Allow default arguments in the middle (#14879)

compiler/src/dmd/dsymbolsem.d

@@ -4146,9 +4146,8 @@ private extern(C++) final class DsymbolSemanticVisitor : Visitor
             else if (dim == 0 && tf.parameterList.varargs != VarArg.none) // allow varargs only ctor
             {
             }
-            else if (dim && tf.parameterList[0].defaultArg)
+            else if (dim && !tf.parameterList.hasArgsWithoutDefault)
             {
-                // if the first parameter has a default argument, then the rest does as well
                 if (ctd.storage_class & STC.disable)
                 {
                     ctd.error("is marked `@disable`, so it cannot have default "~

compiler/src/dmd/mtype.d

@@ -4683,7 +4683,7 @@ extern (C++) final class TypeFunction : TypeNext
         }
 
         // https://issues.dlang.org/show_bug.cgi?id=22997
-        if (parameterList.varargs == VarArg.none && nparams > argumentList.length && !parameterList[argumentList.length].defaultArg)
+        if (parameterList.varargs == VarArg.none && nparams > argumentList.length && !parameterList.hasDefaultArgs)
         {
             OutBuffer buf;
             buf.printf("too few arguments, expected %d, got %d", cast(int)nparams, cast(int)argumentList.length);
@@ -6631,6 +6631,28 @@ extern (C++) struct ParameterList
         // Ensure no remaining parameters in `other`
         return !diff && other[idx] is null;
     }
+
+    /// Returns: `true` if any parameter has a default argument
+    extern(D) bool hasDefaultArgs()
+    {
+        foreach (oidx, oparam, eidx, eparam; this)
+        {
+            if (eparam.defaultArg)
+                return true;
+        }
+        return false;
+    }
+
+    // Returns: `true` if any parameter doesn't have a default argument
+    extern(D) bool hasArgsWithoutDefault()
+    {
+        foreach (oidx, oparam, eidx, eparam; this)
+        {
+            if (!eparam.defaultArg)
+                return true;
+        }
+        return false;
+    }
 }
 
 

compiler/src/dmd/typesem.d

@@ -1333,7 +1333,6 @@ extern(C++) Type typeSemantic(Type type, const ref Loc loc, Scope* sc)
             // extended index), as we need to run semantic when `oidx` changes.
             size_t tupleOrigIdx = size_t.max;
             size_t tupleExtIdx = size_t.max;
-            bool hasDefault;
             foreach (oidx, oparam, eidx, eparam; tf.parameterList)
             {
                 // oparam (original param) will always have the default arg
@@ -1342,7 +1341,6 @@ extern(C++) Type typeSemantic(Type type, const ref Loc loc, Scope* sc)
                 // position to get the offset in it later on.
                 if (oparam.defaultArg)
                 {
-                    hasDefault = true;
                     // Get the obvious case out of the way
                     if (oparam is eparam)
                         errors |= !defaultArgSemantic(eparam, argsc);
@@ -1369,11 +1367,6 @@ extern(C++) Type typeSemantic(Type type, const ref Loc loc, Scope* sc)
                             eparam.defaultArg = (*te.exps)[eidx - tupleExtIdx];
                     }
                 }
-                else if (hasDefault)
-                {
-                    .error(loc, "default argument expected for `%s`", oparam.toChars());
-                    errors = true;
-                }
 
                 // We need to know the default argument to resolve `auto ref`,
                 // hence why this has to take place as the very last step.

compiler/test/compilable/named_arguments.d

@@ -12,6 +12,18 @@ static assert(fun(y: "y",    "z", x: "x") == "xyzW");
 static assert(fun(   "x",    "y", w: "w") == "xyZw");
 static assert(fun(x: "x",    "y", z: "z") == "xyzW");
 
+// Default arguments need not all be at the end anymore
+string fun2(string x = "x", string y, string z = "z")
+{
+	return x ~ y ~ z;
+}
+
+static assert(fun2(y: "y") == "xyz");
+
+// The assumption that first parameter having a default implies all parameters have a default is no longer valid,
+// so this struct constructor shouldn't be mistaken for a default constructor.
+struct SD { this(int x = 1, int y) { } }
+
 // UFCS
 static assert("x".fun("y", w: "w") == "xyZw");
 

compiler/test/fail_compilation/diag3438.d

@@ -7,7 +7,7 @@ fail_compilation/diag3438.d(20): Error: constructor `diag3438.F5.this` is marked
 fail_compilation/diag3438.d(20):        Use `@disable this();` if you want to disable default initialization.
 fail_compilation/diag3438.d(21): Error: constructor `diag3438.F6.this` is marked `@disable`, so it cannot have default arguments for all parameters.
 fail_compilation/diag3438.d(21):        Use `@disable this();` if you want to disable default initialization.
-fail_compilation/diag3438.d(24): Error: default argument expected for `y`
+fail_compilation/diag3438.d(22): Error: constructor `diag3438.F7.this` all parameters have default arguments, but structs cannot have default constructors.
 ---
 */
 
@@ -19,6 +19,4 @@ struct F3 { this(...) { } } // ok
 struct F4 { this(int[] x...) { } }  // ok
 struct F5 { @disable this(int x = 1); }
 struct F6 { @disable this(int x = 1) { } }
-
-// Make sure the deprecation doesn't interfere w/ the check for default arguments
-struct S { this(int x = 1, int y) { } }
+struct F7 { this(int x = 1, int y = 2) { } }