Fix #10699 - std.format with position with omitted second number does not work

std/format/spec.d

@@ -127,14 +127,16 @@ if (is(Unqual!Char == Char))
 
        Counting starts with `1`. Set to `0` if not used. Default: `0`.
      */
-    ubyte indexStart;
+    ushort indexStart;
 
     /**
        Index of the last argument for positional parameter ranges.
 
        Counting starts with `1`. Set to `0` if not used. Default: `0`.
+
+       The maximum value of this field is used as a sentinel to indicate the arguments' length.
     */
-    ubyte indexEnd;
+    ushort indexEnd;
 
     version (StdDdoc)
     {
@@ -851,6 +853,18 @@ if (is(Unqual!Char == Char))
     assert(f.indexStart == 0);
 }
 
+// https://github.com/dlang/phobos/issues/10699
+@safe pure unittest
+{
+    import std.array : appender;
+    auto f = FormatSpec!char("%1:$d");
+    auto w = appender!(char[])();
+
+    f.writeUpToNextSpec(w);
+    assert(f.indexStart == 1);
+    assert(f.indexEnd == ushort.max);
+}
+
 /**
 Helper function that returns a `FormatSpec` for a single format specifier.
 

std/format/write.d

@@ -648,9 +648,16 @@ uint formattedWrite(Writer, Char, Args...)(auto ref Writer w, const scope Char[]
                     break SWITCH;
             }
         default:
-            throw new FormatException(
-                text("Positional specifier %", spec.indexStart, '$', spec.spec,
-                     " index exceeds ", Args.length));
+            if (spec.indexEnd == spec.indexEnd.max)
+                break;
+            else if (spec.indexEnd == spec.indexStart)
+                throw new FormatException(
+                    text("Positional specifier %", spec.indexStart, '$', spec.spec,
+                    " index exceeds ", Args.length));
+            else
+                throw new FormatException(
+                    text("Positional specifier %", spec.indexStart, ":", spec.indexEnd, '$', spec.spec,
+                    " index exceeds ", Args.length));
         }
     }
     return currentArg;
@@ -1199,6 +1206,16 @@ if (isSomeString!(typeof(fmt)))
     formattedWrite(stream, "%s", aa);
 }
 
+// https://github.com/dlang/phobos/issues/10699
+@safe pure unittest
+{
+    import std.array : appender;
+    auto w = appender!(char[])();
+
+    formattedWrite(w, "%1:$d", 1, 2, 3);
+    assert(w.data == "123");
+}
+
 /**
 Formats a value of any type according to a format specifier and
 writes the result to an output range.