Fix bug which created dead and incorrectly typed blocks into IR, trig…

…gering a verification failure. (FuelLabs#3210)

If both arms of an `if` (or all arms of a `match` which are converted to
`if`s) were terminated with `return` then we still created a merge block
with an unknown arg, which ended up being dead code and incorrectly
typed.

sway-core/src/ir_generation/compile.rs

@@ -277,12 +277,17 @@ fn compile_fn_with_args(
     let mut compiler = FnCompiler::new(context, module, func, returns_by_ref, logged_types_map);
     let mut ret_val = compiler.compile_code_block(context, md_mgr, body)?;
 
-    // Special case: if the return type is unit but the return value type is not, then we have an
-    // implicit return from the last expression in the code block having a semi-colon.  This isn't
-    // codified in the AST explicitly so we need to make a unit to return here.
-    if ret_type.eq(context, &Type::Unit) && !matches!(ret_val.get_type(context), Some(Type::Unit)) {
-        ret_val = Constant::get_unit(context);
-    }
+    // Special case: sometimes the returned value at the end of the function block is hacked
+    // together and is invalid.  This can happen with diverging control flow or with implicit
+    // returns.  We can double check here and make sure the return value type is correct.
+    ret_val = match ret_val.get_type(context) {
+        Some(ret_val_type) if ret_type.eq(context, &ret_val_type.strip_ptr_type(context)) => {
+            ret_val
+        }
+
+        // Mismatched or unavailable type.  Set ret_val to a correctly typed Undef.
+        _otherwise => Value::new_constant(context, Constant::get_undef(ret_type)),
+    };
 
     // Another special case: if the last expression in a function is a return then we don't want to
     // add another implicit return instruction here, as `ret_val` will be unit regardless of the

sway-ir/src/constant.rs

@@ -77,6 +77,13 @@ impl Constant {
         }
     }
 
+    pub fn get_undef(ty: Type) -> Self {
+        Constant {
+            ty,
+            value: ConstantValue::Undef,
+        }
+    }
+
     pub fn get_unit(context: &mut Context) -> Value {
         Value::new_constant(context, Constant::new_unit())
     }

test/src/e2e_vm_tests/test_programs/should_pass/language/match_expressions_explicit_rets/.gitignore

@@ -0,0 +1,2 @@
+out
+target

test/src/e2e_vm_tests/test_programs/should_pass/language/match_expressions_explicit_rets/Forc.lock

@@ -0,0 +1,8 @@
+[[package]]
+name = 'core'
+source = 'path+from-root-3F456608F1F32854'
+
+[[package]]
+name = 'match_expressions_explicit_rets'
+source = 'root'
+dependencies = ['core']

test/src/e2e_vm_tests/test_programs/should_pass/language/match_expressions_explicit_rets/Forc.toml

@@ -0,0 +1,9 @@
+[project]
+authors = ["Fuel Labs <[email protected]>"]
+entry = "main.sw"
+license = "Apache-2.0"
+name = "match_expressions_explicit_rets"
+implicit-std = false
+
+[dependencies]
+core = { path = "../../../../../../../sway-lib-core" }

test/src/e2e_vm_tests/test_programs/should_pass/language/match_expressions_explicit_rets/src/main.sw

@@ -0,0 +1,15 @@
+script;
+
+// Explicit returns from each arm of a match expression.  Was causing mistyped dead IR to be
+// generated.
+
+fn main() -> bool {
+    match true {
+        true => {
+            return true;
+        },
+        false => {
+            return false;
+        }
+    }
+}

test/src/e2e_vm_tests/test_programs/should_pass/language/match_expressions_explicit_rets/test.toml

@@ -0,0 +1,3 @@
+category = "run"
+expected_result = { action = "return", value = 1 }
+validate_abi = false