diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index b4f360c08f..2af79c4f3d 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -85,7 +85,7 @@ jobs:
         echo $VFLAGS
         sudo ln -s $PWD/thirdparty/tcc/tcc.exe /usr/local/bin/tcc ## TODO: remove
         make -j4
-        ./v -cg -o v cmd/v
+        ./v -cg -cflags -Werror -o v cmd/v
     - name: Test v->c
       run: |
         thirdparty/tcc/tcc.exe -version
@@ -145,7 +145,7 @@ jobs:
         ## brew install sdl2 sdl2_ttf sdl2_mixer sdl2_image
         export LIBRARY_PATH="$LIBRARY_PATH:/usr/local/opt/openssl/lib/"
     - name: Build V
-      run:  make -j4 && ./v -o v cmd/v
+      run:  make -j4 && ./v -cg -cflags -Werror -o v cmd/v
     - name: Build V using V
       run:  ./v -o v2 cmd/v && ./v2 -o v3 cmd/v
     - name: Test symlink
@@ -206,7 +206,7 @@ jobs:
          sudo apt-get install --quiet -y libglfw3 libglfw3-dev  libfreetype6-dev  libxi-dev libxcursor-dev libasound2-dev
          ## sudo apt-get install --quiet -y libsdl2-dev libsdl2-ttf-dev libsdl2-mixer-dev libsdl2-image-dev
     - name: Build V
-      run: make -j4 && ./v -cc gcc -o v cmd/v
+      run: make -j4 && ./v -cc gcc -cg -cflags -Werror -o v cmd/v
 #    - name: Test V
 #      run: ./v -silent test-compiler
 #    - name: Test v binaries
@@ -357,7 +357,7 @@ jobs:
         gcc --version
         .\make.bat -gcc
     - name: Test new v.c
-      run: .\v.exe -o v.c cmd/v && gcc -municode -w v.c
+      run: .\v.exe -o v.c cmd/v && gcc -Werror -municode -w v.c
     - name: Install dependencies
       run: |
         .\v.exe setup-freetype
@@ -392,6 +392,7 @@ jobs:
         echo %VFLAGS%
         echo $VFLAGS
         .\make.bat -msvc
+        .\v.exe -cflags /WX self
     - name: Install dependencies
       run: |
         .\v.exe setup-freetype
@@ -433,7 +434,7 @@ jobs:
         move "${env:ProgramFiles(x86)}\Microsoft Visual Studio\Installer\vswhere.exe" "${env:ProgramFiles(x86)}\Microsoft Visual Studio\Installer\vswhere.exe.old"
         .\make.bat
     - name: Test new v.c
-      run: .\v.exe -o v.c cmd/v && .\thirdparty\tcc\tcc.exe -w -ladvapi32 -bt10 v.c
+      run: .\v.exe -o v.c cmd/v && .\thirdparty\tcc\tcc.exe -Werror -w -ladvapi32 -bt10 v.c
     - name: Install dependencies
       run: |
         .\v.exe setup-freetype
diff --git a/vlib/builtin/builtin_nix.c.v b/vlib/builtin/builtin_nix.c.v
index b305465b8a..c14a3ae302 100644
--- a/vlib/builtin/builtin_nix.c.v
+++ b/vlib/builtin/builtin_nix.c.v
@@ -48,7 +48,7 @@ fn print_backtrace_skipping_top_frames(xskipframes int) bool {
 // so there is no need to have their twins in builtin_windows.v
 fn print_backtrace_skipping_top_frames_mac(skipframes int) bool {
 	$if macos {
-		buffer := [100]byteptr{}
+		buffer := [100]voidptr{}
 		nr_ptrs := C.backtrace(buffer, 100)
 		if nr_ptrs < 2 {
 			eprintln('C.backtrace returned less than 2 frames')
@@ -61,7 +61,7 @@ fn print_backtrace_skipping_top_frames_mac(skipframes int) bool {
 
 fn print_backtrace_skipping_top_frames_freebsd(skipframes int) bool {
 	$if freebsd {
-		buffer := [100]byteptr{}
+		buffer := [100]voidptr{}
 		nr_ptrs := C.backtrace(buffer, 100)
 		if nr_ptrs < 2 {
 			eprintln('C.backtrace returned less than 2 frames')
diff --git a/vlib/os/process_nix.c.v b/vlib/os/process_nix.c.v
index 104ab12add..9710f4dff6 100644
--- a/vlib/os/process_nix.c.v
+++ b/vlib/os/process_nix.c.v
@@ -45,16 +45,16 @@ fn (mut p Process) unix_spawn_process() int {
 	}
 	mut cargv := []charptr{}
 	mut cenvs := []charptr{}
-	cargv << p.filename.str
+	cargv << charptr(p.filename.str)
 	for i in 0 .. p.args.len {
-		cargv << p.args[i].str
+		cargv << charptr(p.args[i].str)
 	}
 	for i in 0 .. p.env.len {
-		cenvs << p.env[i].str
+		cenvs << charptr(p.env[i].str)
 	}
 	cargv << charptr(0)
 	cenvs << charptr(0)
-	C.execve(p.filename.str, cargv.data, cenvs.data)
+	C.execve(charptr(p.filename.str), cargv.data, cenvs.data)
 	// NB: normally execve does not return at all.
 	// If it returns, then something went wrong...
 	eprintln(posix_get_error_msg(C.errno))
diff --git a/vlib/time/time_darwin.c.v b/vlib/time/time_darwin.c.v
index d516961c39..6839e5d252 100644
--- a/vlib/time/time_darwin.c.v
+++ b/vlib/time/time_darwin.c.v
@@ -28,10 +28,10 @@ pub struct C.timeval {
 	tv_usec u64
 }
 
-fn init_time_base() InternalTimeBase {
+fn init_time_base() C.mach_timebase_info_data_t {
 	tb := C.mach_timebase_info_data_t{}
 	C.mach_timebase_info(&tb)
-	return InternalTimeBase{numer:tb.numer, denom:tb.denom}
+	return C.mach_timebase_info_data_t{numer:tb.numer, denom:tb.denom}
 }
 
 fn sys_mono_now_darwin() u64 {
diff --git a/vlib/v/builder/cc.v b/vlib/v/builder/cc.v
index 10380e416f..a8701448da 100644
--- a/vlib/v/builder/cc.v
+++ b/vlib/v/builder/cc.v
@@ -316,7 +316,7 @@ fn (mut v Builder) cc() {
 	//
 	if is_cc_clang {
 		if debug_mode {
-			debug_options = '-g3 -O0 -no-pie'
+			debug_options = '-g3 -O0'
 		}
 		optimization_options = '-O3'
 		mut have_flto := true
diff --git a/vlib/v/gen/cgen.v b/vlib/v/gen/cgen.v
index c60b935f9c..047aa3e2b3 100644
--- a/vlib/v/gen/cgen.v
+++ b/vlib/v/gen/cgen.v
@@ -1191,7 +1191,7 @@ fn (mut g Gen) for_in(it ast.ForInStmt) {
 			g.writeln(' = (*$atmp)[$i];')
 		}
 	} else if it.kind == .map {
-		// `for key, val in map {`
+		// `for key, val in map {
 		g.writeln('// FOR IN map')
 		idx := g.new_tmp_var()
 		atmp := g.new_tmp_var()
@@ -1199,7 +1199,7 @@ fn (mut g Gen) for_in(it ast.ForInStmt) {
 		g.write('$atmp_styp $atmp = ')
 		g.expr(it.cond)
 		g.writeln(';')
-		g.writeln('for (int $idx = 0; $idx < ${atmp}.key_values.len; ++$idx) {')
+		g.writeln('for (int $idx = 0; $idx < (int)${atmp}.key_values.len; ++$idx) {')
 		g.writeln('\tif (${atmp}.key_values.keys[$idx].str == 0) {continue;}')
 		if it.key_var != '_' {
 			key_styp := g.typ(it.key_type)
diff --git a/vlib/v/parser/comptime.v b/vlib/v/parser/comptime.v
index ca2660e448..9e9be823c6 100644
--- a/vlib/v/parser/comptime.v
+++ b/vlib/v/parser/comptime.v
@@ -23,14 +23,14 @@ fn (mut p Parser) hash() ast.HashStmt {
 	val := p.tok.lit
 	kind := val.all_before(' ')
 	p.next()
-	mut main := ''
+	mut main_str := ''
 	mut msg := ''
 	content := val.all_after('$kind ').all_before('//')
 	if content.contains(' #') {
-		main = content.all_before(' #').trim_space()
+		main_str = content.all_before(' #').trim_space()
 		msg = content.all_after(' #').trim_space()
 	} else {
-		main = content.trim_space()
+		main_str = content.trim_space()
 		msg = ''
 	}
 	// p.trace('a.v', 'kind: ${kind:-10s} | pos: ${pos:-45s} | hash: $val')
@@ -38,7 +38,7 @@ fn (mut p Parser) hash() ast.HashStmt {
 		mod: p.mod
 		val: val
 		kind: kind
-		main: main
+		main: main_str
 		msg: msg
 		pos: pos
 	}
diff --git a/vlib/v/parser/parser.v b/vlib/v/parser/parser.v
index 3ad42b4f3c..31a6ae15cc 100644
--- a/vlib/v/parser/parser.v
+++ b/vlib/v/parser/parser.v
@@ -1367,6 +1367,7 @@ fn (mut p Parser) dot_expr(left ast.Expr) ast.Expr {
 		field_name: field_name
 		pos: name_pos
 		is_mut: is_mut
+		mut_pos: mut_pos
 	}
 	mut node := ast.Expr{}
 	node = sel_expr