fix: do not lex types starting with a '!'

Closes: #161

Author: lppedd

src/main/kotlin/com/github/lppedd/cc/language/lexer/conventionalCommit.flex

@@ -79,14 +79,23 @@ FooterType  = [^:\s]+ | BREAKING\ CHANGE
 }
 
 <TYPE> {
-      ^[^(:\r\n]+\!? {
+      // The commit type must be at the beginning of the line, and cannot start with a '!'
+      ^[^!(:\r\n][^(:\r\n]*\!? {
         if (yycharat(yylength() - 1) == '!') {
           yypushback(1);
         }
 
         return ConventionalCommitTokenType.TYPE;
       }
 
+      // This catches the case where we have a lone '!'.
+      // We lex it as BREAKING_CHANGE so that we do not fail lexing
+      // and then let the parser or inspections reason about it.
+      // Note that this rule must have lower priority than '\! / :'.
+      \! {
+        return ConventionalCommitTokenType.BREAKING_CHANGE;
+      }
+
       \( {
         yybegin(SCOPE);
         return ConventionalCommitTokenType.SCOPE_OPEN_PAREN;

src/main/kotlin/com/github/lppedd/cc/parser/strictConventionalCommit.flex

@@ -78,8 +78,8 @@ FooterType  = [^\s:][^:\r\n]*
 }
 
 <TYPE> {
-      // The commit type must be at the start of the line
-      ^[^(:\s]+\!? {
+      // The commit type must be at the beginning of the line, and cannot start with a '!'
+      ^[^!(:\s][^(:\s]*\!? {
         if (yycharat(yylength() - 1) == '!') {
           yypushback(1);
         }

src/test/kotlin/com/github/lppedd/cc/parser/ConventionalCommitParserTest.kt

@@ -7,7 +7,15 @@ import kotlin.contracts.contract
 class ConventionalCommitParserTest {
   @Test
   fun `fail incomplete`() {
-    var result = parseConventionalCommit("fix: ")
+    var result = parseConventionalCommit("f!x")
+    assertIs<ParseResult.Error>(result)
+    assertEquals("The ':' separator is missing after the type/scope", result.message)
+
+    result = parseConventionalCommit("!fix")
+    assertIs<ParseResult.Error>(result)
+    assertEquals("The commit type is missing or invalid", result.message)
+
+    result = parseConventionalCommit("fix: ")
     assertIs<ParseResult.Error>(result)
     assertEquals("The commit subject is missing or invalid", result.message)
 
@@ -56,6 +64,17 @@ class ConventionalCommitParserTest {
     assertEquals("foo", message.subject.trim())
   }
 
+  @Test
+  fun `parse unusual type`() {
+    val result = parseConventionalCommit("bui!ld!: foo")
+    assertIs<ParseResult.Success>(result)
+
+    val message = result.message
+    assertEquals("bui!ld", message.type)
+    assertTrue(message.isBreakingChange)
+    assertEquals("foo", message.subject.trim())
+  }
+
   @Test
   fun `parse empty scope`() {
     var result = parseConventionalCommit("build(): bar")