Fix issue 15653 - IFTI fails for immutable parameter

src/dmd/dtemplate.d

@@ -4324,6 +4324,16 @@ MATCH deduceType(RootObject o, Scope* sc, Type tparam, TemplateParameters* param
                 }
             }
 
+            /* Returns `true` if `t` is a reference type, or an array of reference types
+             */
+            bool isTopRef(Type t)
+            {
+                auto tb = t.baseElemOf();
+                return tb.ty == Tclass ||
+                       tb.ty == Taarray ||
+                       tb.ty == Tstruct && tb.hasPointers();
+            }
+
             Type at = cast(Type)(*dedtypes)[i];
             Type tt;
             if (ubyte wx = deduceWildHelper(e.type, &tt, tparam))
@@ -4335,6 +4345,16 @@ MATCH deduceType(RootObject o, Scope* sc, Type tparam, TemplateParameters* param
             {
                 result = m;
             }
+            else if (!isTopRef(e.type))
+            {
+                /* Bugzilla 15653, In IFTI, recognize top-qualifier conversions
+                 * through the value copy, e.g.
+                 *      int --> immutable(int)
+                 *      immutable(string[]) --> immutable(string)[]
+                 */
+                tt = e.type.mutableOf();
+                result = MATCH.convert;
+            }
             else
                 return; // nomatch
 

test/runnable/template9.d

@@ -4927,6 +4927,56 @@ void test15243()
     s3.apply3(&f3);
 }
 
+/******************************************/
+// 15653
+
+alias TypeTuple15653(T...) = T;
+
+void test15653()
+{
+    void foo(U, T)(const T x)     { static assert(is(T == U)); }
+    void bar(U, T)(immutable T x) { static assert(is(T == U)); }
+
+    struct X { int a; long[2] b; }
+    struct Y { int* a; long[] b; }
+
+    foreach (U; TypeTuple15653!( byte,    short,   int,  long,
+                                ubyte,   ushort,  uint, ulong,
+                                 float,  double,  real,
+                                ifloat, idouble, ireal,
+                                cfloat, cdouble, creal,
+                                void delegate(),
+                                int[2], X, X[2]))
+    {
+        foo!U(U.init);      // OK
+        bar!U(U.init);      // Was error, now OK
+
+        U u;
+        foo!U(u);           // OK
+        bar!U(u);           // Was error, now OK
+    }
+
+    foreach (U; TypeTuple15653!(void*, int**, long[], double*[2]))
+    {
+        foo!U(U.init);      // OK
+        bar!U(U.init);      // Was error, now OK
+
+        U u;
+        foo!U(u);
+        static assert(!__traits(compiles, bar!U(u)), U.stringof);
+    }
+
+    foreach (U; TypeTuple15653!(Object, Y, Y[2], int[int]))
+    {
+        foo!U(U.init);      // OK
+        static assert(!__traits(compiles, bar!U(U.init)), U.stringof);
+
+        U u;
+        foo!U(u);           // OK
+        static assert(!__traits(compiles, bar!U(u)), U.stringof);
+    }
+}
+
 /******************************************/
 
 int main()
@@ -5044,6 +5094,7 @@ int main()
     test16042();
     test16042b();
     test15243();
+    test15653();
 
     printf("Success\n");
     return 0;