 4
-    i = bar.get(); // i == 7
+    int i = bar.baz;
+    assert(i == 4);
+    i = bar.get();
+    assert(i == 7);
 }
 ---
+)
 
         $(P If the $(I Identifier) refers to a property member
         function with no parameters, conversions and undefined
         lookups are forwarded to the return value of the function.
         )
 
+$(SPEC_RUNNABLE_EXAMPLE_RUN
 ---
 struct S
 {
@@ -1097,20 +1109,24 @@ struct S
     alias get this;
 }
 
-void test()
+void main()
 {
     S s;
     s.x = 2;
-    int i = s; // i == 4
+    int i = s;
+    assert(i == 4);
 }
 ---
+)
+
         $(P If an aggregate declaration defines an $(D opCmp) or $(D opEquals)
         method, it will take precedence to that of the aliased this member. Note
         that, unlike an $(D opCmp) method, an $(D opEquals) method is implicitly
         defined for a $(D struct) declaration if a user defined one isn't provided;
         this means that if the aliased this member $(D opEquals) is preferred it
         should be explicitly defined:
         )
+$(SPEC_RUNNABLE_EXAMPLE_RUN
 ---
 struct S
 {
@@ -1128,6 +1144,21 @@ struct T
     alias s this;
 }
 
+void main()
+{
+    S s1, s2;
+    T t1, t2;
+
+    assert(s1 == s2);      // calls S.opEquals
+    assert(t1 == t2);      // calls compiler generated T.opEquals that implements member-wise equality
+
+    assert(s1 == t1);      // calls s1.opEquals(t1.s);
+    assert(t1 == s1);      // calls t1.s.opEquals(s1);
+}
+---
+)
+$(SPEC_RUNNABLE_EXAMPLE_RUN
+---
 struct U
 {
     int a;
@@ -1146,24 +1177,19 @@ struct V
 
 void main()
 {
-    S s1, s2;
-    T t1, t2;
     U u1, u2;
     V v1, v2;
 
-    assert(s1 == s2);      // calls S.opEquals
-    assert(t1 == t2);      // calls compiler generated T.opEquals that implements member-wise equality
     assert(!(u1 < u2));    // calls U.opCmp
     assert(!(v1 < v2));    // calls U.opCmp because V does not define an opCmp method
                            // so the alias this of v1 is employed; U.opCmp expects a
                            // paramter of type U, so alias this of v2 is used
 
-    assert(s1 == t1);      // calls s1.opEquals(t1.s);
-    assert(t1 == s1);      // calls t1.s.opEquals(s1);
     assert(!(u1 < v1));    // calls u1.opCmp(v1.u);
     assert(!(v1 < u1));    // calls v1.u.opCmp(v1);
 }
 ---
+)
         $(P $(GLINK2 attribute, Attribute)s are ignored for $(D AliasThis).
         )