[clang] Fix -Wdouble-promotion in C++ list-initialization

clang/docs/ReleaseNotes.rst

@@ -417,6 +417,7 @@ Bug Fixes to C++ Support
   ``__builtin_addressof``, and related issues with builtin arguments. (#GH154034)
 - Fix an assertion failure when taking the address on a non-type template parameter argument of
   object type. (#GH151531)
+- Suppress ``-Wdouble-promotion`` when explicitly asked for with C++ list initialization (#GH33409).
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaChecking.cpp

@@ -12797,6 +12797,22 @@ static void CheckCommaOperand(
     S.CheckImplicitConversion(E, T, CC);
 }
 
+static Expr *IgnoreExplicitCastForImplicitConversionCheck(ExplicitCastExpr *E) {
+  // In the special case of C++ function-style cast with braces,
+  // CXXFunctionalCastExpr has InitListExpr as direct child with a single
+  // initializer. It basically belongs to the cast itself, so for the purposes
+  // of checking for implicit conversions to warn about it should be skipped
+  // too.
+  if (auto *FCE = dyn_cast<CXXFunctionalCastExpr>(E)) {
+    if (auto *IFCE = dyn_cast<InitListExpr>(FCE->getSubExpr())) {
+      if (IFCE->getNumInits() == 1) {
+        return IFCE->getInit(0);
+      }
+    }
+  }
+  return E->getSubExpr();
+}
+
 /// Data recursive variant of AnalyzeImplicitConversions. Subexpressions
 /// that should be visited are added to WorkList.
 static void AnalyzeImplicitConversions(
@@ -12909,7 +12925,7 @@ static void AnalyzeImplicitConversions(
 
   // Skip past explicit casts.
   if (auto *CE = dyn_cast<ExplicitCastExpr>(E)) {
-    E = CE->getSubExpr()->IgnoreParenImpCasts();
+    E = IgnoreExplicitCastForImplicitConversionCheck(CE)->IgnoreParenImpCasts();
     if (!CE->getType()->isVoidType() && E->getType()->isAtomicType())
       S.Diag(E->getBeginLoc(), diag::warn_atomic_implicit_seq_cst);
     WorkList.push_back({E, CC, IsListInit});

clang/test/Sema/warn-double-promotion.c

@@ -1,4 +1,5 @@
 // RUN: %clang_cc1 -triple x86_64-apple-darwin -verify -fsyntax-only %s -Wdouble-promotion
+// RUN: %clang_cc1 -triple x86_64-apple-darwin -verify -fsyntax-only -x c++ %s -Wdouble-promotion
 
 float ReturnFloatFromDouble(double d) {
   return d;
@@ -24,10 +25,25 @@ long double ReturnLongDoubleFromDouble(double d) {
   return d;  //expected-warning{{implicit conversion increases floating-point precision: 'double' to 'long double'}}
 }
 
+double ReturnDoubleFromFloatWithExplicitCast(float f) {
+  return (double)f;
+}
+
+long double ReturnLongDoubleFromFloatWithExplicitCast(float f) {
+  return (long double)f;
+}
+
+long double ReturnLongDoubleFromDoubleWithExplicitCast(double d) {
+  return (long double)d;
+}
+
 void Assignment(float f, double d, long double ld) {
   d = f;  //expected-warning{{implicit conversion increases floating-point precision: 'float' to 'double'}}
   ld = f; //expected-warning{{implicit conversion increases floating-point precision: 'float' to 'long double'}}
   ld = d; //expected-warning{{implicit conversion increases floating-point precision: 'double' to 'long double'}}
+  d = (double)f;
+  ld = (long double)f;
+  ld = (long double)d;
   f = d;
   f = ld;
   d = ld;
@@ -40,6 +56,9 @@ void ArgumentPassing(float f, double d) {
   DoubleParameter(f); // expected-warning{{implicit conversion increases floating-point precision: 'float' to 'double'}}
   LongDoubleParameter(f); // expected-warning{{implicit conversion increases floating-point precision: 'float' to 'long double'}}
   LongDoubleParameter(d); // expected-warning{{implicit conversion increases floating-point precision: 'double' to 'long double'}}
+  DoubleParameter((double)f);
+  LongDoubleParameter((long double)f);
+  LongDoubleParameter((long double)d);
 }
 
 void BinaryOperator(float f, double d, long double ld) {
@@ -49,12 +68,21 @@ void BinaryOperator(float f, double d, long double ld) {
   f = ld * f; // expected-warning{{implicit conversion increases floating-point precision: 'float' to 'long double'}}
   d = d * ld; // expected-warning{{implicit conversion increases floating-point precision: 'double' to 'long double'}}
   d = ld * d; // expected-warning{{implicit conversion increases floating-point precision: 'double' to 'long double'}}
+  f = (double)f * d;
+  f = d * (double)f;
+  f = (long double)f * ld;
+  f = ld * (long double)f;
+  d = (long double)d * ld;
+  d = ld * (long double)d;
 }
 
 void MultiplicationAssignment(float f, double d, long double ld) {
   d *= f; // expected-warning{{implicit conversion increases floating-point precision: 'float' to 'double'}}
   ld *= f; // expected-warning{{implicit conversion increases floating-point precision: 'float' to 'long double'}}
   ld *= d; // expected-warning{{implicit conversion increases floating-point precision: 'double' to 'long double'}}
+  d *= (double)f;
+  ld *= (long double)f;
+  ld *= (long double)d;
 
   // FIXME: These cases should produce warnings as above.
   f *= d;

clang/test/Sema/warn-double-promotion.cpp

@@ -0,0 +1,145 @@
+// RUN: %clang_cc1 -triple x86_64-apple-darwin -verify -fsyntax-only %s -Wdouble-promotion
+
+using LongDouble = long double;
+
+double ReturnDoubleFromFloatWithExplicitCast(float f) {
+  return static_cast<double>(f);
+}
+
+long double ReturnLongDoubleFromFloatWithExplicitCast(float f) {
+  return static_cast<long double>(f);
+}
+
+long double ReturnLongDoubleFromDoubleWithExplicitCast(double d) {
+  return static_cast<long double>(d);
+}
+
+double ReturnDoubleFromFloatWithExplicitListInitialization(float f) {
+  return double{f};
+}
+
+long double ReturnLongDoubleFromFloatWithExplicitListInitialization(float f) {
+  return LongDouble{f};
+}
+
+long double ReturnLongDoubleFromDoubleWithExplicitListInitialization(double d) {
+  return LongDouble{d};
+}
+
+double ReturnDoubleFromFloatWithFunctionStyleCast(float f) {
+  return double(f);
+}
+
+long double ReturnLongDoubleFromFloatWithFunctionStyleCast(float f) {
+  return LongDouble(f);
+}
+
+long double ReturnLongDoubleFromDoubleWithFunctionStyleCast(double d) {
+  return LongDouble(d);
+}
+
+void InitializationWithParens(float f, double d) {
+  {
+    double d(f);  // expected-warning{{implicit conversion increases floating-point precision: 'float' to 'double'}}
+    long double ld0(f);  // expected-warning{{implicit conversion increases floating-point precision: 'float' to 'long double'}}
+    long double ld1(d);  // expected-warning{{implicit conversion increases floating-point precision: 'double' to 'long double'}}
+  }
+  {
+    double d(static_cast<double>(f));
+    long double ld0(static_cast<long double>(f));
+    long double ld1(static_cast<long double>(d));
+  }
+  {
+    double d(double{f});
+    long double ld0(LongDouble{f});
+    long double ld1(LongDouble{d});
+  }
+  {
+    double d((double(f)));
+    long double ld0((LongDouble(f)));
+    long double ld1((LongDouble(d)));
+  }
+}
+
+void InitializationWithBraces(float f, double d) {
+  {
+    double d{f};  // expected-warning{{implicit conversion increases floating-point precision: 'float' to 'double'}}
+    long double ld0{f};  // expected-warning{{implicit conversion increases floating-point precision: 'float' to 'long double'}}
+    long double ld1{d};  // expected-warning{{implicit conversion increases floating-point precision: 'double' to 'long double'}}
+  }
+  {
+    double d{static_cast<double>(f)};
+    long double ld0{static_cast<long double>(f)};
+    long double ld1{static_cast<long double>(d)};
+  }
+  {
+    double d{double{f}};
+    long double ld0{LongDouble{f}};
+    long double ld1{LongDouble{d}};
+  }
+  {
+    double d{double(f)};
+    long double ld0{LongDouble(f)};
+    long double ld1{LongDouble(d)};
+  }
+}
+
+void Assignment(float f, double d, long double ld) {
+  d = static_cast<double>(f);
+  ld = static_cast<long double>(f);
+  ld = static_cast<long double>(d);
+  d = double{f};
+  ld = LongDouble{f};
+  ld = LongDouble{d};
+  d = double(f);
+  ld = LongDouble(f);
+  ld = LongDouble(d);
+}
+
+extern void DoubleParameter(double);
+extern void LongDoubleParameter(long double);
+
+void ArgumentPassing(float f, double d) {
+  DoubleParameter(static_cast<double>(f));
+  LongDoubleParameter(static_cast<long double>(f));
+  LongDoubleParameter(static_cast<long double>(d));
+  DoubleParameter(double{f});
+  LongDoubleParameter(LongDouble{f});
+  LongDoubleParameter(LongDouble{d});
+  DoubleParameter(double(f));
+  LongDoubleParameter(LongDouble(f));
+  LongDoubleParameter(LongDouble(d));
+}
+
+void BinaryOperator(float f, double d, long double ld) {
+  f = static_cast<double>(f) * d;
+  f = d * static_cast<double>(f);
+  f = static_cast<long double>(f) * ld;
+  f = ld * static_cast<long double>(f);
+  d = static_cast<long double>(d) * ld;
+  d = ld * static_cast<long double>(d);
+  f = double{f} * d;
+  f = d * double{f};
+  f = LongDouble{f} * ld;
+  f = ld * LongDouble{f};
+  d = LongDouble{d} * ld;
+  d = ld * LongDouble{d};
+  f = double(f) * d;
+  f = d * double(f);
+  f = LongDouble(f) * ld;
+  f = ld * LongDouble(f);
+  d = LongDouble(d) * ld;
+  d = ld * LongDouble(d);
+}
+
+void MultiplicationAssignment(float f, double d, long double ld) {
+  d *= static_cast<double>(f);
+  ld *= static_cast<long double>(f);
+  ld *= static_cast<long double>(d);
+  d *= double{f};
+  ld *= LongDouble{f};
+  ld *= LongDouble{d};
+  d *= double(f);
+  ld *= LongDouble(f);
+  ld *= LongDouble(d);
+}