Fix compiler crash from match with extra parens around let in tup…

…le (#4595)

The compiler would crash if an extra set of parens were added to a
single element tuple match of a union type. For example,
if `type Foo is (I32 | (I32, I32))` is being matched, then a match
clause like `| (123, (let x: I32)) => x` would crash the compiler.
This was caused by the codegen for the match not handling the extra
SEQ node in the AST created by the parens. This change now skips
any extra SEQ nodes when performing codegen for tuple matches.

Fixes #4412

.release-notes/4595.md

@@ -0,0 +1,13 @@
+## Fix compiler crash from `match` with extra parens around `let` in tuple
+
+When matching on a tuple element within a union type, the compiler would crash when extra parentheses were present.
+
+The following code fails to compile because of `(123, (let x: I32))` in the `match`. The extra parentheses should be ignored and treated like `(123, let x: I32)`.
+
+```pony
+let value: (I32 | (I32, I32)) = (123, 42)
+
+match value
+| (123, (let x: I32)) => x
+end
+```

src/libponyc/codegen/genmatch.c

@@ -321,6 +321,11 @@ static bool dynamic_tuple_ptr(compile_t* c, LLVMValueRef ptr,
 
     // Skip over the SEQ node.
     ast_t* pattern_expr = ast_child(pattern_child);
+    while(ast_id(pattern_expr) == TK_SEQ)
+    {
+      pony_assert(ast_childcount(pattern_expr) == 1);
+      pattern_expr = ast_child(pattern_expr);
+    }
 
     if(!dynamic_tuple_element(c, ptr, desc, pattern_expr, next_block, i))
       return false;

test/full-program-tests/regression-4412/expected-exit-code.txt

@@ -0,0 +1 @@
+42

test/full-program-tests/regression-4412/main.pony

@@ -0,0 +1,14 @@
+use @pony_exitcode[None](code: I32)
+
+// As long as #4412 is fixed, this program will compile.
+
+actor Main
+  new create(env: Env) =>
+    // A union type with a tuple is required to trigger the crash
+    let z: (I32 | (I32, I32)) = (123, 42)
+
+    match z
+    // This is the line that triggered the crash due to the extra parens around
+    // the let.
+    | (123, (let x: I32)) => @pony_exitcode(x)
+    end