Fix #10713 - std.format string positions affect all further format specifiers (#10714)

Author: rikkimax

phobos/std/format/spec.d

@@ -296,6 +296,8 @@ if (is(Unqual!Char == Char))
         }
 
         width = 0;
+        indexStart = 0;
+        indexEnd = 0;
         precision = UNSPECIFIED;
         nested = null;
         // Parse the spec (we assume we're past '%' already)
@@ -834,6 +836,21 @@ if (is(Unqual!Char == Char))
            == "$ expected after '*10' in format string");
 }
 
+// https://github.com/dlang/phobos/issues/10713
+@safe pure unittest
+{
+    import std.array : appender;
+    auto f = FormatSpec!char("%3$d%d");
+
+    auto w = appender!(char[])();
+    f.writeUpToNextSpec(w);
+    assert(f.indexStart == 3);
+
+    f.writeUpToNextSpec(w);
+    assert(w.data.length == 0);
+    assert(f.indexStart == 0);
+}
+
 /**
 Helper function that returns a `FormatSpec` for a single format specifier.