PR47805: Use a single object for a function parameter in the caller and

callee in constant evaluation.

We previously made a deep copy of function parameters of class type when
passing them, resulting in the destructor for the parameter applying to
the original argument value, ignoring any modifications made in the
function body. This also meant that the 'this' pointer of the function
parameter could be observed changing between the caller and the callee.

This change completely reimplements how we model function parameters
during constant evaluation. We now model them roughly as if they were
variables living in the caller, albeit with an artificially reduced
scope that covers only the duration of the function call, instead of
modeling them as temporaries in the caller that we partially "reparent"
into the callee at the point of the call. This brings some minor
diagnostic improvements, as well as significantly reduced stack usage
during constant evaluation.

Author: zygoloid

clang/lib/AST/ExprConstant.cpp

@@ -10,7 +10,7 @@ namespace M {
 
 struct NonLiteral {
   NonLiteral() {}
-  NonLiteral(int) {} // expected-note 2{{here}}
+  NonLiteral(int) {}
   operator int() const { return 0; }
 };
 struct Literal {
@@ -42,8 +42,8 @@ template<typename ...P> struct ConstexprCtor {
 };
 constexpr ConstexprCtor<> f1() { return {}; } // ok
 constexpr ConstexprCtor<int> f2() { return 0; } // ok
-constexpr ConstexprCtor<NonLiteral> f3() { return { 0 }; } // expected-error {{never produces a constant expression}} expected-note {{non-constexpr constructor 'NonLiteral}}
-constexpr ConstexprCtor<int, NonLiteral> f4() { return { 0, 0 }; } // expected-error {{never produces a constant expression}} expected-note {{non-constexpr constructor 'NonLiteral}}
+constexpr ConstexprCtor<NonLiteral> f3() { return { 0 }; } // expected-error {{never produces a constant expression}} expected-note {{non-literal type 'NonLiteral}}
+constexpr ConstexprCtor<int, NonLiteral> f4() { return { 0, 0 }; } // expected-error {{never produces a constant expression}} expected-note {{non-literal type 'NonLiteral}}
 
 struct VirtBase : virtual S {}; // expected-note {{here}}
 

clang/test/CXX/dcl.dcl/dcl.spec/dcl.constexpr/p6.cpp

@@ -55,7 +55,7 @@ namespace noex {
   struct A {};
 
   void g1() noexcept(A()); // expected-error {{not contextually convertible}}
-  void g2(bool b) noexcept(b); // expected-error {{argument to noexcept specifier must be a constant expression}} expected-note {{read of non-const variable 'b'}} expected-note {{here}}
+  void g2(bool b) noexcept(b); // expected-error {{argument to noexcept specifier must be a constant expression}} expected-note {{function parameter 'b' with unknown value}} expected-note {{here}}
 
 }
 

clang/test/CXX/except/except.spec/p1.cpp

@@ -62,11 +62,11 @@ namespace NonConstExprReturn {
   constexpr const int *address_of(const int &a) {
     return &a;
   }
-  constexpr const int *return_param(int n) { // expected-note {{declared here}}
+  constexpr const int *return_param(int n) {
     return address_of(n);
   }
   struct S {
-    int n : *return_param(0); // expected-error {{constant expression}} expected-note {{read of variable whose lifetime has ended}}
+    int n : *return_param(0); // expected-error {{constant expression}} expected-note {{read of object outside its lifetime}}
   };
 }
 
@@ -427,13 +427,12 @@ namespace PseudoDtor {
     int n : (k.~I(), 1); // expected-error {{constant expression}} expected-note {{visible outside that expression}}
   };
 
-  // FIXME: It's unclear whether this should be accepted in C++20 mode. The parameter is destroyed twice here.
-  constexpr int f(int a = 1) { // cxx11-error {{constant expression}}
+  constexpr int f(int a = 1) { // cxx11-error {{constant expression}} expected-note {{destroying object 'a' whose lifetime has already ended}}
     return (
-        a.~I(), // cxx11-note 2{{pseudo-destructor}}
+        a.~I(), // cxx11-note {{pseudo-destructor}}
         0);
   }
-  static_assert(f() == 0, ""); // cxx11-error {{constant expression}} cxx11-note {{in call}}
+  static_assert(f() == 0, ""); // expected-error {{constant expression}}
 
   // This is OK in C++20: the union has no active member after the
   // pseudo-destructor call, so the union destructor has no effect.

clang/test/CXX/expr/expr.const/p2-0x.cpp

@@ -65,7 +65,7 @@ int tmain(int argc, char **argv) { // expected-note {{declared here}}
   foo();
   #pragma omp critical (name2) hint(+ // expected-error {{expected expression}} expected-error {{expected ')'}} expected-note {{to match this '('}}
   foo();
-  #pragma omp critical (name2) hint(argc) // expected-error {{integral constant expression}} expected-note {{read of non-const variable 'argc' is not allowed in a constant expression}}
+  #pragma omp critical (name2) hint(argc) // expected-error {{integral constant expression}} expected-note 0+{{constant expression}}
   foo();
   #pragma omp critical (name) hint(N) // expected-error {{argument to 'hint' clause must be a strictly positive integer value}} expected-error {{constructs with the same name must have a 'hint' clause with the same value}} expected-note {{'hint' clause with value '4'}}
   foo();
@@ -128,7 +128,7 @@ int main(int argc, char **argv) { // expected-note {{declared here}}
   foo();
   #pragma omp critical (name2) hint(+ // expected-error {{expected expression}} expected-error {{expected ')'}} expected-note {{to match this '('}}
   foo();
-  #pragma omp critical (name2) hint(argc) // expected-error {{integral constant expression}} expected-note {{read of non-const variable 'argc' is not allowed in a constant expression}}
+  #pragma omp critical (name2) hint(argc) // expected-error {{integral constant expression}} expected-note 0+{{constant expression}}
   foo();
   #pragma omp critical (name) hint(23) // expected-note {{previous 'hint' clause with value '23'}}
   foo();

clang/test/OpenMP/critical_messages.cpp

@@ -39,7 +39,7 @@ T tmain(T argc, S **argv) {
 
 #pragma omp target
 #pragma omp teams
-#pragma omp distribute parallel for simd safelen (argc  // expected-note {{to match this '('}} expected-error 2 {{integral constant expression}} expected-note 2 {{read of non-const variable 'argc' is not allowed in a constant expression}} expected-note 0+{{constant expression}} expected-error {{expected ')'}}
+#pragma omp distribute parallel for simd safelen (argc  // expected-note {{to match this '('}} expected-error 2 {{integral constant expression}} expected-note 0+{{constant expression}} expected-error {{expected ')'}}
   for (int i = ST; i < N; i++) 
     argv[0][i] = argv[0][i] - argv[0][i-ST];
 

clang/test/OpenMP/distribute_parallel_for_simd_safelen_messages.cpp

@@ -39,7 +39,7 @@ T tmain(T argc, S **argv) {
 
 #pragma omp target
 #pragma omp teams
-#pragma omp distribute simd safelen (argc  // expected-note {{to match this '('}} expected-error 2 {{integral constant expression}} expected-note 2 {{read of non-const variable 'argc' is not allowed in a constant expression}} expected-note 0+{{constant expression}} expected-error {{expected ')'}}
+#pragma omp distribute simd safelen (argc  // expected-note {{to match this '('}} expected-error 2 {{integral constant expression}} expected-note 0+{{constant expression}} expected-error {{expected ')'}}
   for (int i = ST; i < N; i++) 
     argv[0][i] = argv[0][i] - argv[0][i-ST];
 

clang/test/OpenMP/distribute_simd_safelen_messages.cpp

@@ -39,7 +39,7 @@ T tmain(T argc, S **argv) {
 
 #pragma omp target
 #pragma omp teams
-#pragma omp distribute simd safelen (argc  // expected-note {{to match this '('}} expected-error 2 {{integral constant expression}} expected-note 2 {{read of non-const variable 'argc' is not allowed in a constant expression}} expected-note 0+{{constant expression}} expected-error {{expected ')'}}
+#pragma omp distribute simd safelen (argc  // expected-note {{to match this '('}} expected-error 2 {{integral constant expression}} expected-note 0+{{constant expression}} expected-error {{expected ')'}}
   for (int i = ST; i < N; i++) 
     argv[0][i] = argv[0][i] - argv[0][i-ST];
 

clang/test/OpenMP/distribute_simd_simdlen_messages.cpp

@@ -31,7 +31,7 @@ T tmain(T argc, S **argv) {
 #pragma omp target teams distribute parallel for simd safelen () // expected-error {{expected expression}}
   for (int i = ST; i < N; i++) argv[0][i] = argv[0][i] - argv[0][i-ST];
 
-#pragma omp target teams distribute parallel for simd safelen (argc // expected-note {{to match this '('}} expected-error 2 {{integral constant expression}} expected-note 2 {{read of non-const variable 'argc' is not allowed in a constant expression}} expected-note 0+{{constant expression}} expected-error {{expected ')'}}
+#pragma omp target teams distribute parallel for simd safelen (argc // expected-note {{to match this '('}} expected-error 2 {{integral constant expression}} expected-note 0+{{constant expression}} expected-error {{expected ')'}}
   for (int i = ST; i < N; i++) 
     argv[0][i] = argv[0][i] - argv[0][i-ST];
 

clang/test/OpenMP/target_teams_distribute_parallel_for_simd_safelen_messages.cpp

@@ -31,7 +31,7 @@ T tmain(T argc, S **argv) {
 #pragma omp target teams distribute parallel for simd safelen () // expected-error {{expected expression}}
   for (int i = ST; i < N; i++) argv[0][i] = argv[0][i] - argv[0][i-ST];
 
-#pragma omp target teams distribute parallel for simd safelen (argc // expected-note {{to match this '('}} expected-error 2 {{integral constant expression}} expected-note 2 {{read of non-const variable 'argc' is not allowed in a constant expression}} expected-note 0+{{constant expression}} expected-error {{expected ')'}}
+#pragma omp target teams distribute parallel for simd safelen (argc // expected-note {{to match this '('}} expected-error 2 {{integral constant expression}} expected-note 0+{{constant expression}} expected-error {{expected ')'}}
   for (int i = ST; i < N; i++)
     argv[0][i] = argv[0][i] - argv[0][i-ST];
 

clang/test/OpenMP/target_teams_distribute_parallel_for_simd_simdlen_messages.cpp

@@ -31,7 +31,7 @@ T tmain(T argc, S **argv) {
 #pragma omp target teams distribute simd safelen () // expected-error {{expected expression}}
   for (int i = ST; i < N; i++) argv[0][i] = argv[0][i] - argv[0][i-ST];
 
-#pragma omp target teams distribute simd safelen (argc // expected-note {{to match this '('}} expected-error 2 {{integral constant expression}} expected-note 2 {{read of non-const variable 'argc' is not allowed in a constant expression}} expected-note 0+{{constant expression}} expected-error {{expected ')'}}
+#pragma omp target teams distribute simd safelen (argc // expected-note {{to match this '('}} expected-error 2 {{integral constant expression}} expected-note 0+{{constant expression}} expected-error {{expected ')'}}
   for (int i = ST; i < N; i++) 
     argv[0][i] = argv[0][i] - argv[0][i-ST];
 

clang/test/OpenMP/target_teams_distribute_simd_safelen_messages.cpp

@@ -31,7 +31,7 @@ T tmain(T argc, S **argv) {
 #pragma omp target teams distribute simd safelen () // expected-error {{expected expression}}
   for (int i = ST; i < N; i++) argv[0][i] = argv[0][i] - argv[0][i-ST];
 
-#pragma omp target teams distribute simd safelen (argc // expected-note {{to match this '('}} expected-error 2 {{integral constant expression}} expected-note 2 {{read of non-const variable 'argc' is not allowed in a constant expression}} expected-note 0+{{constant expression}} expected-error {{expected ')'}}
+#pragma omp target teams distribute simd safelen (argc // expected-note {{to match this '('}} expected-error 2 {{integral constant expression}} expected-note 0+{{constant expression}} expected-error {{expected ')'}}
   for (int i = ST; i < N; i++) 
     argv[0][i] = argv[0][i] - argv[0][i-ST];
 

clang/test/OpenMP/target_teams_distribute_simd_simdlen_messages.cpp

@@ -35,7 +35,7 @@ T tmain(T argc, S **argv) {
   for (int i = ST; i < N; i++) argv[0][i] = argv[0][i] - argv[0][i-ST];
 
 #pragma omp target
-#pragma omp teams distribute parallel for simd safelen (argc  // expected-note {{to match this '('}} expected-error 2 {{integral constant expression}} expected-note 2 {{read of non-const variable 'argc' is not allowed in a constant expression}} expected-note 0+{{constant expression}} expected-error {{expected ')'}}
+#pragma omp teams distribute parallel for simd safelen (argc  // expected-note {{to match this '('}} expected-error 2 {{integral constant expression}} expected-note 0+{{constant expression}} expected-error {{expected ')'}}
   for (int i = ST; i < N; i++) 
     argv[0][i] = argv[0][i] - argv[0][i-ST];
 

clang/test/OpenMP/teams_distribute_parallel_for_simd_safelen_messages.cpp

@@ -35,7 +35,7 @@ T tmain(T argc, S **argv) {
   for (int i = ST; i < N; i++) argv[0][i] = argv[0][i] - argv[0][i-ST];
 
 #pragma omp target
-#pragma omp teams distribute parallel for simd safelen (argc  // expected-note {{to match this '('}} expected-error 2 {{integral constant expression}} expected-note 2 {{read of non-const variable 'argc' is not allowed in a constant expression}} expected-note 0+{{constant expression}} expected-error {{expected ')'}}
+#pragma omp teams distribute parallel for simd safelen (argc  // expected-note {{to match this '('}} expected-error 2 {{integral constant expression}} expected-note 0+{{constant expression}} expected-error {{expected ')'}}
   for (int i = ST; i < N; i++) 
     argv[0][i] = argv[0][i] - argv[0][i-ST];
 

clang/test/OpenMP/teams_distribute_parallel_for_simd_simdlen_messages.cpp

@@ -35,7 +35,7 @@ T tmain(T argc, S **argv) {
   for (int i = ST; i < N; i++) argv[0][i] = argv[0][i] - argv[0][i-ST];
 
 #pragma omp target
-#pragma omp teams distribute simd safelen (argc  // expected-note {{to match this '('}} expected-error 2 {{integral constant expression}} expected-note 2 {{read of non-const variable 'argc' is not allowed in a constant expression}} expected-note 0+{{constant expression}} expected-error {{expected ')'}}
+#pragma omp teams distribute simd safelen (argc  // expected-note {{to match this '('}} expected-error 2 {{integral constant expression}} expected-note 0+{{constant expression}} expected-error {{expected ')'}}
   for (int i = ST; i < N; i++) 
     argv[0][i] = argv[0][i] - argv[0][i-ST];
 

clang/test/OpenMP/teams_distribute_simd_safelen_messages.cpp

@@ -35,7 +35,7 @@ T tmain(T argc, S **argv) {
   for (int i = ST; i < N; i++) argv[0][i] = argv[0][i] - argv[0][i-ST];
 
 #pragma omp target
-#pragma omp teams distribute simd safelen (argc  // expected-note {{to match this '('}} expected-error 2 {{integral constant expression}} expected-note 2 {{read of non-const variable 'argc' is not allowed in a constant expression}} expected-note 0+{{constant expression}} expected-error {{expected ')'}}
+#pragma omp teams distribute simd safelen (argc  // expected-note {{to match this '('}} expected-error 2 {{integral constant expression}} expected-note 0+{{constant expression}} expected-error {{expected ')'}}
   for (int i = ST; i < N; i++) 
     argv[0][i] = argv[0][i] - argv[0][i-ST];
 

clang/test/OpenMP/teams_distribute_simd_simdlen_messages.cpp

@@ -5,7 +5,7 @@ void test(int x, double p) { // expected-note {{declared here}}
   dummy = __builtin_expect_with_probability(x > 0, 1, 0.9);
   dummy = __builtin_expect_with_probability(x > 0, 1, 1.1); // expected-error {{probability argument to __builtin_expect_with_probability is outside the range [0.0, 1.0]}}
   dummy = __builtin_expect_with_probability(x > 0, 1, -1); // expected-error {{probability argument to __builtin_expect_with_probability is outside the range [0.0, 1.0]}}
-  dummy = __builtin_expect_with_probability(x > 0, 1, p); // expected-error {{probability argument to __builtin_expect_with_probability must be constant floating-point expression}} expected-note {{read of non-constexpr variable 'p' is not allowed in a constant expression}}
+  dummy = __builtin_expect_with_probability(x > 0, 1, p); // expected-error {{probability argument to __builtin_expect_with_probability must be constant floating-point expression}} expected-note {{function parameter 'p' with unknown value}}
   dummy = __builtin_expect_with_probability(x > 0, 1, "aa"); // expected-error {{cannot initialize a parameter of type 'double' with an lvalue of type 'const char [3]'}}
   dummy = __builtin_expect_with_probability(x > 0, 1, __builtin_nan("")); // expected-error {{probability argument to __builtin_expect_with_probability is outside the range [0.0, 1.0]}}
   dummy = __builtin_expect_with_probability(x > 0, 1, __builtin_inf()); // expected-error {{probability argument to __builtin_expect_with_probability is outside the range [0.0, 1.0]}}

clang/test/Sema/builtin-expect-with-probability-avr.cpp

@@ -43,7 +43,7 @@ void test(int x, double p) { // expected-note {{declared here}}
   dummy = __builtin_expect_with_probability(x > 0, 1, 0.9);
   dummy = __builtin_expect_with_probability(x > 0, 1, 1.1); // expected-error {{probability argument to __builtin_expect_with_probability is outside the range [0.0, 1.0]}}
   dummy = __builtin_expect_with_probability(x > 0, 1, -1); // expected-error {{probability argument to __builtin_expect_with_probability is outside the range [0.0, 1.0]}}
-  dummy = __builtin_expect_with_probability(x > 0, 1, p); // expected-error {{probability argument to __builtin_expect_with_probability must be constant floating-point expression}} expected-note {{read of non-constexpr variable 'p' is not allowed in a constant expression}}
+  dummy = __builtin_expect_with_probability(x > 0, 1, p); // expected-error {{probability argument to __builtin_expect_with_probability must be constant floating-point expression}} expected-note {{function parameter 'p'}}
   dummy = __builtin_expect_with_probability(x > 0, 1, "aa"); // expected-error {{cannot initialize a parameter of type 'double' with an lvalue of type 'const char [3]'}}
   dummy = __builtin_expect_with_probability(x > 0, 1, __builtin_nan("")); // expected-error {{probability argument to __builtin_expect_with_probability is outside the range [0.0, 1.0]}}
   dummy = __builtin_expect_with_probability(x > 0, 1, __builtin_inf()); // expected-error {{probability argument to __builtin_expect_with_probability is outside the range [0.0, 1.0]}}

clang/test/Sema/builtin-expect-with-probability.cpp

@@ -65,7 +65,7 @@ void test10 (int x[*]); /* expected-warning {{variable length arrays are a C99 f
 void test11 (int x[static 4]); /* expected-warning {{static array size is a C99 feature}} */
 
 void test12 (int x[const 4]) { /* expected-warning {{qualifier in array size is a C99 feature}} */
-  int Y[x[1]]; /* expected-warning {{variable length arrays are a C99 feature}} expected-note {{parameter 'x'}} */
+  int Y[x[1]]; /* expected-warning {{variable length arrays are a C99 feature}} */
 }
 
 /* PR4074 */

clang/test/Sema/c89.c

@@ -17,7 +17,7 @@ __host__ __device__ void foo(const T **a) {
   constexpr T e = sizeof(a);
   constexpr T f = **a;
   // expected-error@-1 {{constexpr variable 'f' must be initialized by a constant expression}}
-  // expected-note@-2 {{read of non-constexpr variable 'a' is not allowed in a constant expression}}
+  // expected-note@-2 {{}}
   a[0] = &b;
   a[1] = &c;
   a[2] = &d;
@@ -30,7 +30,7 @@ __device__ void device_fun(const int **a) {
   static constexpr int c = sizeof(a);
   constexpr int d = **a;
   // expected-error@-1 {{constexpr variable 'd' must be initialized by a constant expression}}
-  // expected-note@-2 {{read of non-constexpr variable 'a' is not allowed in a constant expression}}
+  // expected-note@-2 {{}}
   a[0] = &b;
   a[1] = &c;
   foo(a);
@@ -43,7 +43,7 @@ void host_fun(const int **a) {
   static constexpr int c = sizeof(a);
   constexpr int d = **a;
   // expected-error@-1 {{constexpr variable 'd' must be initialized by a constant expression}}
-  // expected-note@-2 {{read of non-constexpr variable 'a' is not allowed in a constant expression}}
+  // expected-note@-2 {{}}
   a[0] = &b;
   a[1] = &c;
   foo(a);
@@ -55,7 +55,7 @@ __host__ __device__ void host_device_fun(const int **a) {
   static constexpr int c = sizeof(a);
   constexpr int d = **a;
   // expected-error@-1 {{constexpr variable 'd' must be initialized by a constant expression}}
-  // expected-note@-2 {{read of non-constexpr variable 'a' is not allowed in a constant expression}}
+  // expected-note@-2 {{}}
   a[0] = &b;
   a[1] = &c;
   foo(a);

clang/test/SemaCUDA/constexpr-variables.cu

@@ -18,12 +18,12 @@ struct POD {
 
 // We allow VLAs of POD types, only.
 void vla(int N) { // expected-note 5{{here}}
-  int array1[N]; // expected-warning{{variable length arrays are a C99 feature}} expected-note {{variable 'N'}}
-  POD array2[N]; // expected-warning{{variable length arrays are a C99 feature}} expected-note {{variable 'N'}}
-  StillPOD array3[N]; // expected-warning{{variable length arrays are a C99 feature}} expected-note {{variable 'N'}}
-  StillPOD2 array4[N][3]; // expected-warning{{variable length arrays are a C99 feature}} expected-note {{variable 'N'}}
+  int array1[N]; // expected-warning{{variable length arrays are a C99 feature}} expected-note {{parameter 'N'}}
+  POD array2[N]; // expected-warning{{variable length arrays are a C99 feature}} expected-note {{parameter 'N'}}
+  StillPOD array3[N]; // expected-warning{{variable length arrays are a C99 feature}} expected-note {{parameter 'N'}}
+  StillPOD2 array4[N][3]; // expected-warning{{variable length arrays are a C99 feature}} expected-note {{parameter 'N'}}
   NonPOD array5[N]; // expected-error{{no matching constructor for initialization of 'NonPOD [N]'}}
-  // expected-warning@-1{{variable length arrays are a C99 feature}} expected-note@-1 {{variable 'N'}}
+  // expected-warning@-1{{variable length arrays are a C99 feature}} expected-note@-1 {{parameter 'N'}}
   // expected-note@-16{{candidate constructor not viable}}
   // expected-note@-18{{candidate constructor (the implicit copy constructor) not viable}}
   // expected-note@-19{{candidate constructor (the implicit move constructor) not viable}}

clang/test/SemaCXX/c99-variable-length-array-cxx11.cpp

@@ -13,6 +13,7 @@ struct POD {
 };
 
 // expected-note@* 1+{{read of non-const variable}}
+// expected-note@* 1+{{function parameter}}
 // expected-note@* 1+{{declared here}}
 
 // We allow VLAs of POD types, only.