Merge #20182: ci: Build with --enable-werror by default, and document exceptions

2f6fe4e4e9e9e35e713c0a20cf891b023592110a ci: Build with --enable-werror by default, and document exceptions (Hennadii Stepanov)

Pull request description:

  This PR prevents introducing of new compiler warnings in the master branch, e.g., #19986, #20162.

ACKs for top commit:
  practicalswift:
    cr ACK 2f6fe4e4e9e9e35e713c0a20cf891b023592110a: patch looks correct
  MarcoFalke:
    re-ACK 2f6fe4e4e9 🏏
  vasild:
    ACK 2f6fe4e

Tree-SHA512: 23b5feb5bc472658c992d882ef61af23496f25adaa19f9c79bfaef5d2db273d44981aa93b1631a7d37cb58755283c1dacf3f2d68e501522d3fa8c965ab646d19

5 files changed, 11 insertions, 4 deletions

diff --git a/ci/test/00_setup_env_arm.sh b/ci/test/00_setup_env_arm.sh
index 610e55c4c3..42783197a9 100644
--- a/ci/test/00_setup_env_arm.sh
+++ b/ci/test/00_setup_env_arm.sh
@@ -25,4 +25,4 @@ export RUN_FUNCTIONAL_TESTS=false
 export GOAL="install"
 # -Wno-psabi is to disable ABI warnings: "note: parameter passing for argument of type ... changed in GCC 7.1"
 # This could be removed once the ABI change warning does not show up by default
-export BITCOIN_CONFIG="--enable-glibc-back-compat --enable-reduce-exports CXXFLAGS=-Wno-psabi --enable-werror --with-boost-process"
+export BITCOIN_CONFIG="--enable-glibc-back-compat --enable-reduce-exports CXXFLAGS=-Wno-psabi --with-boost-process"
diff --git a/ci/test/00_setup_env_mac.sh b/ci/test/00_setup_env_mac.sh
index 4a022f9b39..5bda37d46d 100644
--- a/ci/test/00_setup_env_mac.sh
+++ b/ci/test/00_setup_env_mac.sh
@@ -15,4 +15,4 @@ export XCODE_BUILD_ID=11C505
 export RUN_UNIT_TESTS=false
 export RUN_FUNCTIONAL_TESTS=false
 export GOAL="deploy"
-export BITCOIN_CONFIG="--with-gui --enable-reduce-exports --enable-werror --with-boost-process"
+export BITCOIN_CONFIG="--with-gui --enable-reduce-exports --with-boost-process"
diff --git a/ci/test/00_setup_env_mac_host.sh b/ci/test/00_setup_env_mac_host.sh
index 906f51fc1a..274a0d1b7c 100644
--- a/ci/test/00_setup_env_mac_host.sh
+++ b/ci/test/00_setup_env_mac_host.sh
@@ -9,7 +9,7 @@ export LC_ALL=C.UTF-8
 export HOST=x86_64-apple-darwin18
 export PIP_PACKAGES="zmq"
 export GOAL="install"
-export BITCOIN_CONFIG="--with-gui --enable-reduce-exports --enable-werror --with-boost-process"
+export BITCOIN_CONFIG="--with-gui --enable-reduce-exports --with-boost-process"
 export CI_OS_NAME="macos"
 export NO_DEPENDS=1
 export OSX_SDK=""
diff --git a/ci/test/00_setup_env_win64.sh b/ci/test/00_setup_env_win64.sh
index 72cc3f63c4..affaaaa1aa 100644
--- a/ci/test/00_setup_env_win64.sh
+++ b/ci/test/00_setup_env_win64.sh
@@ -14,3 +14,7 @@ export RUN_FUNCTIONAL_TESTS=false
 export RUN_SECURITY_TESTS="true"
 export GOAL="deploy"
 export BITCOIN_CONFIG="--enable-reduce-exports --disable-gui-tests --without-boost-process"
+
+# Compiler for MinGW-w64 causes false -Wreturn-type warning.
+# See https://sourceforge.net/p/mingw-w64/bugs/306/
+export NO_WERROR=1
diff --git a/ci/test/06_script_a.sh b/ci/test/06_script_a.sh
index 17d765b862..d99068cb10 100755
--- a/ci/test/06_script_a.sh
+++ b/ci/test/06_script_a.sh
@@ -6,7 +6,10 @@
 
 export LC_ALL=C.UTF-8
 
-BITCOIN_CONFIG_ALL="--disable-dependency-tracking --prefix=$DEPENDS_DIR/$HOST --bindir=$BASE_OUTDIR/bin --libdir=$BASE_OUTDIR/lib"
+BITCOIN_CONFIG_ALL="--enable-suppress-external-warnings --disable-dependency-tracking --prefix=$DEPENDS_DIR/$HOST --bindir=$BASE_OUTDIR/bin --libdir=$BASE_OUTDIR/lib"
+if [ -z "$NO_WERROR" ]; then
+  BITCOIN_CONFIG_ALL="${BITCOIN_CONFIG_ALL} --enable-werror"
+fi
 DOCKER_EXEC "ccache --zero-stats --max-size=$CCACHE_SIZE"
 
 BEGIN_FOLD autogen