Fix Issue 15243 - rejects-valid on variadic

changelog/fix15243_PR7403.dd

@@ -0,0 +1,31 @@
+Variadic template arguments no longer require `alias` workaround
+
+Prior to this release, the following code would not compile.
+
+----
+class C(Types...)
+{
+    void apply(U)(U delegate(Types[0]) f0) { }  // Error: cannot deduce function from argument types
+}
+
+void test()
+{
+    C!int c;
+
+    int f(int) { return 0; }
+
+    c.apply(&f);
+}
+----
+
+It could be worked around by `alias`ing the individual template arguments.
+
+----
+class C(Types...)
+{
+    alias Types[0] T0;
+    void apply(U)(U delegate(T0) f0) { }  // OK
+}
+----
+
+Starting with this release, the workaround is no longer required.

src/ddmd/dtemplate.d

@@ -3601,20 +3601,32 @@ MATCH deduceType(RootObject o, Scope* sc, Type tparam, TemplateParameters* param
                     return;
                 }
 
-                size_t nfargs = Parameter.dim(t.parameters);
-                size_t nfparams = Parameter.dim(tp.parameters);
-
-                // https://issues.dlang.org/show_bug.cgi?id=2579
-                // Apply function parameter storage classes to parameter types
-                for (size_t i = 0; i < nfparams; i++)
+                foreach (fparam; *tp.parameters)
                 {
-                    Parameter fparam = Parameter.getNth(tp.parameters, i);
+                    // https://issues.dlang.org/show_bug.cgi?id=2579
+                    // Apply function parameter storage classes to parameter types
                     fparam.type = fparam.type.addStorageClass(fparam.storageClass);
                     fparam.storageClass &= ~(STC_TYPECTOR | STCin);
+
+                    // https://issues.dlang.org/show_bug.cgi?id=15243
+                    // Resolve parameter type if it's not related with template parameters
+                    if (!reliesOnTident(fparam.type, parameters, inferStart))
+                    {
+                        auto tx = fparam.type.typeSemantic(Loc(), sc);
+                        if (tx.ty == Terror)
+                        {
+                            result = MATCH.nomatch;
+                            return;
+                        }
+                        fparam.type = tx;
+                    }
                 }
                 //printf("\t. this   = %d, ", t.ty); t.print();
                 //printf("\t. tparam = %d, ", tparam.ty); tparam.print();
 
+                size_t nfargs = Parameter.dim(t.parameters);
+                size_t nfparams = Parameter.dim(tp.parameters);
+
                 /* See if tuple match
                  */
                 if (nfparams > 0 && nfargs >= nfparams - 1)

test/runnable/template9.d

@@ -4848,6 +4848,35 @@ void test15781()
     static assert(is(typeof(foo(cs, cs)) == const S));
 }
 
+/******************************************/
+// https://issues.dlang.org/show_bug.cgi?id=15243
+
+struct S15243(Types...)
+{
+    void apply1(U)(U delegate(Types[0]) f0) {}
+
+    void apply2(U)(U delegate(Types) f0) {}
+
+    void apply3(U)(U delegate(Types[1..$]) f0) {}
+}
+
+void test15243()
+{
+    int f1(int) { return 0; }
+    int f2(int, long) { return 0; }
+    int f3(long, string) { return 0; }
+
+    S15243!(int) s1;
+    s1.apply1(&f1);
+    s1.apply2(&f1);
+
+    S15243!(int, long) s2;
+    s2.apply2(&f2);
+
+    S15243!(int, long, string) s3;
+    s3.apply3(&f3);
+}
+
 /******************************************/
 
 int main()
@@ -4962,6 +4991,7 @@ int main()
     test14735();
     test14802();
     test15116();
+    test15243();
 
     printf("Success\n");
     return 0;