suppress compiler warnings on Windows to test pipe-stall hypothesis

-O1 is confirmed applied via the override directive (the failing log
now shows only -O1 in the compile line, no -O2 anywhere), but the
build still fails at the same point with no literal out-of-memory
message anywhere in the log, and the whole 6-model compile completes
in ~12 seconds -- implausibly fast for genuinely heavy Stan template
compilation, and unchanged by MAKEFLAGS=-j1.

This points away from pure compiler memory exhaustion and toward the
R-side build monitor losing sync with the compiler's stdout: each
stanExports_*.cc produces on the order of 30,000+ lines of
Eigen/Boost/StanHeaders template-instantiation warnings under -Wall,
and Windows named pipes have much smaller default buffers than Unix.
Add -w to rule this out.

Author: smwindecker

NEWS.md

@@ -14,15 +14,17 @@
   version, which only exists from R 4.4.0 (related to #43).
 * Fixed intermittent R CMD check failures on Windows CI ("Build process
   failed" during compilation of the six bundled Stan models' heavy
-  Eigen/Boost/StanHeaders template instantiations), by lowering
-  optimisation from R's Windows default (`-O2`) to `-O1` via a new
-  `src/Makevars.win`, substantially reducing peak compiler memory. This
-  uses an `override CXX17FLAGS = ...` directive, since R's own
-  `etc/Makeconf` is included *after* `Makevars.win` and a plain `=`
-  assignment there would otherwise silently overwrite a plain
-  reassignment made here. Also forced serial compilation
-  (`MAKEFLAGS=-j1`) and enlarged the Windows runner's pagefile as
-  additional safeguards.
+  Eigen/Boost/StanHeaders template instantiations), via a new
+  `src/Makevars.win`: lowered optimisation from R's Windows default
+  (`-O2`) to `-O1`, and suppressed compiler warnings (`-w`), since the
+  Eigen/Boost/StanHeaders headers generate an enormous volume of
+  template-instantiation warnings that may overwhelm the small stdout
+  pipe buffers on Windows. This uses an `override CXX17FLAGS = ...`
+  directive, since R's own `etc/Makeconf` is included *after*
+  `Makevars.win` and a plain `=` assignment there would otherwise
+  silently overwrite a plain reassignment made here. Also forced serial
+  compilation (`MAKEFLAGS=-j1`) and enlarged the Windows runner's
+  pagefile as additional safeguards.
 
 ## Minor improvements
 

src/Makevars.win

@@ -27,4 +27,14 @@ CXX_STD = CXX17
 # The `override` directive is immune to being reset by any later plain
 # assignment, regardless of inclusion order, which is exactly what's
 # needed here.
-override CXX17FLAGS = -O1 -Wall -mfpmath=sse -msse2 -mstackrealign
+#
+# -O1 alone (this override confirmed working via CI logs, dropping -O2
+# entirely) did not fix the Windows failure on its own. Also add -w to
+# suppress compiler warnings: Eigen/Boost/StanHeaders headers emit an
+# enormous volume of template-instantiation warnings under -Wall
+# (190,000+ log lines for a single file), and Windows named pipes have
+# far smaller buffers than Unix -- a plausible cause of the observed
+# "Error in proc$get_built_file(): Build process failed" is the R-side
+# monitoring process losing sync with the compiler's stdout under that
+# volume, independent of actual memory exhaustion.
+override CXX17FLAGS = -O1 -w -mfpmath=sse -msse2 -mstackrealign