Control transfers and loops in constant evaluation [GH #401, GH #402, GH #403, GH #404, GH #411]

Author: daveedvdv-nvidia

src/interpret.c

@@ -0,0 +1,22 @@
+[GH #401]
+break and continue in GNU statement expressions in constant evaluation
+
+In GNU modes, a break or continue statement inside a statement expression
+did not correctly transfer control out of the enclosing full-expression
+during constant evaluation: the evaluation of the enclosing expression
+continued with an indeterminate value for the statement expression, and the
+enclosing loop or switch statement was not exited.  For example, with
+--c++20 --gnu_version 130000:
+
+  constexpr int f(int v) {
+    int n = 0;
+    for (int i = 0; i < 5; ++i) {
+      n += ({ if (i == v) break; i; });
+    }
+    return n;
+  }
+  static_assert(f(3) == 3);  // Previously failed.
+
+This is now fixed.  To that end, the interpretation of control transfers
+(break, continue, and return statements, and the entry of a switch statement
+at a case label) and of loops was reorganized.

src/pending_changes/gh-401-statement-expression-break.txt

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+[GH #402]
+Variables declared before a case label in constant evaluation
+
+During constant evaluation, a variable declared (without initializer) after
+one case label of a switch statement and used after a subsequent case label
+was treated as inaccessible when the switch statement transferred control
+to the subsequent label.  The same happened for a variable declared without
+initializer in the init-statement of an if- or for-statement whose
+substatement contains the case label.  For example, with --c++20:
+
+  constexpr int f() {
+    switch (1) {
+      for (int x; ;) {
+      case 1:
+        x = 3;
+        return x;
+      }
+    }
+    return 0;
+  }
+  static_assert(f() == 3);  // Previously failed.
+
+This is now fixed.

src/pending_changes/gh-402-switch-case-variable.txt

@@ -0,0 +1,23 @@
+[GH #403]
+Lifetime of condition variables in constant evaluation
+
+During constant evaluation, a variable declared in the condition of an if-,
+switch-, while-, or for-statement was destroyed before the substatement
+(and, for a for-statement, the increment expression) was evaluated, instead
+of at the end of the statement (or, for a loop, of the iteration).  The same
+was true for a variable declared in the init-statement of an if- or
+switch-statement.  For example, with --c++20:
+
+  struct S {
+    int* p;
+    constexpr ~S() { *p = *p * 10 + 2; }
+    constexpr explicit operator bool() const { return true; }
+  };
+  constexpr int f() {
+    int n = 0;
+    if (S s{&n}) n = n * 10 + 1;
+    return n;
+  }
+  static_assert(f() == 12);  // Previously failed.
+
+This is now fixed.

src/pending_changes/gh-403-condition-variable-lifetime.txt

@@ -0,0 +1,17 @@
+[GH #404]
+Lifetime of the variable of a range-based for-statement in constant evaluation
+
+During constant evaluation, the variable declared by the for-range-declaration
+of a range-based for-statement was not destroyed at the end of each
+iteration.  For example, with --c++20:
+
+  struct R { int* p; constexpr ~R() { ++*p; } };
+  constexpr int g() {
+    int n = 0;
+    R a[2] = {{&n}, {&n}};
+    for (R r : a) {}
+    return n;
+  }
+  static_assert(g() == 2);  // Previously failed.
+
+This is now fixed.

src/pending_changes/gh-404-range-for-variable-lifetime.txt

@@ -0,0 +1,25 @@
+[GH #411]
+For loops entered through a case label in constant evaluation
+
+During constant evaluation, when a switch statement transferred control to a
+case label in the body of a for loop that was followed by other statements,
+the loop was not continued after the body completed: The loop condition was
+not evaluated again.  For example, with --c++20:
+
+  constexpr int f(int k) {
+    bool b = false;
+    int z = 0;
+    switch (k) {
+      for (; b;) {
+        return 5;
+      case 1:
+        b = true;
+        z = 9;
+      }
+      return z;
+    }
+    return -1;
+  }
+  static_assert(f(1) == 5);  // Previously failed.
+
+This is now fixed.

src/pending_changes/gh-411-loop-entered-through-case-label.txt

@@ -0,0 +1 @@
+fe_only -DTEST_NUMBER=1 --c++20 --gnu_version 130000 Test_name.c

tests/tests/changes/.gh-401.rto/default.1.1.txt

@@ -0,0 +1,5 @@
+fe_only -DTEST_NUMBER=1 --c++20 Test_name.c
+"Test_name.c", line 45: warning: statement is unreachable
+      if (int x; false) {
+      ^
+

tests/tests/changes/.gh-402.rto/default.1.1.txt

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+fe_only -DTEST_NUMBER=1 --c++20 Test_name.c

tests/tests/changes/.gh-403.rto/default.1.1.txt

@@ -0,0 +1 @@
+fe_only -DTEST_NUMBER=1 --c++20 Test_name.c