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);
 }
 
 /*******************************************/