Merge pull request #4426 from 9rnsr/fix14210

Issue 14210 - invalid merging of template instances

src/template.c

@@ -1768,11 +1768,15 @@ MATCH TemplateDeclaration::deduceFunctionTemplateMatch(
     for (size_t i = 0; i < dedtypes->dim; i++)
     {
         Type *at = isType((*dedtypes)[i]);
-        if (at && at->ty == Tnone)
+        if (at)
         {
-            TypeDeduced *xt = (TypeDeduced *)at;
-            (*dedtypes)[i] = xt->tded;  // 'unbox'
-            delete xt;
+            if (at->ty == Tnone)
+            {
+                TypeDeduced *xt = (TypeDeduced *)at;
+                at = xt->tded;  // 'unbox'
+                delete xt;
+            }
+            (*dedtypes)[i] = at->merge2();
         }
     }
     for (size_t i = ntargs; i < dedargs->dim; i++)

test/runnable/functype.d

@@ -253,6 +253,26 @@ void test8579()
     static assert(typeof(fn4).stringof == "int function(int i, double j = " ~ (1.0).stringof ~ ")");
 }
 
+/***************************************************/
+// 14210
+
+string foo14210a(DT)(string name, DT dg)
+{
+    return name ~ " :: " ~ typeof(dg).stringof;
+}
+string foo14210b(DT)(string name, DT dg)
+{
+    return name ~ " :: " ~ typeof(dg).stringof;
+}
+void test14210()
+{
+    assert(foo14210a("1", (int a)    => a+0) == "1 :: int function(int) pure nothrow @nogc @safe");
+    assert(foo14210a("2", (int a=40) => a+2) == "2 :: int function(int) pure nothrow @nogc @safe");
+
+    assert(foo14210b("2", (int a=40) => a+2) == "2 :: int function(int) pure nothrow @nogc @safe");
+    assert(foo14210b("1", (int a)    => a+0) == "1 :: int function(int) pure nothrow @nogc @safe");
+}
+
 /***************************************************/
 // 10734
 
@@ -288,6 +308,7 @@ int main()
     test3646();
     test3866();
     test8579();
+    test14210();
 
     printf("Success\n");
     return 0;