Reject invalid integer constants in unevaluated preprocessor operands (#134884)

Clang was previously accepting invalid code like:
```
  #if 1 ? 1 : 999999999999999999999
  #endif
```
because the integer constant (which is too large to fit into any
standard or extended integer type) was in an unevaluated branch of the
conditional operator. Similar invalid code involving || or && was also
accepted and is now rejected.

Fixes #134658

clang/docs/ReleaseNotes.rst

@@ -354,6 +354,14 @@ Bug Fixes in This Version
 - Defining an integer literal suffix (e.g., ``LL``) before including
   ``<stdint.h>`` in a freestanding build no longer causes invalid token pasting
   when using the ``INTn_C`` macros. (#GH85995)
+- Clang no longer accepts invalid integer constants which are too large to fit
+  into any (standard or extended) integer type when the constant is unevaluated.
+  Merely forming the token is sufficient to render the program invalid. Code
+  like this was previously accepted and is now rejected (#GH134658):
+  .. code-block:: c
+
+    #if 1 ? 1 : 999999999999999999999
+    #endif
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Lex/PPExpressions.cpp

@@ -345,9 +345,7 @@ static bool EvaluateValue(PPValue &Result, Token &PeekTok, DefinedTracker &DT,
     // Parse the integer literal into Result.
     if (Literal.GetIntegerValue(Result.Val)) {
       // Overflow parsing integer literal.
-      if (ValueLive)
-        PP.Diag(PeekTok, diag::err_integer_literal_too_large)
-            << /* Unsigned */ 1;
+      PP.Diag(PeekTok, diag::err_integer_literal_too_large) << /* Unsigned */ 1;
       Result.Val.setIsUnsigned(true);
     } else {
       // Set the signedness of the result to match whether there was a U suffix

clang/test/Preprocessor/constants.c

@@ -0,0 +1,40 @@
+// RUN: %clang_cc1 -E -verify %s
+
+// C++ [lex.icon]p4 and C 6.4.4.1p2 + 6.4.4.2p7 both require C and C++ to
+// validate the integer constant value when converting a preprocessing token
+// into a token for semantic analysis, even within the preprocessor itself.
+
+// Plain integer constant.
+#if 999999999999999999999 // expected-error {{integer literal is too large to be represented in any integer type}}
+#endif
+
+// These cases were previously incorrectly accepted. See GH134658.
+
+// Integer constant in an unevaluated branch of a conditional.
+#if 1 ? 1 : 999999999999999999999 // expected-error {{integer literal is too large to be represented in any integer type}}
+#endif
+
+// Integer constant in an unevaluated operand of a logical operator.
+#if 0 && 999999999999999999999 // expected-error {{integer literal is too large to be represented in any integer type}}
+#endif
+
+#if 1 || 999999999999999999999 // expected-error {{integer literal is too large to be represented in any integer type}}
+#endif
+
+// Make sure we also catch it in an elif condition.
+#if 0
+#elif 1 || 999999999999999999999 // expected-error {{integer literal is too large to be represented in any integer type}}
+#endif
+
+// However, if the block is skipped entirely, then it doesn't matter how
+// invalid the constant value is.
+#if 0
+int x = 999999999999999999999;
+
+#if 999999999999999999999
+#endif
+
+#if 0 && 999999999999999999999
+#endif
+
+#endif