fix: requesting layout.form should not touch shape (#3189)

src/awkward/_nplikes/typetracer.py

@@ -320,6 +320,12 @@ def shape(self) -> tuple[ShapeItem, ...]:
         self.touch_shape()
         return self._shape
 
+    @property
+    def inner_shape(self) -> tuple[ShapeItem, ...]:
+        if len(self._shape) > 1:
+            self.touch_shape()
+        return self._shape[1:]
+
     @property
     def form_key(self) -> str | None:
         return self._form_key
@@ -1332,8 +1338,8 @@ def concat(self, arrays, *, axis: int | None = 0) -> TypeTracerArray:
         for x in arrays:
             assert isinstance(x, TypeTracerArray)
             if inner_shape is None:
-                inner_shape = x.shape[1:]
-            elif inner_shape != x.shape[1:]:
+                inner_shape = x.inner_shape
+            elif inner_shape != x.inner_shape:
                 raise ValueError(
                     f"inner dimensions don't match in concatenate: {inner_shape} vs {x.shape[1:]}"
                 )

src/awkward/contents/numpyarray.py

@@ -188,9 +188,14 @@ def _raw(self, nplike=None):
         return to_nplike(self.data, nplike, from_nplike=self._backend.nplike)
 
     def _form_with_key(self, getkey: Callable[[Content], str | None]) -> NumpyForm:
+        if hasattr(self._data, "inner_shape"):
+            inner_shape = self._data.inner_shape
+        else:
+            inner_shape = self._data.shape[1:]
+
         return self.form_cls(
             ak.types.numpytype.dtype_to_primitive(self._data.dtype),
-            self._data.shape[1:],
+            inner_shape,
             parameters=self._parameters,
             form_key=getkey(self),
         )