libstdc++: Enable _GLIBCXX_ASSERTIONS by default for -O0 [PR112808]

Too many users don't know about -D_GLIBCXX_ASSERTIONS and so are missing
valuable checks for C++ standard library preconditions. This change
enables libstdc++ assertions by default when compiling with -O0 so that
we diagnose more bugs by default.

When users enable optimization we don't add the assertions by default
(because they have non-zero overhead) so they still need to enable them
manually.

For users who really don't want the assertions even in unoptimized
builds, defining _GLIBCXX_NO_ASSERTIONS will prevent them from being
enabled automatically.

libstdc++-v3/ChangeLog:

	PR libstdc++/112808
	* doc/xml/manual/using.xml (_GLIBCXX_ASSERTIONS): Document
	implicit definition for -O0 compilation.
	(_GLIBCXX_NO_ASSERTIONS): Document.
	* doc/html/manual/using_macros.html: Regenerate.
	* include/bits/c++config [!__OPTIMIZE__] (_GLIBCXX_ASSERTIONS):
	Define for unoptimized builds.

3 files changed, 29 insertions, 8 deletions

diff --git a/libstdc++-v3/doc/html/manual/using_macros.html b/libstdc++-v3/doc/html/manual/using_macros.html
index 67623b5..c1406ec 100644
--- a/libstdc++-v3/doc/html/manual/using_macros.html
+++ b/libstdc++-v3/doc/html/manual/using_macros.html
@@ -82,9 +82,15 @@
 	This is described in more detail in
 	<a class="link" href="ext_compile_checks.html" title="Chapter 16. Compile Time Checks">Compile Time Checks</a>.
       </p></dd><dt><span class="term"><code class="code">_GLIBCXX_ASSERTIONS</code></span></dt><dd><p>
-	Undefined by default. When defined, enables extra error checking in
-        the form of precondition assertions, such as bounds checking in
-        strings and null pointer checks when dereferencing smart pointers.
+	Defined by default when compiling with no optimization, undefined
+	by default when compiling with optimization.
+	When defined, enables extra error checking in the form of
+	precondition assertions, such as bounds checking in strings
+	and null pointer checks when dereferencing smart pointers.
+      </p></dd><dt><span class="term"><code class="code">_GLIBCXX_NO_ASSERTIONS</code></span></dt><dd><p>
+	Undefined by default.  When defined, prevents the implicit
+	definition of <code class="code">_GLIBCXX_ASSERTIONS</code> when compiling
+	with no optimization.
       </p></dd><dt><span class="term"><code class="code">_GLIBCXX_DEBUG</code></span></dt><dd><p>
 	Undefined by default. When defined, compiles user code using
 	the <a class="link" href="debug_mode.html" title="Chapter 17. Debug Mode">debug mode</a>.
diff --git a/libstdc++-v3/doc/xml/manual/using.xml b/libstdc++-v3/doc/xml/manual/using.xml
index 89119f6..7ca3a3f 100644
--- a/libstdc++-v3/doc/xml/manual/using.xml
+++ b/libstdc++-v3/doc/xml/manual/using.xml
@@ -1247,9 +1247,19 @@ g++ -Winvalid-pch -I. -include stdc++.h -H -g -O2 hello.cc -o test.exe
     <varlistentry><term><code>_GLIBCXX_ASSERTIONS</code></term>
     <listitem>
       <para>
-	Undefined by default. When defined, enables extra error checking in
-        the form of precondition assertions, such as bounds checking in
-        strings and null pointer checks when dereferencing smart pointers.
+	Defined by default when compiling with no optimization, undefined
+	by default when compiling with optimization.
+	When defined, enables extra error checking in the form of
+	precondition assertions, such as bounds checking in strings
+	and null pointer checks when dereferencing smart pointers.
+      </para>
+    </listitem></varlistentry>
+    <varlistentry><term><code>_GLIBCXX_NO_ASSERTIONS</code></term>
+    <listitem>
+      <para>
+	Undefined by default.  When defined, prevents the implicit
+	definition of <code>_GLIBCXX_ASSERTIONS</code> when compiling
+	with no optimization.
       </para>
     </listitem></varlistentry>
     <varlistentry><term><code>_GLIBCXX_DEBUG</code></term>
diff --git a/libstdc++-v3/include/bits/c++config b/libstdc++-v3/include/bits/c++config
index 29d795f..b87a352 100644
--- a/libstdc++-v3/include/bits/c++config
+++ b/libstdc++-v3/include/bits/c++config
@@ -586,9 +586,14 @@ namespace std
 #pragma GCC visibility pop
 }
 
+#ifndef _GLIBCXX_ASSERTIONS
+# if defined(_GLIBCXX_DEBUG)
 // Debug Mode implies checking assertions.
-#if defined(_GLIBCXX_DEBUG) && !defined(_GLIBCXX_ASSERTIONS)
-# define _GLIBCXX_ASSERTIONS 1
+#  define _GLIBCXX_ASSERTIONS 1
+# elif ! defined(__OPTIMIZE__) && ! defined(_GLIBCXX_NO_ASSERTIONS)
+// Enable assertions for unoptimized builds.
+#  define _GLIBCXX_ASSERTIONS 1
+# endif
 #endif
 
 // Disable std::string explicit instantiation declarations in order to assert.