[gcc r17-3388] libstdc++: Do not overwrite module definition files during build [PR126786]

[Jonathan Wakely]

https://gcc.gnu.org/g:e87a450f3ead2d6278a54e93409ff67853de93ef

commit r17-3388-ge87a450f3ead2d6278a54e93409ff67853de93ef
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon Aug 17 19:04:34 2026 +0100

    libstdc++: Do not overwrite module definition files during build [PR126786]
    
    As described in the bug report, when compilation of std.cc or
    std.compat.cc fails we overwrite the source file with an empty file, and
    then compile that (so that bootstrap doesn't fail). Then when the module
    definition files are installed, we install the empty file.
    
    We should install the original source, not an empty file. We can create
    an empty file and compile it, but leave the original source untouched so
    that it can still be installed.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/126786
            * src/c++23/Makefile.am (empty.cc): New target.
            (std.lo, std.o, std.compat.lo, std.compat.o): Use empty.cc as
            fallback instead of overwriting original sources.
            * src/c++23/Makefile.in: Regenerate.
    
    Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>

Diff:
---
 libstdc++-v3/src/c++23/Makefile.am | 24 ++++++++++++------------
 libstdc++-v3/src/c++23/Makefile.in | 24 ++++++++++++------------
 2 files changed, 24 insertions(+), 24 deletions(-)

diff --git a/libstdc++-v3/src/c++23/Makefile.am b/libstdc++-v3/src/c++23/Makefile.am
index 92691c502a94..e09c4bd54da2 100644
--- a/libstdc++-v3/src/c++23/Makefile.am
+++ b/libstdc++-v3/src/c++23/Makefile.am
@@ -54,6 +54,10 @@ std.cc: std.cc.in std-clib.cc.in
 std.compat.cc: std.compat.cc.in std-clib.cc.in
 	cat $^ > $@
 
+# Empty file used in case std.cc or std.compat.cc fails to compile.
+empty.cc:
+	echo > $@
+
 # Also put the interface units in the build-includes bits directory.
 stamp-modules-bits: $(includebits_DATA)
 	@-mkdir -p $(top_builddir)/include/bits
@@ -100,33 +104,29 @@ MODULES_FLAGS = -isystem ${glibcxx_srcdir}/include/backward -fmodules \
 	-Wno-unknown-pragmas
 
 # These go into libmodulesconvenience.la and are linked into libstdc++
-std.lo: std.cc
+std.lo: std.cc empty.cc
 	if ! $(LTCXXCOMPILE) $(MODULES_FLAGS) -c $< ; then \
 	  echo "Cannot compile std module" >&2; \
 	  echo "Module initialization function will be missing" >&2; \
-	  echo > $<.tmp && mv $<.tmp $< && \
-	  $(LTCXXCOMPILE) $(MODULES_FLAGS) -c $< ; \
+	  $(LTCXXCOMPILE) $(MODULES_FLAGS) -c empty.cc -o $@; \
 	fi
-std.o: std.cc
+std.o: std.cc empty.cc
 	if ! $(CXXCOMPILE) $(MODULES_FLAGS) -c $< ; then \
 	  echo "Cannot compile std module" >&2; \
 	  echo "Module initialization function will be missing" >&2; \
-	  echo > $<.tmp && mv $<.tmp $< && \
-	  $(CXXCOMPILE) $(MODULES_FLAGS) -c $< ; \
+	  $(CXXCOMPILE) $(MODULES_FLAGS) -c empty.cc -o $@; \
 	fi
-std.compat.lo: std.compat.cc std.lo
+std.compat.lo: std.compat.cc std.lo empty.cc
 	if ! $(LTCXXCOMPILE) $(MODULES_FLAGS) -c $< ; then \
 	  echo "Cannot compile std.compat module" >&2; \
 	  echo "Module initialization function will be missing" >&2; \
-	  echo > $<.tmp && mv $<.tmp $< && \
-	  $(LTCXXCOMPILE) $(MODULES_FLAGS) -c $< ; \
+	  $(LTCXXCOMPILE) $(MODULES_FLAGS) -c empty.cc -o $@; \
 	fi
-std.compat.o: std.compat.cc std.o
+std.compat.o: std.compat.cc std.o empty.cc
 	if ! $(CXXCOMPILE) $(MODULES_FLAGS) -c $< ; then \
 	  echo "Cannot compile std.compat module" >&2; \
 	  echo "Module initialization function will be missing" >&2; \
-	  echo > $<.tmp && mv $<.tmp $< && \
-	  $(CXXCOMPILE) $(MODULES_FLAGS) -c $< ; \
+	  $(CXXCOMPILE) $(MODULES_FLAGS) -c empty.cc -o $@; \
 	fi
 
 # AM_CXXFLAGS needs to be in each subdirectory so that it can be
diff --git a/libstdc++-v3/src/c++23/Makefile.in b/libstdc++-v3/src/c++23/Makefile.in
index 0ce93f812d25..f3ea90ce2478 100644
--- a/libstdc++-v3/src/c++23/Makefile.in
+++ b/libstdc++-v3/src/c++23/Makefile.in
@@ -862,6 +862,10 @@ std.cc: std.cc.in std-clib.cc.in
 std.compat.cc: std.compat.cc.in std-clib.cc.in
 	cat $^ > $@
 
+# Empty file used in case std.cc or std.compat.cc fails to compile.
+empty.cc:
+	echo > $@
+
 # Also put the interface units in the build-includes bits directory.
 stamp-modules-bits: $(includebits_DATA)
 	@-mkdir -p $(top_builddir)/include/bits
@@ -884,33 +888,29 @@ print.o: print.cc
 	$(CXXCOMPILE) -std=gnu++26 -c $<
 
 # These go into libmodulesconvenience.la and are linked into libstdc++
-std.lo: std.cc
+std.lo: std.cc empty.cc
 	if ! $(LTCXXCOMPILE) $(MODULES_FLAGS) -c $< ; then \
 	  echo "Cannot compile std module" >&2; \
 	  echo "Module initialization function will be missing" >&2; \
-	  echo > $<.tmp && mv $<.tmp $< && \
-	  $(LTCXXCOMPILE) $(MODULES_FLAGS) -c $< ; \
+	  $(LTCXXCOMPILE) $(MODULES_FLAGS) -c empty.cc -o $@; \
 	fi
-std.o: std.cc
+std.o: std.cc empty.cc
 	if ! $(CXXCOMPILE) $(MODULES_FLAGS) -c $< ; then \
 	  echo "Cannot compile std module" >&2; \
 	  echo "Module initialization function will be missing" >&2; \
-	  echo > $<.tmp && mv $<.tmp $< && \
-	  $(CXXCOMPILE) $(MODULES_FLAGS) -c $< ; \
+	  $(CXXCOMPILE) $(MODULES_FLAGS) -c empty.cc -o $@; \
 	fi
-std.compat.lo: std.compat.cc std.lo
+std.compat.lo: std.compat.cc std.lo empty.cc
 	if ! $(LTCXXCOMPILE) $(MODULES_FLAGS) -c $< ; then \
 	  echo "Cannot compile std.compat module" >&2; \
 	  echo "Module initialization function will be missing" >&2; \
-	  echo > $<.tmp && mv $<.tmp $< && \
-	  $(LTCXXCOMPILE) $(MODULES_FLAGS) -c $< ; \
+	  $(LTCXXCOMPILE) $(MODULES_FLAGS) -c empty.cc -o $@; \
 	fi
-std.compat.o: std.compat.cc std.o
+std.compat.o: std.compat.cc std.o empty.cc
 	if ! $(CXXCOMPILE) $(MODULES_FLAGS) -c $< ; then \
 	  echo "Cannot compile std.compat module" >&2; \
 	  echo "Module initialization function will be missing" >&2; \
-	  echo > $<.tmp && mv $<.tmp $< && \
-	  $(CXXCOMPILE) $(MODULES_FLAGS) -c $< ; \
+	  $(CXXCOMPILE) $(MODULES_FLAGS) -c empty.cc -o $@; \
 	fi
 
 # Tell versions [3.59,3.63) of GNU make to not export all variables.