fix: an unmatched strong run before emphasis stays literal (#4008)

Author: spokodev

src/Tokenizer.ts

@@ -746,7 +746,12 @@ export class _Tokenizer<ParserOutput = string, RendererOutput = string> {
       const lLength = [...match[0]].length - 1;
       let rDelim, rLength, delimTotal = lLength, midDelimTotal = 0;
 
-      const endReg = match[0][0] === '*' ? this.rules.inline.emStrongRDelimAst : this.rules.inline.emStrongRDelimUnd;
+      const delimChar = match[0][0];
+      // A mid-run opener (for example the second star of an unmatched `**`) must
+      // only pair with a delimiter that can only close, otherwise it steals the
+      // opener of a later span (`**a*b*c` must be `**a<em>b</em>c`).
+      const midRun = prevChar === delimChar;
+      const endReg = delimChar === '*' ? this.rules.inline.emStrongRDelimAst : this.rules.inline.emStrongRDelimUnd;
       endReg.lastIndex = 0;
 
       // Clip maskedSrc to same section of string as src (move to lexer?)
@@ -767,6 +772,11 @@ export class _Tokenizer<ParserOutput = string, RendererOutput = string> {
             midDelimTotal += rLength;
             continue; // CommonMark Emphasis Rules 9-10
           }
+          if (midRun) {
+            // A mid-run opener cannot close against an ambiguous delimiter that
+            // can also open; that delimiter opens its own emphasis span instead.
+            break;
+          }
         }
 
         delimTotal -= rLength;

test/specs/new/em_strong_unclosed_orphan.html

@@ -0,0 +1 @@
+<p>**a<em>b</em>c</p>

test/specs/new/em_strong_unclosed_orphan.md

@@ -0,0 +1 @@
+**a*b*c