fix(ssa): Do not use inner else condition when collapsing IfElse based upon the then condition

compiler/noirc_evaluator/src/ssa/ir/dfg/simplify.rs

@@ -229,15 +229,14 @@ pub(crate) fn simplify(
                 if let Instruction::IfElse {
                     then_condition: inner_then_condition,
                     then_value: inner_then_value,
-                    else_condition: inner_else_condition,
                     ..
                 } = dfg[*instruction]
                 {
                     if then_condition == inner_then_condition {
                         let instruction = Instruction::IfElse {
                             then_condition,
                             then_value: inner_then_value,
-                            else_condition: inner_else_condition,
+                            else_condition,
                             else_value,
                         };
                         return SimplifiedToInstruction(instruction);
@@ -250,7 +249,6 @@ pub(crate) fn simplify(
             if let Value::Instruction { instruction, .. } = &dfg[else_value] {
                 if let Instruction::IfElse {
                     then_condition: inner_then_condition,
-                    else_condition: inner_else_condition,
                     else_value: inner_else_value,
                     ..
                 } = dfg[*instruction]
@@ -259,7 +257,7 @@ pub(crate) fn simplify(
                         let instruction = Instruction::IfElse {
                             then_condition,
                             then_value,
-                            else_condition: inner_else_condition,
+                            else_condition,
                             else_value: inner_else_value,
                         };
                         return SimplifiedToInstruction(instruction);

compiler/noirc_evaluator/src/ssa/opt/flatten_cfg.rs

@@ -648,6 +648,7 @@ impl<'f> Context<'f> {
                 self.inserter.function.dfg.make_constant(FieldElement::one(), NumericType::bool())
             }
         };
+
         let enable_side_effects = Instruction::EnableSideEffectsIf { condition };
         let call_stack = self.inserter.function.dfg.get_value_call_stack_id(condition);
         self.insert_instruction_with_typevars(enable_side_effects, None, call_stack);

compiler/noirc_evaluator/src/ssa/ssa_gen/mod.rs

@@ -687,6 +687,7 @@ impl FunctionContext<'_> {
     /// ```
     fn codegen_if(&mut self, if_expr: &ast::If) -> Result<Values, RuntimeError> {
         let condition = self.codegen_non_tuple_expression(&if_expr.condition)?;
+
         if let Some(result) = self.try_codegen_constant_if(condition, if_expr) {
             return result;
         }

test_programs/execution_success/nested_if_then_block_same_cond/Nargo.toml

@@ -0,0 +1,6 @@
+[package]
+name = "nested_if_then_block_same_cond"
+type = "bin"
+authors = [""]
+
+[dependencies]

test_programs/execution_success/nested_if_then_block_same_cond/Prover.toml

@@ -0,0 +1,4 @@
+a = 0
+b = [[1, 0], [1, 0], [1, 1]]
+
+return = [1, 0]

test_programs/execution_success/nested_if_then_block_same_cond/src/main.nr

@@ -0,0 +1,12 @@
+// Regression taken from issue #7961 (https://github.com/noir-lang/noir/issues/7961)
+fn main(a: bool, b: [[bool; 2]; 3]) -> pub [bool; 2] {
+    if (!!a) {
+        if (!!a) {
+            b[0]
+        } else {
+            b[0]
+        }
+    } else {
+        b[0]
+    }
+}

test_programs/execution_success/nested_if_then_block_same_cond/stdout.txt

@@ -0,0 +1 @@
+[nested_if_then_block_same_cond] Circuit output: Vec([Field(1), Field(0)])