compiler: fix endless loop on syntax errors in template placeholders

compiler.c

@@ -237,7 +237,15 @@ uc_compiler_set_srcpos(uc_compiler_t *compiler, size_t srcpos)
 	size_t delta;
 
 	/* ensure that lines counts are strictly increasing */
-	assert(srcpos == 0 || srcpos >= compiler->current_srcpos);
+	assert(srcpos == 0 || srcpos >= compiler->current_srcpos ||
+	       compiler->parser->error);
+
+	/* after a syntax error, code may be emitted for tokens in front of the
+	 * current position, e.g. after the expression of a template literal
+	 * placeholder has been terminated prematurely. The generated code is
+	 * discarded, but a negative offset would be taken for an enormous one */
+	if (srcpos < compiler->current_srcpos)
+		srcpos = 0;
 
 	delta = srcpos ? srcpos - compiler->current_srcpos : 0;
 	compiler->current_srcpos += delta;

tests/custom/99_bugs/60_template_placeholder_syntax_error_hang

@@ -0,0 +1,49 @@
+A syntax error within the expression of a template literal placeholder
+made the compiler spin until it ran out of memory. The statement
+terminating the placeholder expression had advanced the source position
+beyond the last token of the expression, and instructions emitted for
+that token afterwards moved the position backwards. The negative offset
+was taken for an enormous one when recording the source position of the
+instruction, which then took practically forever and a lot of memory.
+
+1. Excess token in a placeholder.
+
+-- Testcase --
+x = `a${ 1 2 }`;
+-- End --
+
+-- Args --
+-R
+-- End --
+
+-- Expect stderr --
+Syntax error: Unexpected token
+Expecting '}'
+In line 1, byte 12:
+
+ `x = `a${ 1 2 }`;`
+  ~~~~~~~~~~~^
+
+
+-- End --
+
+
+2. Unterminated placeholder at the end of the source.
+
+-- Testcase --
+x = `tpl${
+-- End --
+
+-- Args --
+-R
+-- End --
+
+-- Expect stderr --
+Syntax error: Expecting expression
+In line 2, byte 1:
+
+ `x = `tpl${`
+            ^-- Near here
+
+
+-- End --