Fix Bugzilla 21564 - Allow assignment syntax for instantiating mixin …

…templates (#16387)

Following the [alias this change](https://dlang.org/changelog/2.105.0.html#dmd.alias-this-syntax),
I think mixin instantiation was the last place not using assignment
syntax.

compiler/src/dmd/parse.d

@@ -1707,19 +1707,28 @@ class Parser(AST, Lexer = dmd.lexer.Lexer) : Lexer
      *      mixin Foo;
      *      mixin Foo!(args);
      *      mixin a.b.c!(args).Foo!(args);
-     *      mixin Foo!(args) identifier;
      *      mixin typeof(expr).identifier!(args);
+     *      mixin Foo!(args) identifier;
+     *      mixin identifier = Foo!(args);
      */
     private AST.Dsymbol parseMixin()
     {
         AST.TemplateMixin tm;
-        Identifier id;
+        Identifier id, name;
         AST.Objects* tiargs;
 
         //printf("parseMixin()\n");
         const locMixin = token.loc;
         nextToken(); // skip 'mixin'
 
+        // mixin Identifier = MixinTemplateName TemplateArguments;
+        if (token.value == TOK.identifier && peekNext() == TOK.assign)
+        {
+            name = token.ident;
+            nextToken();
+            nextToken();
+        }
+
         auto loc = token.loc;
         AST.TypeQualified tqual = null;
         if (token.value == TOK.dot)
@@ -1782,14 +1791,14 @@ class Parser(AST, Lexer = dmd.lexer.Lexer) : Lexer
             nextToken();
         }
 
-        id = null;
-        if (token.value == TOK.identifier)
+        // mixin MixinTemplateName TemplateArguments Identifier;
+        if (!name && token.value == TOK.identifier)
         {
-            id = token.ident;
+            name = token.ident;
             nextToken();
         }
 
-        tm = new AST.TemplateMixin(locMixin, id, tqual, tiargs);
+        tm = new AST.TemplateMixin(locMixin, name, tqual, tiargs);
         if (token.value != TOK.semicolon)
             error("`;` expected after `mixin`");
         nextToken();

compiler/test/runnable/mixin1.d

@@ -100,13 +100,15 @@ template Foo2(T)
 
 mixin Foo2!(uint) B2;
 mixin Foo2!(long) C2;
+mixin D2 = Foo2!(wchar);
 mixin Foo2!(int);
 
 void test2()
 {
     B2.x2 = 3;
     assert(B2.x2 == 3);
     assert(C2.x2 == long.sizeof);
+    assert(D2.x2 == 2);
 //    assert(x2 == int.sizeof);
 }
 
@@ -284,6 +286,9 @@ void test11()
     int y = 8;
     mixin Foo11!(y) B;
     assert(B.abc() == 8);
+
+    mixin C = Foo11!2;
+    assert(C.abc() == 2);
 }
 
 /*******************************************/