Fix Issue 8687 - Variadic templates do not work properly with default…

… arguments

changelog/default_after_variadic.dd

@@ -0,0 +1,17 @@
+Function parameters with default values are now allowed after variadic template parameters, and always take their default values. This allows using special tokens (eg __FILE__) after variadic parameters, which was previously impossible
+
+Eg:
+---
+string log(T...)(T a, string file = __FILE__, int line = __LINE__)
+{
+  return text(file, ":", line, " ", a);
+}
+
+assert(log(10, "abc") == text(__FILE__, ":", __LINE__, " 10abc"));
+---
+
+This should be preferred to the previous workaround, which caused template bloat:
+---
+string log(string file = __FILE__, int line = __LINE__, T...)(T a);
+---
+

src/dmd/dtemplate.d

@@ -1342,13 +1342,17 @@ extern (C++) final class TemplateDeclaration : ScopeDsymbol
                         declaredTuple = new Tuple();
                         (*dedargs)[parameters.dim - 1] = declaredTuple;
 
-                        /* Count function parameters following a tuple parameter.
-                         * void foo(U, T...)(int y, T, U, int) {}  // rem == 2 (U, int)
+                        /* Count function parameters with no defaults following a tuple parameter.
+                         * void foo(U, T...)(int y, T, U, double, int bar = 0) {}  // rem == 2 (U, double)
                          */
                         size_t rem = 0;
                         for (size_t j = parami + 1; j < nfparams; j++)
                         {
                             Parameter p = Parameter.getNth(fparameters, j);
+                            if(p.defaultArg)
+                            {
+                               break;
+                            }
                             if (!reliesOnTident(p.type, parameters, inferStart))
                             {
                                 Type pt = p.type.syntaxCopy().typeSemantic(fd.loc, paramscope);

test/runnable/testdefault_after_variadic.d

@@ -0,0 +1,62 @@
+/*
+PERMUTE_ARGS:
+*/
+
+import std.typecons : tuple;
+import std.conv : text;
+
+void fun0(U, T...)(U gold, int b_gold, T a, int b)
+{
+    assert(tuple(a) == gold);
+    assert(b == b_gold);
+}
+
+void fun(U, T...)(U gold, T a, int b = 1)
+{
+    assert(tuple(a) == gold);
+    assert(b == 1);
+}
+
+void fun2(U, V, T...)(U gold, V gold2, T a, string file = __FILE__, int line = __LINE__)
+{
+    assert(tuple(a) == gold);
+    assert(tuple(file, line) == gold2);
+}
+
+// 
+void fun3(int[] gold, int[] a...)
+{
+    assert(gold == a);
+}
+
+/+
+NOTE: this is disallowed by the parser:
+
+void fun4(int[] gold, int[] a ..., int b = 1)
+{
+    assert(gold==a);
+    assert(b==1);
+}
++/
+
+// Example in changelog
+string log(T...)(T a, string file = __FILE__, int line = __LINE__)
+{
+    return text(file, ":", line, " ", a);
+}
+
+void main()
+{
+    fun0(tuple(10), 7, 10, 7);
+
+    fun(tuple());
+    fun(tuple(10), 10);
+    fun(tuple(10, 11), 10, 11);
+
+    fun2(tuple(10), tuple(__FILE__, __LINE__), 10);
+
+    fun3([1, 2, 3], 1, 2, 3);
+    // fun4([1,2,3], 1,2,3);
+
+    assert(log(10, "abc") == text(__FILE__, ":", __LINE__, " 10abc"));
+}