Detect STR()+STR() as string concat and convert + to ||

_convert_string_concat now also matches CAST(... AS VARCHAR)
adjacent to + operators, so STR([x]) + STR([y]) produces
CAST(x AS VARCHAR) || CAST(y AS VARCHAR) instead of using +
which fails in DuckDB/Postgres for text operands.

Author: nicosuave

sidemantic/adapters/tableau.py

@@ -417,13 +417,19 @@ def _translate_iif(text: str) -> str:
 def _convert_string_concat(text: str) -> str:
     """Convert Tableau's + string concatenation to SQL ||.
 
-    Replaces + with || when at least one adjacent operand is a string literal.
-    Uses a simple heuristic: if a + is preceded or followed by a single-quoted
-    string (possibly with whitespace), replace it with ||.
+    Replaces + with || when at least one adjacent operand is a string-producing
+    expression: a string literal ('...') or a CAST(... AS VARCHAR).
     """
-    # Match: 'string' + or + 'string'
-    result = re.sub(r"('\s*)\+(\s*)", r"\1||\2", text)
-    result = re.sub(r"(\s*)\+(\s*')", r"\1||\2", result)
+    result = text
+    prev = None
+    while prev != result:
+        prev = result
+        # 'string' + ... or ... + 'string'
+        result = re.sub(r"('\s*)\+(\s*)", r"\1||\2", result)
+        result = re.sub(r"(\s*)\+(\s*')", r"\1||\2", result)
+        # CAST(... AS VARCHAR) + ... or ... + CAST(... AS VARCHAR)
+        result = re.sub(r"(AS\s+VARCHAR\)\s*)\+(\s*)", r"\1||\2", result, flags=re.IGNORECASE)
+        result = re.sub(r"(\s*)\+(\s*CAST\()", r"\1||\2", result, flags=re.IGNORECASE)
     return result
 
 

tests/adapters/tableau/test_formula.py

@@ -267,6 +267,15 @@ def test_string_concat_plus_to_pipes():
     assert "+" not in sql
 
 
+def test_string_concat_str_functions():
+    """STR() + STR() uses || not +."""
+    sql, ok = _translate_formula("STR([x]) + STR([y])")
+    assert ok
+    assert "||" in sql
+    assert "+" not in sql
+    assert "CAST" in sql
+
+
 def test_arithmetic_plus_preserved():
     """Arithmetic + is NOT converted to ||."""
     sql, ok = _translate_formula("[x] + [y]")