cmd: validate explicit C compiler before dispatch

cmd/v/v.v

@@ -183,6 +183,17 @@ fn main() {
 	if prefs.is_help {
 		invoke_help_and_exit(args)
 	}
+	$if windows {
+		// An invalid `-cc` setting should take precedence over an unknown command,
+		// but must not affect commands that do not compile code.
+		if builder.should_find_windows_host_c_compiler(prefs) && prefs.ccompiler_set_by_flag
+			&& prefs.ccompiler != 'msvc' {
+			mut probe := builder.Builder{
+				pref: unsafe { prefs }
+			}
+			probe.find_win_cc() or { builder.verror(err.msg()) }
+		}
+	}
 
 	other_commands := ['run', 'crun', 'build', 'build-module', 'help', 'version', 'new', 'init',
 		'install', 'link', 'list', 'outdated', 'remove', 'search', 'show', 'unlink', 'update',

cmd/v/v_windows_test.v

@@ -0,0 +1,24 @@
+module main
+
+import os
+
+fn test_invalid_c_compiler_is_reported_before_command_dispatch() {
+	$if !windows {
+		return
+	}
+	missing_compiler := 'missing_compiler_27868'
+	expected_error := 'builder error: C compiler `${missing_compiler}` was requested with `-cc`, but was not found.'
+	for command in ['', 'not-a-command'] {
+		result := os.execute('${os.quoted_path(@VEXE)} -cc ${missing_compiler} ${command}')
+		assert result.exit_code == 1
+		assert result.output.contains(expected_error), result.output
+	}
+	help_result := os.execute('${os.quoted_path(@VEXE)} -cc ${missing_compiler} help')
+	assert help_result.exit_code == 1
+	assert help_result.output.contains('provide only one help topic.'), help_result.output
+	assert !help_result.output.contains(expected_error), help_result.output
+
+	version_result := os.execute('${os.quoted_path(@VEXE)} -cc ${missing_compiler} version')
+	assert version_result.exit_code == 0, version_result.output
+	assert !version_result.output.contains(expected_error), version_result.output
+}