c++: Extend -Wrange-loop-construct for binding-to-temp [PR94695]

This patch finishes the second half of -Wrange-loop-construct I promised
to implement: it warns when a loop variable in a range-based for-loop is
initialized with a value of a different type resulting in a copy.  For
instance:

  int arr[10];
  for (const double &x : arr) { ... }

where in every iteration we have to create and destroy a temporary value
of type double, to which we bind the reference.  This could negatively
impact performance.

As per Clang, this doesn't warn when the range returns a copy, hence the
glvalue_p check.

gcc/ChangeLog:

	PR c++/94695
	* doc/invoke.texi: Update the -Wrange-loop-construct description.

gcc/cp/ChangeLog:

	PR c++/94695
	* parser.c (warn_for_range_copy): Warn when the loop variable is
	initialized with a value of a different type resulting in a copy.

gcc/testsuite/ChangeLog:

	PR c++/94695
	* g++.dg/warn/Wrange-loop-construct2.C: New test.

1 files changed, 17 insertions, 1 deletions

diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 02abac3..26372a2 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -3681,7 +3681,23 @@ void fn () @{
 @end smallexample
 
 It does not warn when the type being copied is a trivially-copyable type whose
-size is less than 64 bytes.  This warning is enabled by @option{-Wall}.
+size is less than 64 bytes.
+
+This warning also warns when a loop variable in a range-based for-loop is
+initialized with a value of a different type resulting in a copy.  For example:
+
+@smallexample
+void fn() @{
+  int arr[10];
+  for (const double &x : arr) @{ @dots{} @}
+@}
+@end smallexample
+
+In the example above, in every iteration of the loop a temporary value of
+type @code{double} is created and destroyed, to which the reference
+@code{const double &} is bound.
+
+This warning is enabled by @option{-Wall}.
 
 @item -Wredundant-tags @r{(C++ and Objective-C++ only)}
 @opindex Wredundant-tags